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    Laser Instruments and Applications

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  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Introduction to Laser Instruments and Applications

    When the laser was invented in 1960, it was amazingly, a solution looking for a problem. While the laser's weapons potential was clear, most of the uses of lasers that have changed the World were not foreseen even by the so-called experts of the time. In this chapter, we touch on perhaps one tenth of one percent of those where lasers are now indispensable, or at least have the potential to be in the future.

    But lasers are not the solution to every problem. There are applications where lasers are not useful and probably never will be. Among the short list of idiotic proposals for lasers are (in no particular order): grass and tree trimming, insect extermination, and advertising on the moon. For more details and a few chuckles, see the section: Laser Humor.

  • Back to Laser Instruments and Applications Sub-Table of Contents.


    Using a Laser to Measure Distance, Position, or Speed

    There are a variety of ways of using lasers to measure distance. The precise 3-D shape or profile of solid objects can be determined using laser scanning techniques. Common approaches include: Laser Atlanta Optics is an example of a company that specializes in laser based distance and speed measureing technology.

    Manufactures/suppliers of devices used in laser rangefinders include: E-O Devices and Analog Modules.

    Optical Rangefinders

    This is the basic principle used in 35 mm rangefinder cameras and other devices where you view the distance scene and turn a knob to line up two images that are either superimposed or split top/bottom half. In the case of the camera, turning the lens focus ring adjusts the angle of mirror A below.

              To distant scene.
              ^               ^
              |               |
              |       C/------/D    
              |A       |      
              \--------\       (B is partially silvered or a half mirror to
             adjust   B|        permit viewing of both sides from the scene.)
             angle     ^
                   view here
              |               |
              |<- baseline -->|

    The further apart the mirrors are (size of baseline), the greater the useful range. Adjust the angle of mirror A or D until the images are superimposed. Calibrate the angular setting to distance.

    The distance from A to the scene is then: tan(angle A) * baseline.

    For long distances, C and D can be eliminated - they compensate for the difference in path lengths of the two views - else the sizes would not be the same. (Even this doesn't work perfectly in any case. Can you figure out why?)

    You can add telescopes and other optics if you like - this is just the basics.

    Look Ma, no electronics. :-)

    Note that SLR cameras do NOT use this approach as they are entirely optical (meaning that adjusting the focus only controls the lens - nothing else!). With SLRs, a pair of shallow prisms oriented in opposite directions (or many in the case of a 'microscreen' type) are cemented onto a clear area of the ground glass. When the image is precisely focused onto the ground glass, the prisms have no effect. However, when the image is in front or behind, they divert the rays such that the two halves of the image move apart (or the image breaks up in the case of the 'microscreen').

    There were some "Amateur Scientist" articles in Scientific American a few decades ago on constructing several types of optical range finders. These were included in the book, "Light and Its Uses". See the section: A HREF="laserclt.htm#cltsi">Scientific American Articles on Lasers and Related Topics.

    Simple Laser Rangefinder Based on Triangulation

    (Portions from: Mike Cimorosi (

    My students construct a simple laser rangefinder using a few basic parts:


    Basic procedure:

    1. Place the laser to the left of the optical bench. Follow standard safety procedures for using 1/2 mW lasers.

    2. About 3 inches to the right of the laser aperture (opening), place the beam splitter at an angle of 45 degrees with respect to (wrt) the incident beam. This will split the beam into two different paths. Most of the beam will pass through the splitter. Some will be reflected at a right angle wrt the incident beam.

    3. About 6 feet to the right of the splitter, place the rotary table with the mirror on it and face it toward the beam that passes through the splitter.

    4. Now, before you turn on the laser, make sure you have a safe place to aim the beam for the distance you want to determine.

    5. Now fire up the laser. Note where the first reflected beam strikes the target (a wall maybe?). Now, slowly and carefully rotate the rotary table until the beam reflected from the mirror coincides with first reflected beam. You now have formed a right triangle made of laser light! Pretty neat! Remember to respect the beam, especially with respect to your eyes!!!

    6. Finally, you can use the trig relation: distance = 6 ft x tan(angle) to determine the distance. How's your trig? :-)

    7. It's not the most precise rangefinder - i.e., the equation is pretty sensitive to the angular precision of the rotary table. However, it does demonstrate the basic principle. Maybe the diagram below will help with setting up the laser rangefinder.

    Rough diagram of rangefinder setup:

                   To wall                    To wall
                     ^                           ^
                     |                             \ 
            distance | first reflected beam          \ second reflected beam
                     |                                 \
                     |                             angle \
        Laser --3"---/------------------------------------/
                Beamsplitter                    Rotary table with mirror
                     |<------------- 6 feet ------------->|

    Of course, you can make the non-laser version of this type of rangefinder (but this is a laser FAQ! --- sam). My students also make that one as well. Both are pretty neat and demonstrate the power of trig to determine distances!

    Comments on Laser Rangefinders

    (From: Andrzej Hanczak).

    I am just finishing the development of a range finder based on the TOF (pulse-Time-Of-Flight) measurement method. There are also different methods like phase-shift method which compares the phase shift between outgoing modulated beam and reflected light.

    The Pulse TOF method has some advantages which make it very useful: you can use relatively high pulse power and still be in the Class I safety range.

    While building such a range finder there are two crucial components which have influence on its accuracy: the time measurement circuits and the receiver. Our aim was to build a laser scanner with the resolution of 1 cm which means that you have to be able to measure the time with the resolution of 67 ps. The range of the scanner should be approx. 30m. We are not ready yet but there are some results.

    For the first prototype we used a 1.25 GHz oscillator and special microstrip design to get the resolution of 70 ps. In the current prototype we use a special prototype IC which should deliver 50 ps resolution.

    The problems are on the receiver side, a relatively large jitter (which I'm fighting now) destroys my high time measurement precision. The jitter on the input results in the distance differences of approximately 10 cm). This can be filtered out by averaging of a number of measurements and that is what we are doing now. Our measurement frequency is at present 100 kHz, but we will probably perform the averaging over 10 measurements so that effective measurement rate will be 10 kHz.

    (From: jfd (

    The problem is getting simultaneous long standoff range and extremely accurate range. You can phase detect with accuracies in the sub-inch range using direct detected RF modulated LIDARS or you can use an interferometric technique with a reference to get sub-micron distances.

    (From: Robert (

    For much better resolution than would be possible with simple sampling while still maintaining low cost, digital TOF rangefinders can combine a precision analog temporal interpolator with say a CMOS system running at 100 MHz. The analog circuitry to accomplish this is in many production units (for different applications) - but 5 ps resolution has been achieved with low-cost components and in production for 15 years from at least one manufacturer. The idea is interpolate between the digital count periods with a precision time-to-voltage converter which is then sampled by microcontroller and combined with the digital counter results.

    (From: Bill Sloman (

    You may be able to achieve this at low unit cost, but getting a precision analog temporal interpolator to work well next to CMOS running at 100 MHz isn't something I'd describe as easy.

    We developed a system of this sort at Cambridge Instruments between 1988 and 1991 using a mixture of 100K ECL and GigaBit Logic's GaAs for the digital logic. Any digital signal going to or from the analog temporal interpolator was routed as a balanced pair on adjacent tracks, and we were very careful about the layout, but we still had to work at getting the noise on the interpolator output down to the 60 picosecond jitter on our 800 MHz master clock (getting a better master clock was the next priority).

    Current-steering logic (like ECL and GaAs) is a lot quieter than voltage-steering logic (like TTL and CMOS), which is why very fast DACs and ADCs use ECL interfaces. Precision analog interpolators are no less sensitive.

    Do you know who has actually achieved that 5 ps resolution and for what application? Tektronix and time domain reflectometers come to mind, though Tektronix isn't exactly cheap. IIRR Triquint was originally their in-house analog foundry and I think Tektronix has been using GaAs ASICs in their faster gear for quite some time now.

    The hybrid approach certainly isn't new, but getting it to work is a fair test of one's analog skills.

    Of course, using phase-shift not only makes for easier circuit design, but also lets you run your LED at a 50% duty cycle, giving you a lot more reflected photons to work with than the 0.01% you get with TOF.

    (From: Lou Boyd (

    The Texas Instruments book "Optoelectronics: Theory and Practice" published by McGraw-Hill had a chapter (23) on the design of an LED/Si Diode rangefinder with schematics of the transmitter, receiver, and timing section. This was a phase modulated design but obsolete by todays standards. Low cost modern rangefinders like those by Leica or even Bushnell are far more advanced in the detection circuit than that in the TI book. Most eye-safe commercial rangefinders use phase modulated techniques. This gives good accuracy but limited range, usually less than 1 kilometer with measurement times typically 1/10 second.

    Most military rangefinders use a much higher power transmitter with a time of flight method. A time of flight rangefinder just sends a single pulse and receives it. Some use multiple pulses for improved resolution and range but that typically isn't necessary. A counter is started on the rising edge of the transmitted pulse and stopped when the rising edge of the receive pulse is detected. If the counter is measuring a 150 MHz (approx) clock the range will be displayed in meters. Unfortunately that fast of counter requires at least a few high speed chips beyond the capability of standard CMOS or TTL logic. Since the round trip takes only 6.667 microseconds per kilometer you don't even need blanking on the displays. They can be attached directly to the counters or just read by a computer. A four or five digit counter suffices for most purposes. There is a little added complexity on sophisticated units for making the sensitivity of the receiver increase with time after the pulse is transmitted. This is sometimes done by charging a capacitor attached to a gain control which increases the gain with the square of time out to the maximum the unit is capable of. These rangefinders tend to be expensive because of the technology but the electronics is simple in concept. Ranges are limited only by the transmit power which can be extremely high using solid state Q switched lasers.

    Surplus lasers and the associated electronics from military rangefinders have been showing up on the surplus market in the $300 range. Unfortunately the receivers have not.

    For some insight on the level of complexity involved look at the boards sold by E-O Devices These are time of flight pulsed laser rangefinder components designed for use primarily with LED's or diode lasers. Also check Analog Modules for examples of state of the art variable gain rangefinder receivers. If you want one of their modules plan on spending between $1,000 and $2,000. :-(

    Phase shift methods allow achieving high precision in distance resolution with lower power and lower speed circuitry. That equates to lower cost and higher precision. Which type is best depends on what properties are needed.

     Parameter      Single Pulse           Phase Shift
     Range          100 m to 100 km        1 m to 10 km
     Resolution     1 m any target         1 mm corner cube to 1 m any
     Cost           $5000 and up           $100 and up
     Power level    10 w to 1 MW           1 mW to 1 W
     Time to read   sub-ms                 0.01 to 10 seconds
     Applications   artillery, navigation  surveying, hunting

    Single pulse rangefinders typically use YAG or erbium lasers while most of the phase shift type use diode lasers.

    (From: Don Stauffer

    Which type to use depends a bit on what range resolution you are looking for. If you want high resolution, you will be working with a high modulation frequency. Then you may find many circuits designed for receiving audio modulation may not provide enough bandwidth.

    Also, there is the range ambiguity problem. If you go high enough in frequency, you may find some range ambiguity.

    You will also likely be needing very accurate phase measurement circuits if you are using moderate modulation frequency, so study carefully high accuracy phase detectors. These are not trivial circuits. In order for them to work well, you need a pretty good SNR.

    (From: A. E. Siegman (

    Adding to what others have said, hand-held laser rangefinders using low-power RF-modulated CW lasers (a.k.a. diode lasers) together with phase-detection techniques are simpler, cheaper, smaller, *much* more battery efficient, and much safer; and are more or less replacing the pulsed hand-held versions of yore.

    These techniques are also moderately old. Coherent (maybe Spectra also) were making widely used laser surveying instruments ("Geodolite"?) that worked this way a couple decades or more ago (and there may have been incoherent light source versions even further back).

    I suppose that compared to TOF, one disadvantage is that it takes longer to integrate up the signal to get a range finding, and if you're in a tank battle and want to get off the first shot before alerting the enemy that you're illuminating him and giving him a chance to duck, the pulsed type may still be better.

    Do some web searching: You can buy binoculars with a built-in diode laser rangefinder from Amazon, and use it to measure the distance to the pin on your next golf outing.

    (From: Louis Boyd (

    Prior to laser diodes (1960's) there were optical geodimeters which used a tungsten lamp, a Kerr shutter (which modulates light at multi-megahertz rates using polarizers and high voltage rf driven nitrobenzene), and photomultiplier receivers. These could measure distances to a few centimeters at ranges of several kilometers. They were large, expensive, and a bi*ch to calibrate. They used phase shift techniques similar to modern diode rangefinders, but without the aid of microprocessors. They switched modulation frequencies to resolve phase ambiguities.

    Modern rangefinders often use pseudorandom modulation and cross-correlation computation to give the round-trip delay which is proportional to distance. Distance resolution can be much finer than the length of the shortest pulse.

    With modern geodimeters the distance accuracy is primarily limited by uncertainty of light propagation velocity in the air since it's not practical to measure the pressure and humidity at all points along the path, but can be accurate to better than 1 part in 10^6 with care. Tape and chain is difficult to get better than 1 part in 10^3 which is the typical accuracy of $200 pocket laser rangefinders.

    (From: Mike Poulton (

    Using pulses is not very practicable - if you want to achieve a resolution of a few mm over a distance of 100 m or so, you find that you'd need extremely short pulses (recall that 1 ns corresponds to 30 cm or 12 inches, approximately, so you's need pulses of a few ps); you could do this with a W-switched SS laser, but those little hand-held devices, who do have a resolution in this order of magnitude, cannot work in this way. They use a RF-modulated CW signal from a laser diode, say with 100 MHz, and measure the phase shift of the 100 MHz signal between outgoing and incoming beams. This phase shift can be very accurately measured by first converting the 100 MHz down to a few 100 kHz (like a superheterodyne receiver).

    Some while ago I had been interested in such a circuit myself (for measuring optical path lengths) but didn't find anything useful on the web.

    (From: Repeating Rifle (

    Equipment of this ilk is called *distance measuring equipment* or DME and has all but replaced the use of chains in surveying practice. Various implementations have been used. Some use high frequencies to obtain precision and lower frequencies for range ambiguity resolution. Others use inconmensurate frequencies that are not all that different from one another. I you match the filtering to the transmission, you pretty much get the same signal to noise ration for all kinds of devices. The broad-band pulses mentioned above use short pulses. The CW devices use narrow band filters.

    The first items of this nature used RF directly without light.

    Trade names that come to mind quickly are tellurometer and geodimeter.

    For the military rangefinders that use high power pulses, signal processing is less than optimum. An error of 5 meters will usually not be a big deal. For surveying, that kind of error will usually be unacceptable. In both cases extended (in range) targets will introduce error.

    Almost all of the inexpensive hand-held rangefinders on the market use a simplified form of phase detection with relatively low modulation rates. Phase sensing rangefinders uses a variable pulse width modulated laser diode. It would use use thousands of on/off transitions in determining each distance measurement by comparing the modulation pattern to the returned signal using cross-correlation techniques. Resolution is a function of measurement time, speed and size of the registers, and instrument stability. Single pulse TOF rangefinders on the other hand are generally used for very long ranges (several km and up) with very high pulse power (kilowatts to megawatts peak) and range resolution rarely better than a meter. Low power single pulse rangefinders are rare as the expense of the detection circuits isn't justified for the low resolution.

    The accuracy of quality surveying distance meters is limited primarily by the uncertainty of the velocity of propagation of light through the atmosphere. That varies of with air pressure and humidity which can't easily be determined over the entire path. Still, they're orders of magnitude better than a tape or chain.

    (From: Phil Hobbs (

    Modulated CW measurements also allow you to use very narrow measurement bandwidths very easily (e.g. with a PLL), which helps the SNR very much. In shorter range units, sinusoidal modulation can also be used to prevent back-reflections from causing mode hopping. You choose delta-f so that the phase modulation of the back-reflection (in radians) is at a null of the zero-order Bessel function J0. This can make a huge difference (3 orders of magnitude) in the back-reflection sensitivity.

    Building a Time-of-Flight Laser Rangefinder

    The following is what I would suggest for a relatively low cost approach achieving 15 to 50 cm resolution and 100 meter or more range. However, also see the next section for a much simpler approach that may be adequate.

    A Q-switched solid state laser will give you short pulses with minimal fuss. A unit like the small surplus Nd:YAG laser (SSY1) described in chapter: Solid State Lasers was originally part of the M-1 tank rangefinders and thus should be ideal. It is quite trivial to build a suitable power supply these laser heads since a passive Q-switch is used and this doesn't require any electrical control.

    A few mJ should be sufficient. (SSY1 is probably in the 10 to 30 mJ range using the recommended pulse forming network.) With a Q-switched laser, the required short pulse if created automagically eliminating much of the complexity of the laser itself.

    Diode laser assemblies from the Chieftain tank rangefinder are also available on the surplus market but you probably would have to build a pulsed driver for them which would be more work.

    For the detector, a PIN photodiode or avalanche photodiode (APD) would be suitable. The preamp is the critical component to get the required ns response time. You need to sample both the pulse going out and the return since the delay from firing the flashlamp (if you are using a solid state laser) to its output pulse is not known or constant.

    15 cm resolution requires a time resolution of about 1 ns (twice what you might think because the pulse goes out and back). GHz class counters are no big deal these days.

    However, approaches that are partially analog (ramp and A/D) which don't require such high speed counters are also possible. In fact, if your digital design skills aren't so great, this is probably the easiest way to get decent resolution, if possibly not the greatest accuracy/consistency. All you need is a constant current source and an A/D (Analog to Digital converter). This can be as simple as a FF driving a transistor buffer to turn the voltage to charge the capacitor on and off with a transistor set up with emitter feedback for as a constant current source. Or, it can just be an exponential charge with non-linear correction done in software. The A/D doesn't need to be fast as long as its output word has enough bits for your desired resolution. For a typical exponential charging waveform, add 1 bit to the required A/D word size. For example, determining distance over 100 meters to to 5 cm resolution would require that the full voltage ramp be about 700 ns in duration (a bit over maximum round trip time, cut off sooner if there is a return pulse) and then sampled with a 12 bit A/D.

    Another even simpler way of doing this is to charge the capacitor as above but then discharge it with a much longer time constant and determine how long it takes to reach a fixed voltage. By making the discharge time constant sufficiently large, any vanilla flavored microprocessor could be used for control and timing.

    All in all, these are non-trivial but doable projects.

    See the previous sections on laser rangefinders for more info.

    Here is a Web site that appears to go into some detail on the design of TOF laser rangefinders:

    Resonant Time-of-Flight Laser Rangefinder

    This is a slightly modified approach and may be made to work with relatively simple inexpensive circuitry. The idea is to use a normal IR or visible laser diode (e.g., such as from a CD or DVD player) in conjunction with a common photodiode to form an oscillator whose frequency will depend on the path delay between them - i.e., the distance to the "target". Basically, the laser diode is turned on which sends out a leading edge of a light pulse. The light hits the target and is reflected back into the photodiode, which turns the laser diode off. The loss of signal then turns the laser diode on and the cycle repeats continuously. The oscillating frequency is then equal to 1 over (4 times the distance to the target plus 2 times the internal circuit delay). A simple frequency to voltage converter drives an analog meter. No really high speed components are needed.

    This was seen as a project in a Dutch book: "Lasers in Theorie en Praktijk: Experimenten - Meten - Holografie", by Dirk R. Baur, Uitgeverij Elektuur/Segment B.V., Postbus 75, 6190 AB, Beek (L) The Netherlands.

    I'm not convinced that the circuit as presented works - there is at least one part value (C4, 100 uF) which would appear to be much larger than desired inside the feedback loop. The principle appears valid though.

    Time-of-Flight Laser Rangefinder using CCD Camera

    Each pixel of a CCD-based image sensor accumulates charge proportional to the light intensity and shutter open or "gate time". For normal video, the electronic shutter is open for a duration which is a large fraction of a video frame to maximize sensitivity and minimize aliasing in moving images. For stop motion photography, much shorter shutter open times are used. If it were possible to synchronize the electronic shutter with the generation of a light pulse illuminating the scene, then the amount of charge in each CCD cell would also depend on how long it takes for the light to reach the CCD (since the shutter would close before the light from more distant points returned). One problem, of course, is that this is possible only under very special conditions. A way to get around this would be to do the measurement in two steps:

    In order for this to be implemented with a normal CCD camera, either direct control of the electronic shutter is needed, bypassing any synchronous logic, or a "sync" output from the camera must be available. Also note that the charge integration times involved - 10s or 100s of ns - are orders of magnitude smaller than those normally used on all but very specialized CCD cameras, even with a fast shutter. So, sensitivity is going to be very low. A high power pulsed laser may be needed to generate adequate photons and even then, the CCD may not be able to supply enough charge.

    However, there are CCD image sensors that have been designed specifically for this application. They include logic on each pixel to enable the arrival time to be determined and stored. This permits an entire depth map to be captured with a single TOF pulse. See, for example: CSEM Optical Time-Of-Flight Imaging - A Technology for Multiple Applications.

    Using a CD or DVD Optical Pickup for Distance Measurements

    The simplist way of doing this may be to use the existing focusing mechanism of the pickup. Focus in a CD or DVD device depends on a reflection from a relatively flat smooth surface (the metalized information layer of the disc/k) to produce an elliptical spot back at the photodiode array. The major axis of the ellipse lies on a diagonal (45 or 135 degrees) and depends on the distance above or below optimal focus - at that point, it is a perfect circle. A four quadrant photodetector takes the difference of the amplitude of the return signals from the two pairs of diagonally opposed quadrants to determine the focus error. See the document: Notes on the Troubleshooting and Repair of Compact Disc Players and CDROM Drives for more on how optical pickups actually work.

    If the surface is smooth and flat over a scale of 5 to 10 um, this could work as a way of determining distance to the pickup. In other words, the dominant return from the surface has to be a specular reflection back to the source in order for the focus servo to lock properly. (The width and depth of the pits/lands of the CD or DVD disc is small compared to the beam so they are mostly ignored by the focus servo.) I don't know how much angular deviation could be tolerated.

    The output would be an analog voltage roughly proportional to focus error which could be mapped to lens height (assuming the device is in a fixed orientation with respect to gravity - more complex if you want to do this while on a roller coaster or in microgravity!). The total range would be 1 to 2 mm with an accuracy of a few um.

    Also see the section: Can I Use the Pickup from a CD/DVD Player or CD/DVDROM Drive for Interferometry?, which would be even more precise but more complex. The practical issues of using the guts of these devices are also discussed there.

    Using a CD or DVD Optical Pickup in a Precision Position or Angle Encoder

    Conventional optical encoders - whether they are the dirt-cheap variety inside your computer mouse or the precision type found in industrial robots and other machine tools - consist of a light source or sources, some means of interrupting or varying the light intensity based on linear position or rotation angle, and photodetectors to convert the light to an electrical signals. By using various patterns on film or glass strips or discs, relative (2 bits) or absolute (many bits) measurements can be made with a computer or dedicated logic calculating position or angle, speed or rotation rate, acceleration, and so forth from this data. Through clever design and careful manufacturing, extremely high resolution is possible using conventional LEDs or incandescent lamps for the light source(s). However, lasers can be used as well with some potential advantages - even higher precision and stand-off (some distance between the moving parts) operation.

    Since the 'stylus' of a CD player has an effective size of around 1 um (DVD would be even less), it could in principle be used to implement a very high resolution optical encoder for use in linear, rotary, or other sensing application. The stand-off distance (from objective lens to focal point) can be a couple of mm which may be an advantage as well. While this is probably somewhat less difficult than turning a CD player into an interferometer (see below), it still is far from trivial. You will have to create an encoder disc or strip with a suitable reflective pattern with microscopic dimensions. Without access to something like a CD/DVD mastering unit or semiconductor wafer fab, this may be next to impossible. Your servo systems will need to maintain focus (at least, possibly some sort of tracking as well) to the precision of the pattern's feature size. To obtain direction information, the 'track' would need to have a gray code pattern similar to that of a normal optical encoder - but laid down with um accuracy in such a way that the photodiode array output would pick it up. (Implementing an absolute encoding scheme would probably require so many changes to the pickup as to make it extremely unlikely to be worth the effort.) Of course, you also need laser diode driver circuitry and the front-end electronics to extract the data signal. Not to mention the need for a suitable enclosure to prevent contamination (like lathe turnings) from gumming up the works. And, with your device in operation, any sort of vibration or mechanical shock could cause a momentarily or longer term loss of focus and thus loss of your position or angle reference.

    If you are still interested, see the section: Can I Use the Pickup from a CD/DVD Player or CD/DVDROM Drive for Interferometry? since some of the practical issues of using the guts of these devices are discussed there.

    Measuring Speed with a Laser

    Speed is just the rate of change of position so any of the approaches that measure position can be adapted for speed measurements by simply taking a pair of readings and computing their difference with respect to time. More direct methods using CW lasers depend on using some form of the doppler shift of the reflected beam, usually of a subcarrier imposed on the the laser beam by amplitude modulation.

    For example, if the outgoing laser beam is modulated at 1 GHz and the reflected beam is combined with this same reference 1 GHz in the sensor photodiode or a mixer, for relative speeds small compared to c (the velocity of light), the difference frequency will be approximately 1 Hz per 0.5 foot/second.

  • Back to Laser Instruments and Applications Sub-Table of Contents.

    General Interferometers

    Basics of Interferometry and Interferometers

    The dictionary definition goes something like:
    "INTERFEROMETER: An instrument designed to produce optical interference fringes for measuring wavelengths, testing flat surfaces, measuring small distances, etc."
    As an example of an interferometer for making precise physical measurements, split a beam of monochromatic coherent light from a laser into two parts, bounce the beams around a bit and then recombine them at a screen, optical viewer, or sensor array. The beams will constructively or destructively interfere with each-other on a point-by-point basis depending on the net path-length difference between them. This will result in a pattern of light and dark fringes. If one of the beams is reflected from a mirror or corner reflector mounted on something whose position you need to monitor extremely precisely (like a multi-axis machine tool), then as it moves, the pattern will change. Counting the passage of the fringes can provide measurements accurate to a few nanometers!

    A simple version of a Michelson interferometer is shown below:

                                    _____ Mirror 1 (Moving)
                                      |  Beam
                                      |  Splitter
                   +-------+          | /          |
                   | Laser |=========>/<---------->| Mirror 2 (Fixed)
                   +-------+        / |            |
                                      v    Screen (or optical viewer,
                                   -------    magnifier, sensor, etc.)

    1. The laser produces a coherent monochromatic beam which is expanded and collimated by a pair of positive lenses (not shown).

    2. Part of the laser beam is reflected up by the Beamsplitter (half silvered mirror), reflects off of Mirror 1 and back down. A portion of this passes through the Beamsplitter to the Screen.

    3. The remainder of the laser beam passes through the Beamsplitter and is reflected from Mirror 2. Part of this is reflected down by the Beam Splitter to the Screen.

    4. The two beams combine at the Screen resulting in an interference pattern of light and dark fringes or a full field varying between light and dark as the path length is changed. A magnifier, microscope, or other optical system imaging to a human observer or electronic sensor may be provided in place of the screen to view the fringe pattern in more detail or provide input to an electronic measurement system.

      In a perfectly symmetric Michelson interferometer, the fringe pattern should uniformly vary between bright and dark (rather than stripes or concentric circles of light) depending on the phase difference between the two beams that return from the two arms. A circular pattern is expected if the two curvatures of the wavefront are not identical due to a difference in arm-lengths or differently curved optics. Stripes (straight or curved) in any direction) would be an indication of a misalignment of some part of the interferometer (i.e. the beams do not perfectly overlap or one is tilted with respect to the other).

    5. A microscopic shift in position or orientation of either mirror will result in a change to the pattern. Presumably, the mirror designated as 'Moving' is mounted on some equipment such as a disk drive head positioner that is being tested or calibrated. For these applications, setting up the interferometer is set up to produce a fringe pattern with at least two sensors to determine direction and velocity in a sophisticated version of the A-B quadrature decoder used in your typical computer mouse. :)

    (Yes, about 50 percent of the light gets reflected back toward the laser and is wasted with this particular configuration. This light may also destabilize laser action if it enters the resonator. Both of these problems can be easily dealt with using slightly different optics than what are shown.)

    A long coherence length laser producing a TEM00 beam is generally used for this application. HeNe lasers have excellent beam characteristics especially when frequency stabilized to operate in a single longitudinal mode. However, some types of diode lasers (which are normally not thought of as having respectable coherence lengths or stability) may also work. See the section: Interferometers Using Inexpensive Laser Diodes. Even conventional light sources (e.g., gas discharge lamps producing distinct emission lines with narrow band optical filters) have acceptable performance for some types of interferometry.

    Such a setup is exceedingly sensitive to EVERYTHING since positional shifts of a small fraction of a wavelength of the laser light (10s of nm - that's nanometers!) will result in a noticeable change in the fringe pattern. This can be used to advantage in making extremely precise position or speed measurements. However, it also means that setting up such an instrument in a stable manner requires great care and isolated mountings. Walking across the room or a bus going by down the street will show up as a fringe shift!

    Interferometry techniques can be used to measure vibrational modes of solid bodies, the quality (shape, flattness, etc.) of optical surfaces, shifts in ground position or tilt which may signal the precursor to an earthquake, long term continental drift, shift in position of large suspended masses in the search for gravitational waves, and much much more. Very long base-line interferometry can even be applied at cosmic distances (with radio telescopes a continent or even an earth orbit diameter apart, and using radio emitting stars or galaxies instead of lasers). And, holography is just a variation on this technique where the interference pattern (the hologram) stores complex 3-D information.

    NASA has some information on interferometry oriented toward cosmic measurements at the NASA Interferometry Page. And you can try your hands at aligning a Michelson interferometer at the NASA Interactive Interferometer Page.

    This isn't something that can be explained in a couple of paragraphs. You need to find a good book on optics or lasers. Here are some suggestions for further study:

  • Gordon McComb's: "The Laser Cookbook [1} and the Scientific American collection: "Light and its Uses [5]" include various type of interferometers which can be built with (relatively) readily available parts.

  • Agilent (among others) manufacture 'Laser Interferometry Measurement Systems' based on these techniques. Information and application notes are available by searching for the key words: "Laser" or "Dimensional Measurement". For Agilent in particular, searching for "5501" or "5517" will find information on their specific systems.

  • The Astroweb Internet Resources for Astronomy Web site (and others). There are links to people interested in designing, building, and operating various types of laser interferometers. Much of the information relates directly to the testing of optical components for astronomical telescopes but there should be much of general interest as well.

    Where Does All the Energy Go?

    Suppose we have a Michelson interferometer (see the section: Basics of Interferometry and Interferometers) set up with a perfectly collimated (plane wave source) and perfectly plane mirrors adjusted so that they are perfectly perpendicular to the optical axis (for each mirror) and the beamsplitter is also of perfect construction and oriented perfectly. In this case, there won't be multiple fringes but just a broad area whose intensity will be determined by the path-length difference between the two beams. Where this is exactly 1/2 wavelength (180 degrees), the result will be nothing at all and the screen will be absolutely dark! So, where is all the energy going? No, it doesn't simply vanish into thin air or the ether, vacuum, the local dump, or anywhere else. :-)

    Your initial response might be: "Well, no system is ideal and the beams won't really be perfectly planar so, perhaps the energy will appear around the edges or this situation simply cannot exist - period". Sorry, this would be incorrect. The behavior will still be true for the ideal case of perfect non-diverging plane wave beams with perfect optics.

    Perhaps, it is easier to think of this in terms of an RF or microwave, acoustic, or other source:

    Hint: From the perspective of either of the two signals, how is this different (if at all) than imposing a node (fixed point) on a transmission line? Or at the screen of the interferometer? After all, a nodal point is just an enforced location where the intensity of the signal MUST be 0 but here it is already exactly 0. For the organ pipe, such a nodal point is a closed end; for the string, just an eye-hook or a pair of fingers!

    OK, I know the anticipation is unbearable at this point. The answer is that the light is reflected back to the source (the laser) and the entire optical path of the interferometer acts like a high-Q resonator in which the energy can build up as a standing wave. Light energy is being pumped into the resonator and has nowhere to go. In practice, unavoidable imperfections of the entire system aside, the reflected light can result in laser instability and possibly even damage to the laser itself. So, there is at least a chance that such an experiment could lead to smoke!

    (From: Art Kotz (

    We don't have to to think all that hard to figure out where all the energy is dissipated in a Michelson interferometer. Nor do we have to refer to imperfect components either. The thought experiment of perfect non-absorbing components still renders a physically correct solution.

    To summarize a (correct) previous statement, in a Michelson interferometer with flat surfaces, you can get a uniform dark transmissive exit beam. The power is not dissipated as heat. There is an alternate path that light can follow, and in this case, it exits the way it came in (reflected back out to the light source).

    In fact, with a good flat Fabry-Perot interferometer, you can actually observe this (transmission and reflection from the interferometer alternate as you scan mirror spacing).

    In the electrical case, imagine a transmitter with the antenna improperly sized so that most of the energy is not emitted. It is reflected back to the output stage of the transmitter. If the transmitter can't handle dissipating all that energy, then it will go up in smoke. Any Ham radio operators out there should be familiar with this.

    (From: Don Stauffer (

    Many of the devices mentioned have been at least in part optical resonators. It may be instructive to look at what happens in an acoustic resonator like an organ pipe or a Helmholtz resonator.

    Let's start with a source of sound inside a perfect, infinite Q resonator. The energy density begins to build up with a value directly proportional to time. So we can store, theoretically, an infinite amount of acoustic energy within the resonator.

    Of course, it is impossible to build an infinite Q resonator, but bear with me a little longer. It is hard to get an audio sound source inside the resonator without hurting the Q of the resonator. So lets cut a little hole in the resonator so we can beam acoustic energy in. Guess what, even theoretically, this hole prevents the resonator from being perfect. It WILL resonate.

    No optical resonator can be perfect. Just like in nature there IS no perfectly reflecting surface (FTIR is about the closest thing we have). Every time an EM wave impinges on any real surface, energy is lost to heat. With any source of light beamed at any surface, light will be turned into heat. In fact, MOST of the energy is immediately turned to heat. By the laws of thermodynamics, even that that is not converted instantaneously into heat, but goes into some other form of energy, will eventually turn up as heat. You pay now, or you pay later, but you always pay the entropy tax.

    (From: Bill Vareka (

    And, something else to ponder:

    If you combine light in a beamsplitter there is a unavoidable phase relation between the light leaving one port and the light leaving the other.

    So, if you have a perfect Mach-Zehnder interferometer like the following

                +-------+      BS          M
                | Laser |=====>[\]---------\
                +-------+       |          |         M = Mirror
                                |          |        BS = Beamsplitter
                                |       BS |
                              M \---------[\]---->A
    If you set it up so that there is total cancellation out of, say, port A, then Port B will have constructive interference and the intensity coming out port B will equal the combined intensity coming in the two input ports of that final beamsplitter. This is due to the phase relation between the light which is reflected at the beamsplitter. That which is reflected and goes out port A will be 180 degrees out of phase with that which is reflected and goes out port B. The transmitted part of port A and port B are the same. Hence the strict phase relationship between the light from the two output ports. This is an unavoidable result of the time-reversal symmetry of the propagation of light.

    (From: A. Nowatzyk (

    A beam-splitter (say a half silvered mirror) is fundamentally a 4 port device. Say you direct the laser at a 45 degree angle at an ideal, 50% transparent mirror. Half of the light passes through straight, the rest is reflected at a 90 degree angle. However, the same would happen if you beam the light from the other side, which is the other input port here. If you reverse the direction of light (as long as you stay within the bounds of linear optics, the direction of light can always be reversed), you will see that light entering either output branch will come out 50/50 on the two input ports. An optical beam-splitter is the same as a directional coupler in the RF or microwave realm. Upon close inspection, you will find that the two beams of a beam-splitter are actually 90deg. out of phase, just like in an 1:1 directional RF coupler.

    In an experiment where you split a laser beam in two with one splitter and then combine the two beams with another splitter, all light will either come out from one of the two ports of the second splitter, depending on the phase. It is called a Mach-Zehnder interferometer.

    Ideal beam-splitters do not absorb any energy, whatever light enters will come out one of the two output ports.

    Interference between E/M Radiation of Different Wavelengths

    We all know that light from a single coherent source can create interference patterns and such. What about arbitrary uncorrelated sources?

    There will be interference but you won't see any visible patterns unless the two sources are phase locked to each-other since even the tiny differences in wavelength between supposedly identical lasers (HeNe, for example) translate into beat frequencies of MHz or GHz!

    (From: Charles Bloom (

    The short answer is yes.

    Let's just do the math. For a wave-number k (2pi over wavelength), ordinary interference from two point-like apertures goes like:

    Psi = (e^(ik(L+a).) + e^(ik(L-a).))/2
        = e^(ikL) * cos(ka)
    I = Psi^* Psi = cos^2(ka)
    (a is actually like (x-d)^2/L where 2d is the slit separation, and x is the position along the screen; L is the distance from the center of the slits to our point on the screen).

    Now for different wavenumbers:

    Psi = ( e^(ik(L+a).)+ e^(iK(L-a).))/2
    I = Psi^* Psi = 1/2 [ 1 + Re{ e^(i ( k(L+a) - K(L-a) ).)} ]
    	      = 1/2 [ 1 + cos( L(k-K) + a(k+K) ) ]
    	      = cos^2[ 1/2( L(k-K) + a(k+K) ) ]
    This is almost a nice interference pattern as we vary 'a', but we've got some nasty L dependence, and in the regime L >> a where our approximations are valid, the L dependence will dominate the a dependence (unless (k-K) is very small; in particular, we'll get interference roughly when a(k+K) ~ 10 and L(k-K) ~ 1 , and L >> a , which implies |k-K| << |k+K| , nearly equal wavelengths.)

    The L dependence is the usual phenomenon of "beats" which is also a type of interference, but not the nice "fringes" we get with equal wavelengths (the L dependence is like a Michelson-Morely experiment to compare wavelengths of light, by varying L (the distance between the screen and the sources) I can count the frequency of light and dark flashes to determine k-K.

    What about Hobbyist Interferometry?

    Building something that demonstrates the principles of interferometry may not be all *that* difficult (see the comments below). However, constructing a useful interferometer based measurement system is likely to be another matter.

    So you would like to add a precision measurement system to that CNC machining center you picked up at a garage sale or rewrite the servo tracks on all your dead hard drives. :) If you have looked at Agilent's products - megabucks (well 10s of K dollars at least), it isn't surprising that doing this may be a bit of a challenge. As noted in the section: Basics of Interferometry and Interferometers, a high quality (and expensive) frequency stabilized single mode HeNe laser is often used. For home use without one of these, a short HeNe laser with a short random polarized tube (e.g., 5 or 6 inches) will probably be better than a high power long one because it's possible only 2 longitudinal modes will be active and they will be orthogonally polarized with stable orientation fixed by the slight birefringence in the mirror coatings. As the tube heats up, the polarization will go back and forth between the two orientations but should remain constant for a fair amount of time after the tube warms up and stabilizes. Also see the section: Inexpensive Home-Built Frequency or Intensity Stabilized HeNe Laser.

    The problem with cheap laser diodes is that most have a coherence length that is in the few mm range - not the several cm or meters needed for many applications (but see the section: Can I Use the Pickup from a CD Player or CDROM Drive for Interferometry?). There may be exceptions (see the section: Interferometers Using Inexpensive Laser Diodes) and apparently the newer shorter wavelength (e.g., 640 to 650 nm) laser pointers are much better than the older ones but I don't know that you can count on finding inexpensive long coherence length laser diodes. Even if you find that a common laser diode has adequate beam quality when you test it, the required stability with changes in temperature and use isn't likely to be there.

    The detectors, front-end electronics, and processing, needed for an interferometer based measurement system are non-trivial but aren't likely to be the major stumbling block both technically and with respect to cost. But the laser, optics, and mounts could easily drive your cost way up. And, while it may be possible to use that $10 HeNe laser tube, by the time you get done stabilizing it, the effort and expense may be considerable.

    Note that bits and pieces of commercial interferometric measurings systems like those from HP do show up on eBay and other auction sites from time to time as well as from laser surplus dealers. The average selling prices are far below original list but complete guaranteed functional systems or rare.

    (From: Randy Johnson (

    I'm an amateur telescope maker and optician and interferometry is a technique and method that can be used to quantify error in the quality of a wavefront. The methods used vary but essentially the task becomes one of reflecting a monochromatic light source, (one that is supplied from narrow spectral band source i.e ., laser light) off of, or transmitting the light through a reference element, having the reference wavefront meet the wavefront from the test element and then observing the interference pattern (fringes) that are formed. Nice straight, unwavering fringe patterns indicate a matched surface quality, curved patterns indicate a variation from the reference element. By plotting the variation and feeding the plot into wavefront analysis software (i.e ., E -Z Fringe by Peter Ceravolo and Doug George), one can assign a wavefront rating to the optic under test.

    The simplest interference test would involve two similar optical surfaces in contact with each other, shining a monocromatic light source off the two and observing the faint fringe pattern that forms. This is known as a Newton contact interferometer and the fringe pattern that forms is known as Newton's rings or Newton's fringes, named for its discoverer, you guessed it, Sir Issac Newton. If you would like to demonstrate the principle for yourself, try a couple of pieces of ordinary plate glass in contact with each other, placed under a fluorescent light. Though not perfectly monochromatic, if you observe carefully you should be able to observe a fringe pattern.

    Non-contact interferometry is much tougher as it involves the need to get a concentrated amount of monochromatic light through or reflected off of the reference, positioning it so it can be reflected off of the test piece, and then positioning the eye or imaging device so that the fringe pattern can be observed, all this while remaining perfectly still, for the slightest vibration will render the fringe pattern useless.

    (From: Bill Sloman (

    An interferometer is a high precision and expensive beast ($50,000?). You use a carefully stabilized mono-mode laser to launch a beam of light into a cavity defined by a fixed beamsplitter and a moving mirror. As the length of the cavity changes, the round-trip length changes from an integral number of wavelengths of light - giving you constructive interference and plenty of light - to a half integral number of wavelengths - giving you destructive interference and no light.

    This fluctuation in your light output is the measured signal. Practical systems produce two frequency-modulated outputs in quadrature, and let you resolve the length of a cavity to about 10 nm while the length is changing at a couple of meters per second. The precision is high enough that you have to correct for the changes in speed of light in air caused by the changes temperature and pressure in an air-conditioned laboratory.

    Hewlett-Packard invented the modern interferometer. When I was last involved with interferometers, Zygo was busy trying to grab a chunk of the market from them with what looked liked a technically superior product. Both manufacturers offered good applications literature.

    (From: Mark Kinsler (

    You can get interferometer kits from several scientific supply houses. They are not theoretically difficult to build since they consist mostly of about five mirrors and a lens or two. But it's not so easy to get them to work right since they measure distances in terms of wavelengths of light, and that's *real* sensitive. You can't just build one on a table and have it work right. One possible source is: Central Scientific Company.

    (From: Bill Wainwright (

    Yes, you can build one on a table top. I have done it. I was told it could not be done but tried it anyway. The info I read said you should have an isolation table to get rid of vibrations I did not, and even used modeling clay to hold the mirrors. The main problem I had was that the image was very dark and I think I will use a beamsplitter in place of one of the mirrors next time. The setup I had was so sensitive that lightly placing your finger on the table top would make the fringes just fly. To be accurate you need to take into account barometric presure and humidity.

    Interferometers Using Inexpensive Laser Diodes

    The party line has tended to be that the coherence length of diode lasers is too short for interferometry or holography. (See the sections beginning with: General Interferometers.) While I was aware of CD laser optics being used with varying degrees of success for relatively short range interferometry (a few mm or cm - see the section: Can I Use the Pickup from a CD Player or CDROM Drive for Interferometry?), the comments below are the first I have seen to suggest that performance using some common laser diodes may be at least on par with that of a system based on a typical HeNe laser (though not a high quality and expensive frequency stabilized single mode HeNe laser).

    While I don't know how to select a laser diode to guarantee an adequate coherence length, it certainly must be a single spatial (transverse) mode type which is usually the case for lower power diodes but those above 50 to 100 mW are generally multimode. So, forget about trying to using a 1 W laser diode of any wavelength for interferometry or holography. However, single spatial mode doesn't guarantee that the diode operates with a single longitudinal mode or has the needed stability for these applications. And, any particular diode may operate with the desired mode structure only over a range of current/output power and/or when maintained within a particular temperature range.

    (From: Steve Rogers (

    I have been involved with laser diodes for the last 15 years or so. My first was a pulsed (only ones available at that time) monster that peaked 35 watts at 2 kHz with 40 A pulses! It was a happy day when they could operate CW and visible to say the least. Anyway, in the course of my working travels, I have built numerous Twymann-Green double pass interferometers for the wave front distortion analysis of laser rods, i.e ., Nd:Yag, Ruby, Alexandrite, etc. The standard reference light source for this instrument has always been the 632.8 nm HeNe laser. Good coherence length and relatively stable frequency was its strong suit.

    When visible diode lasers came out I often wondered aloud about their suitability as a replacement for the HeNe. I despise HeNe lasers. They are bulky and I have been shocked too many times from their power supplies.

    I assumed that since CD player laser diodes at 780 nm could have coherence lengths on the order of tens of centimeters or into the meters (!!, see, for example: Katherine Creath, "Interferometric Investigation of a Diode Laser Source", Applied Optics (24 1-May-1985) pp. 1291-1293), Visible Laser Diodes (VLDs) could make excellent replacements. As it turned out, VLDs tend to have coherence lengths which are considerably shorter according to the latest technical literature and I held off on experimenting with them. Last week, I went through my shop and found enough mirrors, beamsplitter, assorted optics to throw together my own double-pass interferometer for home use. This coincided with my acquisition of a 635 nm 5 mw diode module - a good one from Laserex.

    To make a longer story shorter, I assembled said equipment with the VLD and WOW! excellent fringe contrast (a test cavity of four inches using a .250" x 4.0" Nd:Yag rod as the test sample.) When a HeNe laser was substituted for the VLD, virtually no difference in the manual calculation of wave front distortion (WFD) and fringe curvature/fringe spacing. The only drawback with the VLD is that it produces a rectangular output beam. When collimated you have a LARGE rectangular beam rather than a nice round HeNe style beam. My interferometer now occupies a space of 10" x 10" and is fully self contained. It probably could even be made smaller. Not only that, but it runs on less than 3 V!!!

    I am just as surprised as you are with the results that I achieved. This is one reason why it took me so long to attempt this experiment (something like 4 to 5 years). I have always assumed that a HeNe laser would be FAR superior in this configuration than a VLD would be. Perhaps others may know more about the physics than I do. One thing is certain, these are "single mode" index guided laser diodes and typically exhibit the classic gaussian intensity distribution which is not so evident with the "gain guided" diodes. This in turn implies a predominant lasing mode which in turn would imply a (somewhat) stable frequency output. Purists would note that this VLD has a nominal wavelength of 635 nm +/- 10 nm while the HeNe laser is pretty much fixed at 632.8 nm. This variable could account for extremely minor WFD differences.

    (From: W. Letendre (

    There's an outfit in Israel selling a diode based laser interferometer enough cheaper than Zeeman split HeNe units to suggest that they are using a laser diode in the 'CD player' class, or perhaps a little better. They are able to measure, 'single pass' (retro rather than plane mirror) over lengths of up to about 0.5 m, suggesting that as an upper limit for coherence length.

    Can I Use the Optical Pickup from a CD/DVD Player or CD/DVDROM for Interferometry?

    With the nice precision optics, electromechanical actuators, laser diode, and photodiode array present in the mass produced pickup of a CD/DVD player, CD/DVDROM drive, or other optical disc/k drive, one would think that alternative uses could be found for this assembly after it has served for many years performing its intended functions - or perhaps, much earlier, depending on your relative priorities. :-) (Also see the section: Using a CD or DVD Optical Pickup in a Precision Position or Angle Encoder.

    People sometimes ask about using the focused laser beam for for scanning or interferometry. This requires among other things convincing the logic in the CD/DVD player or CD/DVDROM drive to turn the laser on and leave it on despite the possible inability to focus, track, or read data. The alternative is to remove the optical pickup entirely and drive it externally.

    If you keep the pickup installed in the CD player (or other equipment), what you want to do isn't going to be easy since the microcontroller will probably abort operation and turn off the laser based on a failure of the focus as well as inability to return valid data after some period of time.

    However, you may be able to cheat:

    Where such a feature is not provided:

    CAUTION: Take care around the lens since the laser will be on even when there is no disc in place and its beam is essentially invisible. See the section: Diode Laser Safety before attempting to power a naked CD player or simlar device.

    It may be easier to just remove the pickup entirely and drive it directly. Of course you need to provide a proper laser diode power supply to avoid damaging it. See the chapter: Diode Laser Power Supplies for details. You will then have to provide the focus and/or tracking servo front-end electronics (if you need to process their signals or drive their actuators) but these should not be that complex.

    Some people have used intact CD player, CDROM, and other optical disc/k drive pickup assemblies to construct short range interferometers. While they have had some success, the 'instruments' constructed in this manner have proven to be noisy and finicky. I suspect this is due more to the construction of the optical block which doesn't usually take great care in suppressing stray and unwanted reflections (which may not matter that much for the original optical pickup application but can be very significant for interferometry) rather than a fundamental limitation with the coherence length or other properties of the diode laser light source itself as is generally assumed.

    In any case, some of the components from the optical block of that dead CD/DVD player may be useful even if you will be substituting a nice HeNe laser for the original laser diode in your experiments. Although CD optics are optimized for the IR wavelength (generally 780 nm), parts like lenses, diffraction grating (if present and should you need it), and the photodiode array, will work fine for visible light. However, the mirrors and beamsplitter (if present) may not be much better than pieces of clear glass! (DVDs lasers are 635 to 650 nm red, so the optics will be fine in any case.)

    Unfortunately, everything in a modern pickup is quite small and may be a bit a challenge to extract from the optical block should this be required since they are usually glued in place.

    If what you want is basic distance measurements, see the section: Using a CD or DVD Optical Pickup for Distance Measurements which discusses the use of the existing focusing mechanism for this purpose - which could be a considerably simpler approach.

    Also see the section: Basics of Interferometry and Interferometers.

  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Interferometers Using Two Frequency Lasers

    The interferometers described in the previous sections and found in physics labs (assuming such topics are even taught with hands-on experience!) all use CW lasers and look at the fringe shifts as the relative path lengths of the two arms is changed. While this works in principle and has been used widely, modern commercial measurement systems based on interferometry often use more sophisticated techniques to reduce susceptibility to signal amplitude changes and noise and improve measurement accuracy, stability, and convenience.

    If you've used a CD or DVD or a harddrive, in all likelihood, the equipment that defined their track position and spacing was controlled by a dimensional measurement system using a two-frequency interferometer. Additional applications include semiconductor steppers, multiaxis precision machine tools, and others where very accurate non-contact measurements or submicron positioning are required.

    Interferometer-based measurements systems typically use some type of low power stabilized helium-neon laser to produce the "yardstick" beam of light. By stabilizing the laser with reference to the neon gain curve, the accuracy of the optical frqeuency/wavelength can easily be known to better than +/-0.1 ppm. A basic system may use such a laser in a Michelson or similar interferometer, using a quadrature detector to count fringes representing changes in path length. Problems with such a system are that changes in light intensity will result in measurement errors, alignment is very critical to obtain adequate fringe contrast, and they are more susceptible to noise.

    In two frequency interferometers, a special stabilized HeNe laser is used that produces a beam consisting of two very slightly different frequencies (wavelengths) of light simultaneously. This may be achieved by various techniques. HP/Agilent lasers employ a special tube which uses a magnet to perform Zeeman splitting while Zygo uses an external acousto-optic modulator. As above, both types of lasers are locked in such a way that the optical frequency is very precisely known.

    A diagram of the general approach is shown in Interferometer Using Two Frequency HeNe Laser.

    The following description applies to the HP/Agilent approach using Zeeman splitting. With Zygo, the method of generating the two frequencies differ, but their use in the interferometer is the same.

    In the Zeeman split approach, the two-frequency laser consists of a HeNe laser tube surrounded by permanent magnets which produce a constant axial magnetic field. The laser tube is short enough that without a magnetic field, only a single longitudinal mode will normally oscillate if it is located near the center of the neon gain curve. (Those on either side will not see enough gain.) The net result of the magnetic field is that instead of a single longitudinal mode, two modes are produced that differ very slightly in frequency and have right and left circular polarization. The difference between the two frequencies is typically in the 1.5 to 4 MHz range (though some go up to 6 MHz or more), which makes the resulting signals extremely easy to process electronically. The actual difference frequency is determined by the strength of the magnetic field, length of the internal laser cavity, and other physical details, as well as the exact place on the Zeeman-split neon gain curve where the laser has been locked.

    To stabilize the laser, there is a piezo element and/or heater to precisely adjust cavity length. A feedback control system is used to adjust the cavity length to maintain the position of the Zeeman-split frequencies - and thus the wavelengths - constant. The feedback is generally based on the simple approach of forcing the orthogonally polarized outputs to be equal, which results in the highest beat frequency and most stable optical frequency.

    The wavelength of the laser is the measurement increment and will remain essentially unchanged for the life of the instrument. For example, with the doppler broadened gain curve for the HeNe laser being about 1.5 GHz FWHM (1 part in about 300,000 with respect to the 474 THz optical frequency at 633 nm) and a 1 percent accuracy within the gain curve, the absolute wavelength accuracy will then be better than 1 part in 30 million! Not too shabby for what is basically a very simple system. In practice it's even better. :)

    Note that the exact value of the difference frequency does not need to be very precisely controlled over the long term. Rather, it is the difference between the reference difference frequency and the measurement difference frequency that matters, and the latter only depends on the motion of the target reflector - and the speed of light. Thus, the exact beat frequency of each laser is not precisely controlled or even precisely measured and recorded or used anywhere in the calculations.

    Since the output of the laser is a beam consisting of a pair of circularly polarized components, a quarter waveplate and half waveplate is used to separate these into two orthogonal linearly polarized components called F1 and F2, and to orient them such that they are parallel to the horizontal or vertical axes.

    The beam consisting of F1 and F2 is split into two parts with a non-polarizing beam-splitter: One part goes through a polarizer at 45 degrees (to recover a signal with both F1 and F2 linearly polarized in the same direction) to a photodiode which generates a local copy of the reference frequency (REF, the difference between F1 and F2) for the measurement electronics; the second is the measurement beam which exits the laser. The return beam is called MEAS.

    The purpose of the remainder of the interferometer is essentially to measure the path length change between two points. In a typical installation, the beam consisting of F1 and F2 is sent through a polarizing beamsplitter. F1 goes to a cube-corner (retro-reflector) on the tool whose position is being measured and F2 goes to a cube-corner fixed with respect to the beamsplitter and laser. However, differential measurements could be made as well using F2 in some other manner. Various "widgets" are available for making measurements of rotary position, monitoring multi-axis machine tools, etc.

    The return from the object corner reflector is F1+ΔF1 which is recombined with F2 and sent to an "optical receiver" module - a photodiode behind a polarizer at 45 degrees and preamp which generates a new difference frequency, F2-(F1+ΔF1). This signal, called "MEAS" is compared with REF to produce an output which is then simply ΔF1. (A mixer is shown in the diagram but in practice, it might be implemented with digital logic like counters and subtractors.) A change in the position of the object by 316 nm (1/2 the laser wavelength) results in ΔF1 going through a whole cycle. By simply keeping track of the number of complete cycles of ΔF1, this provides measurements of object position down to a resolution of a few hundred nm with an accuracy of +/-0.02 ppm! And the typical implementation will multiply the REF and MEAS frequencies by 16 or 32 or more using a pair of phase-locked loops to produce a corresponding improvement in resolution down to a few nanometers or better!

    More information on the two-frequency HeNe laser can be found in the sections: Hewlett-Packard HeNe Lasers and Two Frequency HeNe Lasers Based on Zeeman Splitting. Searching on the Agilent Web site will yield product specific information and application notes on two frequency interferometers. A comprehesive but not too hairy description of the two frequency approach can be found in the Hewlett-Packard`Journal, February, 1976. Yes, this is an old technique (actually much older)! Searching at HP Archive for "Interferometer" and similar terms will turn up many more interesting articles.

    Zygo, another manufacturer of interferometer measurement systesms using two-frequency lasers has an excellent tutorial at A Primer on Displacement Measuring Interferometers and Sam's Copy of Zygo's Primer on Displacement Measuring Interferometers.

    Lasers for Interferometers Using Two-Frequency Lasers

    There are generally low power HeNe lasers with either specially designed (and expensive) laser tubes or an external modulator (also expensive) to produce the two (relatively) closely spaced optical frequencies with orthogonal polarization. Depending on technique, the difference frequency can be anywhere from a few hundred kHz to 20 MHz or more. Since the beat frequency between the reference and measurement signals decreases with one direction of motion and can't go below 0 Hz, a higher difference frequency translates into higher maximum speed of position change in the measurement system. Therefore, depending on the specific application, a higher difference frequency may be essential.

    Optics for Interferometers Using Two-Frequency Lasers

    The following discusses the various types of optical components supplied by Hewlett Packard (now Agilent). Those from other companies like Excell and Zygo are similar.

    The most basic application (for a single axis measurement) will consist of the following optical components:

    Since the optical frequency/wavelength is being used as the "measuring stick" in these systems, it must be known to a high degree of precision and anything that affects it must also be taken into account. In particular, the temperature, pressure, and humidity of the air must be factored into the measurement calculations. Or, if part or all of the measurement setup is in a vacuum, this will affect it. These corrections can be done at least partially automatically with sensors, or by manually entering parameters into the measurement computer.

    These systems generally allow a single laser to be used with installations where the motion of multiple access needs to be measured. So in addition to the actual measurement optics, there will be components to split and redirect the original beam from the two-frequency laser to each axis.


    The heart of all of these systems are the interferometers. The three most common configurations are shown in Types of Hewlett Packard/Agilent Interferometers. There can be various permutations of the individual components that are optically and functionally equivalent. Combinations of multiple interferometers mounted on a common platform are also available for compact multi-axis applications. What HP calls the "interferometer" consists of all the components in the center of each diagram - the Polarizing Beam Splitter (PBS), 1 or 2 Retro Reflectors (RRs), and 0, 1, or 2 Quarter Wave Plates (QWPs). There will also be a RR or Plane Mirror (PM) on the "tool" whose position is to be measured, and an Optical Receiver (OR) for the return beam. The Two-Frequency Laser (TWL) can be shared among all the axes of the machine by distributing its beam using non-polarizing beam splitters and 45 degree mirrors ("Beam Benders", but all beam orientations must be a multiple of 90 degrees to the original TFL).

    As noted, all of these interferometers contain a high quality Polarizing Beam Splitter (PBS) cube as their central component. What gets added on to the PBS depends on the specific type and may include Retro-Reflectors (RRs, which are solid cube-corners) and/or Quarter Wave Plates (QWPs). Please refer to the diagram, above. For HP-5517 lasers, F1 is the lower frequency component and is horizontally polarized, while F2 is the higher frequency component and is vertically polarized. (HP-5501 lasers are the opposite, but the only effect is a sign change in the measurement calculation.)

    The Agilent prices are also interesting in that they are at least 5 times what similar optical components would cost from a supplier like Melles Griot or Newport. I do not know how much of this is actually due to the required optical precision compared to less demanding applications.

    See Links to Agilent Laser and Optics User's Manual for general information on all the current Agilent lasers and interferometer optics. For much more on these systems, go to Agilent and search for "Interferometers".

    Electronics for Interferometers Using Two-Frequency Lasers

    The most basic requirement is to convert the phase shift between REF and MEAS to a count or position. Conceptually, this is nearly trivial, being simply the different between the total cycles of the REF and MEAS signals. And in fact, the original HP-5505A display unit did this by brute force with a pair of counters and a subtractor. As will be shown below, this turns out to be a clean, but rather hardware intensive solution. In general, care needs to be taken in the design of the processing hardware and/or software to avoid possible errors ultimately resulting from the Uncertainty Principle.

    Possible approaches

    Sam's Measurement Display for Two-Frequency Interferometers

    I was tired of searching for something inexpensive on eBay and anyhow, wanted something interesting to do. I also miss the days of 0s and 1s a bit (no pun...) and my drawer of ancient TTL chips was getting rather lonely. :)

    I decided on the approach using the up/down counter due to its simplicity. The schematic except for the up/down counter is shown in Sam's Pulse Converter for Two-Frequency Interferometer. It consists of:

    While the laser is warming up and there is no REF signal, only the green LED is lit. Once REF appears but if the MEAS beam is misaligned or blocked, only the RED LED is lit. Once everything is stable and aligned, neither is lit if there is no movement. I also breadboarded a version using a PAL to generate the control signals. This eliminates most of the discrete logic chips.

    Before I added the display, I had to be content watching the green and red LEDs. Any vibration - including moderately loud sounds - result in flickering. I'm not sure what type of music they prefer, but a sustained tone can result in quite impressive activity. :-)

    The multi-digit up/down counter ICs I had found tended to be marginal with respect to their maximum frequency count capability. So I decided to go with the separate chip "brute force" approach for the first version. The prototype shown in Photo of Sam's Measurement Display 1 in Action has been tested and appears to work as expected. This measurement display may be avilable as a kit or fully assembled along with other metrology components for educational purposes in the future. See Installation and Operation Manual for Sam's Interferometer Measurement Display System 1 (SGMD1) for more info including complete specifications.

    The next version of a measurement display will probably be based on an FPGA for the front-end logic with an on-board microprocessor to handle the units/format conversion, user interface, and communications with a PC. But I don't expect that to happen any time soon, if ever. :)

    Sources of Measurement Error in HP/Agilent Metrology Systems

    While it sounds really impressive to be basing precision measurement on the wavelength of light, and HP/Agilent lists the nominal wavelength of the 5501 and 5517 lasers to 9 significant figures, there are many environmental and installation factors that impact what is actually useful. The following is the briefest of summaries of accuracy issues. A large portion of the operation manuals for these systems is devoted to this topic. More than you could ever hope to know can be found in the links in the section: Agilent Laser and Optics User's Manual. The following is a brief summary.

    Laser wavelength accuracy

    A common question that comes up with respect to these systems is: "Since these are based on two-frequency lasers, which frequency is used as the wavelength specification?". The quick answer is: It's not clear. :)

    A phase change of 360 degrees of the difference frequency of the measurement beam with respect to the reference beam represents a change in position of of the moving part of the measurement system (e.g., the "tool") by 1/2 wavelength (linear or single beam interferometer) or 1/4 wavelength (plane mirror interferometer). Thus the component (F1 or F2) that goes to the tool is the actual "yardstick" wavelength. If both F1 and F2 are used in a differential measurement, then each contributes to the measurement based on its wavelength. So, strictly speaking, one should use those wavelengths in the calculation. But as a practical matter, it really doesn't matter as the difference in frequency between the two components F1 and F2 is so small compared to the optical frequency, that the error introduced by using one or the other is way below the accuracy specification for even the military calibrated versions of these systems.

    It's not clear (at least to me) where the value of 632.991372 nm for the 5501B and 5517A/B or 632.991354 nm for the 5517C/D comes from. My assumption would be that it's the theoretical lasing line center of the Zeeman-split neon gain curve. Various sources list other slightly different values for HeNe lasers. Wikipedia has a page on the Meter Measuring Unit that gives a value of λHeNe=632.99139822 nm. And an HP Journal article on the 5528A gives yet another value of 632.991393 nm for the 5518A, but that tube should be essentially identical to the tube in the 5517A! The actual fill pressure, ratio of He:Ne and their isotopic mix, temperature, and other factors will affect the exact wavelength. But why the different values even for essentially similar lasers from HP/Agilent including lasers that are still in production? The corresponding difference in optical frequency between 632.991372 nm and 632.991354 nm is about 13.5 MHz, so it's way beyond the error due to whether line center or either F1 or F2 is used. Perhaps, it has something to do with the newer tubes being filled with pure Ne20 or Ne22 and the older ones having a mixture to guarantee compatibility in legacy applications. How's that for a wild guess? :) The difference is still under 0.03 ppm, so it should generally not be a huge issue in any case.

    The normal HP/Agilent lasers have a long term stability specification of +/-0.1 ppm. The laser stabilization depends on the condition of the electronics and may result in a small variation in this wavelength. Even one power cycle to the next does not result in a precise return to the exact same conditions due to the particular temperature, and thus cavity mode number, at which lock occurs. But any variation will only be a few MHz at most, well below the +/-0.1 ppm specification.

    The Agilent info indicates that they will certify a laser to MIL STD-45662 for long term stability of +/-0.02 ppm. MIL STD-45662 requires traceability to a (wavelength/frequency) reference, which I would assume to be something like an iodine-stabilized laser. However, this doesn't sound like the laser design is necessarily any different, just that the optical frequency of the specific laser is measured precisely, and the exact value to even more significant digits than the those given above, is included in the calibration report. With that number known, +/-0.02 ppm corresponding to about +/-9.4 MHz, really shouldn't be hard to maintain.

    Velocity of light

    Ambient temperature, air pressure, and humidity all have a very significant effect on the measurement. Approximately a 1 part-per-million change in the velocity of light and thus measurement wavelength will result from:

    Since 1 ppm is 10X of the basic measurement specification for accuracy, it is clear that these factors must be taken into account. The HP/Agilent measurement systems have sensor options to allow this to be done automatically, or the corrections can be entered manually.

    And, if the "tool" is in a vacuum, that's roughly a 3 part in 10,000 error due to the difference in the index of refraction of a vacuum compared to air!

    Material effects

    Depending on the construction of the equipment on which the interferometer optics are mounted, the change due to thermal expansion and other effects can be very significant resulting in serious errors if not taken into consideration.

    Determining the Exact Laser Wavelength or Frequency

    So, how accurately can the wavelength or frequency of one of these lasers be determined outside of a NIST calibration laboratory or Agilent test facility? It all really comes down to having a standard of reference. Commercial instruments called wavelength meters available from companies like TOPTICA Photonics AG can have very high accuracy, some down to 2 MHz (0.0000027 nm) or better. But that is only if a more accurate reference is used for calibration not to long before making measurements of the unknown laser. 2 MHz isn't very much when it comes to optical frequency! So, even if you could afford one of these expensive instruments or could borrow one, it would still need to be calibrated against some reference!

    The most common reference of relevance for this testing would be a mode stabilized HeNe laser like a Spectra-Physics 117A. Its frequency will have a long term stability of 10 MHz or less, but whether the absolute accuracy can be nailed down to better than 50 or 100 MHz - about 0.1 to 0.2 parts-per-million (ppm) - is questionable due to factors like the exact neon isotope(s) in the gas fill and the precise point on the neon gain curve where the laser is locked. So, a higher precision reference would be needed to calibrate that!

    However, some of the HP/Agilent lasers themselves may have their frequency known to extremely high accuracy. For the normal commercial versions of the 5517 lasers, the spec'd accuracy is *only* +/-0.1 ppm, which is equivalent to +/-0.0000633 nm (+/-47.4 MHz in optical frequency) with a nominal wavelength of 632.991354 nm (for the 5517C/D; 632.991372 nm for the 5501B and 5517A/B and some others even though the tube is virtually identical in all of these lasers).

    But there are some 5517 lasers that come with a "pedigree" - a report including the wavelength of the specific laser measured to very high accuracy under controlled environmental conditions.

    The report for one particular 5517D laser included the following: Temperature and humidity test conditions of 22.8 °C and 36.2%, respectively, a locked output power of 409 uW (spec is 180 uW minimum), a split (REF) frequency of 2.75 MHz (spec'd range is 2.4 to 3.0 MHz), and the actual vacuum wavelength of 632.9913662 nm (nominal is 632.9913540 nm).

    That difference of 0.0000122 nm is actually a rather large error, equivalent to about 9.4 Mhz in optical frequency or about 0.02 ppm.

    I don't know if it is even remotely possible to obtain information for a specific serial number laser from Agilent unless you're the original buyer, and I rather doubt it. After all, your good fortune on eBay is hardly something that Agilent is likely to care much about! :) But, it may be possible to obtain the information from the original buyer if they can be tracked down. I was able to have access to one of these lasers that I was repairing, and they provided the above data from the calibration report. With this data, I was hoping to be able to measure the exact wavelength/frequence of one of my HP/Agilent lasers and then use it as the reference when I return the other one. However, it turned out that the wavelength report didn't apply to this particular sample but a similar one. Oh well.

    However, age and use of the laser will affect the optical frequency by enough to matter. So unless the laser is relatively new, any data may be unreliable. The drift may be predominantly due to changes in the He and Ne partial pressure. These both alter the width of the Doppler broadened Ne split gain curves, and shift the center optical frequency. End-of-life lasers - the type often found on eBay - would be most prone to such effects. But even relatively young lasers will see a drop in pressure that may be significant. One tip-off to a potential discrepancy could be the REF frequency. From my observations and the comment by an engineer who tests these lasers, the REF frequency tends to increase slightly with use. This is consistent with a broadening of the gain curves which will increase the mode pulling effect. (But, at least the change is in the beneficial direction - the one that increases the maximum measurement velocity specification, assuming the processing electronics can handle the higher REF frequency!)

    One reference is: "Frequency stability measurements on polarization-stabilized He-Ne lasers", T. M. Niebauer, James E. Faller, H. M. Godwin, John L. Hall, and R. L. Barger, Applied Optics, vol. 27, no. 7, 1 April 1988, pp. 1285-1289.

    However, assuming you found a new-in-box Agilent laser with complete documentation of optical frquency:

    Some relevant numbers: 633 nm is about 474 THz based on the speed of light of 299,792,458 m/s. So, 1 nm is 0.749 THz or 749 GHz and 0.0000001 nm is 74.9 kHz. 100 kHz is 0.000000134 nm or 1 MHz is 0.00000134 nm. The difference between 632.9913540 nm and 632.9913662 nm is then around: 9.137 MHz.

    Comparing the Optical Frequencies

    With one of these lasers in hand, it's really quite straightforward at least in principle to determine the exact frequency offset of another similar laser by beating (heterodyning) the two outputs in a high speed photodiode and measuring the difference frequency. Since frequency doesn't depend on environmental conditions, nothing that happens outside the laser will affect it. The resulting optical frequency value can then be divided into the speed of light in the relevant medium (e.g., air at STP or vacuum) to compute the exact wavelength.

    My Two-Frequency Interferometer Laser Tester (See Diagram of Two-Frequency Interferometer Laser Tester and Photo of Two-Frequency Interferometer Laser Tester) was modified to permit the beam from a second HP/Agilent laser to be combined and sent to an optical receiver or biased photodiode. A diagram is shown in Diagram of Test Setup for HP/Agilent Laser Optical Frequency Comparison. A Polarizing Beam-Splitter (PBS) was added, at first only on a somewhat adjustable mount (normally used for Beam Splitters or Beam Benders in these systems) and could easily be installed and removed without tools. However, this proved to have nowhere near the estimated 1/10th of a mR precision needed to align the beams to obtain a beat signal. So, a Newport MM1 kinematic mount with the PBS attached to it was installed in its place. The second laser itself is on a fully adjustable platform (3 screws), so the beams can be lined up precisely.

    The distance from each laser to the PBS is relatively closely matched, so if they have the same optics (e.g., 6 mm), the wavefront curvature should be similar resulting in minimal alignment issues and decent signal signal amplitude. How much mismatched optics (e.g., 6 mm with 9 mm) will affect this is not known though. But in the far field, the more significant issue would probably be the loss of available optical power from the size mismatch since the wavefronts should be quite close to planar.

    A half waveplate which may be installed in front of either laser will rotate the polarization if needed to pass it through the PBS, with the PBS itself serving to eliminate the unwanted F1 or F2 component. Then, F1 or F2 from Laser 1 can be beat with F1 or F2 from Laser 2. However, not knowing the range of possible difference frequencies likely to exist for any given pair of lasers, the highest bandwidth HP/Agilent 10780 may not be adequate to capture the difference frequency without a lot of luck, especially if comparing lasers with different spec'd nominal optical frequencies like the 5517A and 5517C (12 MHz difference). So my original intent was to replace the HP optical receiver with a Zygo 7080 which is used with their lasers having a REF frequency of 20 MHz. Assuming a maximum of a +/-0.1 ppm frequency offset with respect to nominal for each laser, the difference could be up to 95 MHz, though I highly doubt even half of this is at all likely. Nonetheless, some higher speed photodiode detector may be needed in general. And my poor old 10 MHz oscilloscope which had been dedicated to monitoring of HP/Agilent REF and MEAS frequency signals was augmented with another equally old 50 MHz scope. :) (As it turned out, for the tests I actually performed on two specific 5517B lasers, the original 10780A was quite adequate.)

    Since F1 (the lower optical frequency) and F2 (the higher optical frequency) have known orientations, being able to select each one will make it possible to unambiguously determine whether the difference frequency between the two lasers represents Laser 1 or Laser 2 being the one that is higher in frequency.

    Another approach would be to monitor the difference frequency as the cube-corner or plane mirror ("tool") in the interferometer is moved; whether it decreases or increases for a given direction of motion will also unambiguously determine which laser's frequency is the higher one.

    For maximum accuracy, both lasers need to warm up from a cold start (not restarted) for several hours in an environment with a fairly constant temperature. But of course, the activity during warmup is in itself quite exciting. :)

    Note that the stray magnetic field from one laser will change the REF frequency of the other laser if they are close together, especially if they are parallel to each-other. The frequency offset that is introduced is only order of 1 or 2 percent of the reference frequency at most. It may in fact not affect the optical frequency very much, if at all. And even if it does, the magnitude should be of negligible consequence. But any disturbance is still worth minimizing. A bit more on this below.

    There may also be second-order effects of external magnetic fields that are not aligned with the laser's magnetic field. This will not only change the strength of the axial magnetic field, but will also introduce a transverse component to the magnetic field with unknown consequences.

    Although it turned out that I don't have a laser with a known optical frequency and had to return the one that was probably close, I used a pair of healthy 5517Bs to perform the experiment. But nothing is perfect! :) A few of the issues:

    Using a 1/2 waveplate, it was possible to select F1 or F2 of Laser 1 (L1) to beat with F2 of Laser 2 (L2). The difference in optical frequency changed from 5.0 Mhz (no waveplate, L2F2-L1F1) to 2.7 MHz (waveplate oriented at 45 degrees for a 90 degree rotation, L2F2-L1F2), proving that Laser 1 has the lower absolute optical frequency. (Laser 1's REF or split frequency is 2.3 MHz.) It sure feels good when the physics cooperates! :) (For convenience in trying to keep track of things, the difference frequencies here are referenced to F1 of both lasers, rather than the average of F1 and F2, which strictly speaking may be more accurate. In most cases, this results in a shift of less than 1 MHz.)

    The difference frequency between F1 of Laser 1 and F1 of Laser 2 (L2F1-L1F1) after coming Ready can be anywhere from approximately -2 MHz to +6 MHz depending on whether one or both lasers was started from being cold. (Of course, before locking, the difference frequency can be up to 1.6 GHz or so - the FWHM width of the neon gain curve!) The beat then drifts by 4 or 5 MHz over the next hour or so. Typically, if both lasers are started at the same time, having been off for more than an hour, the difference frequency (L2F1-L1F1) just after locking is around -1 MHz, goes down to -2 MHz, and then climbs to a final difference frequency of around +2.6 MHz over the course of an hour or so. Pretty impressive for systems running at 474 THz. That 2.6 MHz is less than 1 part in 100,000,000!

    The temperature of the laser - or more likely the temperature of the control electronics - also affects the optical frequency by slightly shifting the lock point on the split HeNe gain curve. The data above was with the lasers undressed. Installing their covers resulted in the difference frequency dropping to under 1 MHz after an hour but then it went through 0 Hz and seems to have finally settled at around -2.3 MHz after 5 hours. Removing the cover over the PCB on Laser 1 caused the difference frequency to climb back up to +2.6 MHz, so a change of almost 5 MHz. I swapped in the Control PCB from another 5517 laser just for grins and giggles, The optical frequency after 10 hours from a cold start moved to about -1 MHz with at least as wide a variation during warmup as with the original Control PCB. I don't presently have a 5517B with a newer digital Control PCB so I can't compare its stability to that of the common analog Control PCB.

    Finally, I swapped the beam sampler assemblies including the LCD switch on Laser 2 and this resulted in a significant change in the difference frequency - almost 5 MHz. It's possible that an LCD panel that is starting to delaminate or degrade in some other way could do this, but I have no reason to suspect that one of these was bad.

    Given how small the difference frequency between two randomly selected 5517Bs is, could it be that these 5517B lasers really are close to the nominal spec'd value of 632.991372 nm or 473.612234 THz? Possibly, since the least significant digit of 632.991372 or 0.000001 nm is about 0.75 MHz. Where's a NIST calibration lab when you need one?

    I also did some very basic experiments with magnetic fields applied to one of the lasers. Placing another similar laser along-side Laser 1, the difference in optical frequency could be dramatically changed as the distance or orientation of the two lasers was altered. But, most - but not all - of the frequency change was eliminated in a few seconds as the control loop readjusted the locked position. The initial response didn't seem to be instantaneous, so perhaps it was something related to a slight change in laser tube current or something else affecting the temperature of the tube. Given that both the strength and symmetry of the magnetic field was being affected by repositioning, it's not at all clear what was actually changing. But everything else should be intuitively obvious. :-)

    Not content to permit one of these lasers to rest wherever it pleases, I've built a simple network to introduce a small offset into the error signal driving the laser tube heater power amp to fine tune the optical frequency. The circuit consists of a pair of 1K ohm resistors feeding 4.7 V zener diodes from +15 VDC (TP8) and -15 VDC (TP10) to ground (TP1). (These testpoints represent convenient locations to attach the circuit.) The regulated +/-4.7 VDC are probably not really needed but won't hurt. A 25 turn 25K ohm pot connects between +/-4.7 VDC with its wiper feeding a "gain control resistor", whose other end is the offset output. The "Power Amp" test jumper on the Control PCB was removed and replaced with a connector having a 2K ohm resistor to partially isolate the driving op-amp (test header Pins 1 and 2, assuming pin 1 is on the left), and the offest is introduced to pin 4 (which is shorted to pin 2 on the PCB). With a 33K ohm gain control resistor, one turn of the pot changes the optical frequency by about 1 MHz, providing a range of more than +/-10 MHz. A positive offset reduces the optical frequency on the 5517 laser. (It would probably be the opposite for the 5501B.)

    There's no need to bother with this for the 5501A as it has a "Photodiode Offset" pot (R4) on the Lock Reference PCB, which essentially performs the same function. (It's the square pot in the corner.) Normally, the pot is adjusted to maximize the REF/split frequency, which automagically centers the lasing position on the split neon gain curve. But it has quite a wide range - at least +/-50 MHz, possibly much more.

    It would be a simple extension to lock two of these lasers together at 0 Hz with a PLL using the REF frequency of Laser 1 (L1F2-L1F1) as the reference input to a phase/frequency detector (a flip-flip and some simple logic), and the difference frequency between the lasers (L1F2-L2F1) as the "VCO" input. The phase/frequency detector output would be the offset error signal fed into the power amp. The locked state would then have L1F1 equal to L2F1.

    Even simpler and actually more flexible would be to use a monostable and RC filter to convert the L1F2-L2F1 difference frequency to a voltage, and compare that with a set-point value to generate the offset error signal. This would allow the difference frequency to be adjusted over a wide range with closed loop control, including the case where L1F1 equals L2F1 (or with trivial modification, where L1F2 equals L2F2).

    Both of these schemes are left as exercises for the student. :)

    There is still another annoyance. It is a slow variation in the difference frequency with a period of a few seconds. It looks like sort of a dance with the control loops of one or both lasers hunting back and forth a few hundred kHz to and much as +/-1 MHz. (The values for the difference frequencies above were averages.) The period is between 2 and 3 seconds and appears to remain relatively constant. When I first noticed this hunting behavior, I didn't know if was a peculiarity of the locking electronics or from some external influence like the DC power supplies, vibrations, drafts, or aliens attempting to communicate with Earth. :) There is a digital clock with a period of 2.56 seconds on the laser Control PCB but I didn't think it should be doing anything once the laser locks. And it wasn't something associated with the original Control PCBs in either laser since swapping them with Control PCBs from other lasers didn't noticeably change the behavior. Nor did swapping one of the HeNe laser power supplies on the off chance that the switching frequencies were interacting in some peculiar way.

    I also substituted a 5501B for Laser 2, so call it Laser 3. (The final resting place for the frequency difference was around 2.5 MHz for L3F1-L1F1.) The variation in difference frequency was still present, but its deviation seems to be much lower, perhaps +/-100 kHz. The other thing that changed was the power supply since I had to use a different one for the 5501B. So it was possible that power supply fluctuations, origin unknown, could be the cause. However, then I swapped in the power supply originally used for Laser 2 to power Laser 1 and that made no difference. Thus, not the power supply. I then substituted Laser 2 for Laser 1 and the large fluctuations returned. So, they must be either associated with the tube itself or the LCD switch, since everything else of relevance (Control PCB and HeNe laser power supply) had already been swapped with no effect on the frequency fluctuations. And note that the combination of Laser 1 and Laser 3 did have some of this, just not nearly as much with Laser 2. But, even though it isn't pretty, this could still very likely be considered normal since even the much greater variations of Laser 2 are below the allowable specifications for these lasers. However, I then swapped the tube from Laser 2 into another case which meant the HeNe laser power supply and Connector PCB were different. At first, it looked like the difference frequency variation was way down, below +/-50 kHz, but over time as both lasers reached equilibrium, it climbed back up to around +/-300 kHz. Possibly somewhat lower than before but nothing conclusive. The difference frequency had also shifted down by about 5 MHz (L3F1-L2F1 of 6 MHz). Swapping the control PCB made little difference, but swapping in the original beam sampler assembly restored the difference frequency to its previous value of 1.5 MHz but also restored the large fluctuations! So, then yet another beam sampler assembly was installed and now the difference freuquency is around 0.5 MHz, but the fluctuations are also way down at +/-50 kHz and seems to be staying that way. So, it may be that the beam sampler has the most impact on both of these issues. Seriously strange.....

    In frustration, I finally did some of the things I should have done originally - changing the digital clock speed, looking at some control PCB signals and - gasp! - actually reading the service manual in more detail! :) It's easy to select clock speed on 5517B/C/D lasers - there is a convenient jumper for "Normal" and "High". Switching to High immediately caused the hunting to be much faster. Parallelling some key resistors to change the oscillator speed also had a similar effect. So, the hunting was related to the digital circuitry but how? Checking some key test points immediately revealed that my assumptions were totally wrong and that the cause is a direct result of the HP/Agilent implementation of the feedback loop using the LCD switch to alternately select each of the two polarized modes, rather than incorporating a polarizing beam sampler with separate photodiodes. Before realizing the cause, I had assumed that the LCD switch was alternating polarization states at 50 Hz. It's not. The two states are (1) when there is no drive to the LCD and both sides are at the same potential (passive state, polarization rotated 90 degrees) and (2) when the LCD is driven by a 50 Hz squarewave of opposite polarity on each side (active state, polarization unchanged). Apparently, the response of the LCD is slow enough that the active state is essentially DC - it doesn't see the 100 Hz ripple. (At least that's the theory. Given that the behavior of these lasers in terms of exact locked optical frequency and slow speed oscillation in optical frequency is affected by the specific beam sampler with its LCD that is installed, I wonder if there is enough variation in the residual response to be the cause.) The LCD switches from the passive to the active state with a period of 2.56 seconds! Just before each state change, the appropriate sample-and-hold is latched with the photodiode voltage corresponding to that state. This happens every 1.28 seconds. So the hunting is a direct result of this digital artifact in an otherwise analog system. It should be possible to eliminate or greatly reduce the effect. A few of the possibilities are to simply run the system at a higher speed once it's locked, to improve the sample-and-hold circuits (e.g., bigger caps!), or to replace the LCD and S&H circuits entirely with a normal polarizing beam sampler and dual photodiodes.

    At least one mystery remains and it has to do with the REF (or split) frequency behavior after locking. Starting at that time there is a fairly long period of an hour or more where the REF frequency oscillates by 1 or 2 percent up and down (period of several minutes), with a smaller oscillation of a few tenths of a percent period of 20 or 30 seconds). For example, on Laser 1, it starts between 2.279 MHz and 2.308 MHz (change of about 1.25 percent) and the deviation of the faster oscillation is 0.002 MHz (change of about 0.09 percent). The deviation of these oscillations gradually declines until it becomes small or non-existent, or the periods become very long so the changes are not easily seen. The mystery is why there is no obvious corresponding oscillation of the difference frequency between two lasers! One might expect that the origin of the REF frequency oscillations is the optical frequency varying about its nominal value causing the REF frequency to change based on the lasing position on the split gain curve. But while the difference frequency drifts during warmup, there is no obbious correspondance, by eye at least, with the REF frequency variations.

    So, both of these will probably remain unresolved for now.

    See the section: HP/Agilent Laser Wavelength/Optical Frequency for measurements of other lasers and additional comments.

    I then ran a pair of these lasers for many hours to get an idea of the longer term stability. In all cases they are Lasers 2 and 3 from the above link, a healthy 5517B and 5501B. The first set of data is for both from a cold start:

      Lasers      Balanced
      Time on     Frequency
      (hh:mm)    Difference
        0:00     -8.50 MHz*
        0:15     -6.75 MHz
        0:30     -6.65 MHz*
        0:45     -6.55 MHz
        1:00     -3.95 MHz
        1:15     -2.55 MHz
        1:30     -2.05 MHz
        1:45     -1.75 MHz
        2:00     -1.75 MHz
        2:15     -1.69 MHz
        2:30     -1.67 MHz
        2:45     -1.66 MHz
        3:00     -1.61 MHz
        3:15     -1.57 MHz
        3:30     -1.55 MHz
        3:45     -1.57 MHz
        4:00     -1.45 MHz
        5:00     -1.44 MHz
        6:00     -1.32 MHz
        7:00     -1.44 MHz*
        8:00     -1.45 MHz

    The "Balanced Frequency Difference" is the mean optical frequency for Laser 2 [(L2F1+L2F2)/2] minus the mean optical frequency for Laser 3 [(L3F1+L3F2)/2]. * Denotes interpolated values, not measured. Read: Guessed, because I missed recording the value for that time slot! :)

    So, approximately 2 hours was required to get within 0.5 MHz of the final value. However, these are the average of several 10 second measurements and there is an uncertainty of several hundred kHz for each one. There is high frequency jitter at the switching frequencies of the HeNe laser power supplies (40 kHz tyical), as well as at the PWM frequency of the 5501B controller (50 kHz typical), and slow hunting back and forth from the control loops of both lasers (a few seconds or more).

    Next, I turned off Laser 2 for 2 hours to let it return to room temperature without touching Laser 3, and then started Laser 2 up again:

      Laser 2     Balanced
      Time on     Frequency
      (hh:mm)    Difference
        0:00     -8.15 MHz
        0:15     -6.60 MHz*
        0:30     -5.05 MHz
        0:45     -3.52 MHz
        1.00     -2.48 MHz
        1:15     -1.84 MHz
        1:30     -1.61 MHz
        1:45     -1.44 MHz
        2:00     -1.35 MHz
        2:15     -1.20 MHz
        2:30     -1.21 MHz
        2:45     -1.26 MHz*
        3:00     -1.32 MHz
        3:15     -1.39 MHz
        3:30     -1.39 MHz
        3:45     -1.39 MHz

    Finally, I turned Laser 3 off and let Laser 2 run all night:

      Laser 3    Balanced
      Time on    Frequency
      (hh:mm)    Difference
        0:00     -3.25 MHz
        0:05     -1.45 MHz
        0:15     -1.65 MHz
        0:30     -1.55 MHz
        0:45     -1.20 MHz
        1:00     -0.85 MHz
        1:15     -0.97 MHz*
        1:30     -1.10 MHz
        1:45     -1.15 MHz*
        2:00     -1.20 MHz*
        2:15     -1.26 MHz
        2:30     -1.31 MHz*
        2:45     -1.37 MHz
        3:00     -1.46 MHz
        3:15     -1.52 MHz
        3:30     -1.51 MHz
        3:45     -1.46 MHz
        4:00     -1.36 MHz
        8:00     -1.56 MHz
       10:00     -1.95 MHz
       12:00     -1.85 MHz
       24:00     -2.02 MHz
       25:00     -2.05 MHz
       26:00     -1.75 MHz
       29:00     -1.69 MHz
       30:00     -1.94 MHz
       32:00     -1.68 MHz
       33:00     -1.79 MHz
       34:00     -1.79 MHz
       36:00     -1.76 MHz

    So, it would appear that most of the frequency drift with warmup is due to Laser 2, and Laser 3 has very little drift after the first few minutes and for several hours after that. I don't know if this is a characteristic of 5501Bs in general, or whether Laser 2 is simply particularly slow to reach equilibrium. But based on previous testing, other 5517s were not much better. The 5501B control scheme does differ in subtle ways from that of the 5517s, though not in anything fundamental. The cause of the increase in optical frequency difference after about 8 hours is not clear but it does seem to have settled in at the new location. It's unlikely to be anything related to the lasers themselves reaching equilibrium as that should have happened well before this time. The ambient temperature may be changing as I have little control over that in my, um, lab. :) The limit cycle seems to have a total deviation of +/-150 kHz or about 0.0003 ppm.

    Recall that these are averages over multiple 10 second intervals. My next objective was to reduce the short term variations, initially those due to the switching noise from the HeNe laser power supplies in the lasers. So, I fabricated adapters so that the tubes in both lasers could be powered from Spectra-Physics 248 exciters, which are AC line powered HeNe laser power supplies with linear regulators. The residual ripple in the tube current is below the scope noise floor and undetectable, compared to 3 percent p-p from the VMI power supplies HP used. Based on the previous test where I substituted an SP-248 in one of the lasers, I assumed this would clean it up. Well, not quite.

    The result was slightly less messy, but there was still serious frequency modulation with deviation in the MHz range, now seeming to be at a bit over 50 kHz. Since I was beating a 5517B with a 5501B, the last possibility for switching noise of this type was from the 5501B, which uses Pulse Width Modulation (PWM) to drive the heater rather than a linear power amplifier. It runs at around 50 kHz nominal, but that's not very precise as it is simply determined by an RC network.

    So, I swapped in another 5517B (Laser 1) and Voila! The beat is now a really nice sinusoid with only a hint of fuzz. This is what a heterodyne signal should look like! No wonder HP did away with the PWM driven heater in the 5517 lasers! Exactly how the ~50 kHz noise makes its way into the optical frequency is not at all obvious. It would seem to be too high a frequency to be a mechanical effect but the heater coil is bifilar-wound so there should only be a very small magnetic field from it. But perhaps, very small is still enough!

    As further insurance, I have modifed a pair of SP-248s to reduce the already essentially undetectable power line ripple by at least another order of magnitude. For details, see the section: Reducing SP-248 Current Ripple I also plan to add a protection circuit to the SP-248s that will shut the supply down if either the current exceeds 4 mA due to a fault or the current drops out. I don't want a power supply failure to fry a tube!

    Photo of Test Setup for HP/Agilent Laser Optical Frequency Comparison is really ugly but quite functional. The scope on the right shows the actual beat frequency of about 3 MHz between F1 of one 5517B facing left-right and F2 of the 5517B pointing toward the back of the photo, in this case with their covers off. The Variac is powering the two SP-248s sitting under it to reduce the AC input voltage until I add the addtional RC filter. (Although they work at normal line voltage, the voltage across the regulator pass treansistors is pushing their limits at the low current.) The Thorlabs DET-110 can just barely be made out in place of the HP interferometer. In front of that also sitting on the butcher block is a polarizer wheel that may be installed in either beam. And the 1/2 waveplate to select F1 or F2, and bounce mirror for alignment, are on the table in front of the butcher block. The HP-5508A is displaying "PR Error" because it isn't getting the signals it normally expects.

    Most of the high frequency FM modulation is gone but the slow variation persists with the frequency difference still varying by at least 300 kHz and sometimes much more over the course of a few seconds. This is very puzzling. I have substituted a healthy 5517A for each of the 5517Bs with no change and even put the AC power line to the entire setup on a Sola constant voltage transformer, again with no change. The control loop is not quite totally linear due to the 100 Hz sampling of the LCD polarization selector, but this should be a high enough frequency not to matter. But we do know that the specific LCD has at least some effect on this behavior so some interaction there is not out of the question. It sort of looks like a limit cycle in a chaotic system, but part of this is probably because both lasers are doing it at an independent rate. If not due to the control loop impementation, what else can it be besides the aliens attempting to contact us? :)

    Causes of Variation of the Optical Frequency

    These lasers were apparently designed to be good enough for the metrology applications, but are far from what would be considered to have low noise or a super narrow line-width. The following are only some of the possible causes resulting in changes to or frequency modulation of the optical frequency by as much as 1 MHz or even more, over various time scales. The following applies directly to HP/Agilent 5517 and 5501B lasers except as noted, but much of it also applies to other HeNe Metrology lasers:

    Not included here are normal variations in the optical frequency that are a result of the lasing process itself.

  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Scanning Fabry-Perot Interferometers


    While the interferometers described in the previous sections have many applications in diverse areas, the Scanning Fabry-Perot Interferometer (SFPI) is specifically designed to make measurements of the longitudinal (axial) mode structure of CW lasers. It rates it's own set of sections both due to its importance and because it is possible to construct a practical SFPI at low cost without the need for a granite slab or optical table for stability.

    The longitudinal mode structure of a laser is one of those concepts that is often explained but not so often demonstrated. There are a number of indirect ways of showing that it exists including monitoring the beat frequencies between modes and looking at the fringe patterns in a Michelson or other conventional interferometer. One of the clever ways of actually being able to display the modes as they would appear in a textbook is to use an instrument called a Scanning Fabry-Perot Interferometer (SFPI). While conceptually simple, even a basic SFPI can resolve detail in the longitudinal mode structure of a laser that represents better than 1 part in 10,000,000 compared to the frequency of oscillation of the laser.

    Principles of Operation

    An SFPI uses the optical transmission characteristics of a specially designed Fabry-Perot (F-P) resonator as a very selective filter to scan across the optical spectrum of the laser. Any F-P resonator will have a transmission behavior that has peaks and valleys based on optical frequency (or wavelength). The peaks will be located where the distance between mirrors is an integer multiple of one half the laser wavelength. As the reflectivity of the mirrors approaches 100 percent, the peaks become increasingly narrow and the valleys increasingly flat and close to zero transmission. This characteristic looks like that of a "comb" filter which is very selective.

    An SFPI consists of a pair of mirrors with relatively high reflectivity (90% to 99.9% or more is typical) mounted in a rigid frame. In most SFPIs, the laser under test (LUT) is aimed into one end and a photosensor is mounted beyond the other end. The coarse spacing and alignment of the mirrors can be adjusted by micrometer screws. The axial position of one of the mirrors can also be varied very slightly (order of a few half-wavelengths of the LUT) by a linear PieZo Transducer (PZT). (Other methods of moving the mirror can and have been used but the PZT is most popular.) By driving the PZT with a ramp waveform and watching the response of the photosensor on an oscilloscope, the longitudinal modes of the LUT can be displayed in real time. In essence, the comb response of the SFPI is used as a tunable filter (by the PZT) to analyze the fine detail of the optical spectrum of the LUT. As long as the FSR (c/2*L except under certain conditions, described below) of the SFPI is larger than the extent of the lasing mode structure of the LUT, the mode display will be unambiguous. Where this condition isn't satisfied, the mode display will wrap around and may be very confusing. For example, the common helium-neon (HeNe) laser has a gain bandwidth of about 1.5 GHz and longer HeNe laser tubes will generally operate with multiple longitudinal modes covering much of this range. Thus the FSR of an SFPI to be used with such a laser must be greater than 1.5 GHz, corresponding to an SFPI cavity length of less than about 100 mm (assuming c/2*L). For Nd:YAG, the gain bandwidth is about 150 GHz, which results in a required SFPI cavity length of less than 1 mm! However, in practice, lasers don't necessarily lase over their entire gain bandwidth, especially if specific steps have been taken to assure single or dual mode operation (also called single or dual frequency operation). For those - which include many useful lasers - the requirement can be relaxed such that the FSR of the SFPI only needs to be larger than the width of the expected mode structure. And for a single mode laser, this would be only the width of the lasing line itself. Therefore, in these cases, a long cavity low FSR SFPI will result in the highest resolution.

    Commercial scanning Fabry-Perot interferometers usually cost thousands of dollars - or more! But it's possible to construct an SFPI that demonstrates the basic principles - and can be even quite useful - for next to nothing, and one that rivals commercial instruments for less than $100.

    The resolution ("resolvence") of a Fabry-Perot interferometer is determined by the wavelength, mirror reflectance, mirror spacing, and incidence angle of the input beam. For the following, we assume normal incidence (which will be satisfied in most practical situations).

    Consider an SFPI with a mirror spacing (d) of 80 mm and reflectance (R) of 99 percent at a wavelength (Lambda) of 632.8 nm (red HeNe laser):

                     (Lambda)2 * (1-R)        4*10-13 * 0.01
     Delta-Lambda = ------------------- = --------------------- =
                      2*d*pi*sqrt(R)       0.16 * 3.14 * 0.995
      ~8*10-15 m = 0.000008 nm or about 6 MHz.  (633 nm corresponds to 474 THz.)

    Another measure of the performance of an interferometer or laser cavity is the "finesse". This dimensionless quantity is the ratio of the FSR to the resolution. In essence, for the SFPI, finesse determines the how much fine detail is possible within one FSR. The reflectance finesse is equal to pi*sqrt(R)/(1-R) where R is the reflectance of each mirror (which are assumed to be equal). For R near 1 as would be the case in a useful SFPI, this reduces to pi/(1-R). While other factors will affect the finesse, this equation will be reasonably accurate for a properly designed spherical mirror cavity. So, with a reflectivity of 99 percent for both mirrors, the finesse will be roughly 300. If the FSR is 1.875 GHz as in the example above, the resolution will be approximately 6 MHz, which is in agreement with that calculation.

    Other factors will conspire to reduce the useful resolution of a practical SFPI. At modestly high mirror reflectivity (e .g., R=99%), these include alignment, input beam diameter, and input beam collimation. As R is pushed closer to 100%, the quality of the mirrors, their cleanliness, and internal losses become increasingly important. But for the example above, even if the actual finesse is worse by an order of magnitude compared to the theory, it will still be possible to easily resolve the individual modes of any common HeNe laser and probably even the nearly 2 meter long Spectra-Physics model 125 (177 cm resonator, mode spacing of 85 MHz). This is a factor of better than 1 part in 10,000,000 comparing resolution to optical frequency!

    However, note that while textbooks will tell you that the peaks should get through with little attenuation, this is probably not going to be true with practical high finesse SFPIs. (At least not those you're likely to see!) The amplitude of the peaks will depend critically on the quality of the mirrors and of course, on the alignment. For "laser quality" dielectric mirrors, I've gotten as high as 5 to 10 percent peak transmission for a high finesse SFPI using mirrors with a reflectivity of 99.8%. I'm sure this can be improved upon but even so, for a 1 mW laser, there is still more than enough optical power at the output of the SFPI to produce a nice display on most scopes using a 1:1 probe without a preamp.

    (From: A. E . Siegman (

    In evaluating the effect of losses in Fabry-Perot mirrors you really have to distinguish between internal losses (or loss-equivalent effects, like scattering) that are physically located "inside" the mirrors (i.e ., inside the effective reflection plane of each end mirror), and external losses that are physically located "outside" the effective reflection plane, but still within the physical layer of the mirror.

    Losses that are outside the mirrors are effectively just additional external transfer losses in the system, i.e . they have the same effect as if they were separate from the FP, so that they don't affect the FP itself but just weaken the light before or after the FP.

    Losses inside the mirrors (aka "internal" losses) are more serious because they are exposed to the higher-intensity resonant fields inside the FP and therefore can significantly affect the finesse and peak transmission of the FP.

    Just measuring the net reflectivity and net transmission of the mirror itself won't clearly distinguish between these internal and external losses. Also, how you'd describe a situation where the losses are distributed through a moderately thick mirror layer is something I've never thought through; doing this would require a slightly more sophisticated wave calculation of forward and backwave wave propagation inside the finite-thickness partially absorbing mirror layer itself.

    (Too bad I'm no longer actively teaching laser courses; this calculation would make a nice homework problem to torment -- sorry, educate -- students.)

    Mode Degenerate Fabry-Perot Interferometer

    A major disadvantage of the general spherical F-P cavity is that super precise alignment and control of the input beam size and collimation, along with an intracavity aperture, may be needed to suppress higher order transverse modes in the SFPI resonator. Even though not present in a TEM00 laser, higher order modes are almost unavoidable in the SFPI cavity and may in fact dominate the display and render it completely useless. Even if such time consuming steps are taken, there will always be uncertainty as to what is actually being seen. The flat-flat cavity doesn't have this problem but suffers from disadvantages of its own, mainly in the need for a well collimated input and very precise mirror alignment to achieve high finesse and as a result, reflection of the input back directly back into the laser, which may be destabilizing in some cases.

    One way to eliminate the transverse mode problem is to use a cavity configuration called a Mode Degenerate Interferometer (MDI) in which the higher order transverse modes have the same frequency/wavelength as some of the TEM00 (longitudinal) modes and thus simply fall on top of them in the display. Even though each peak in the display representing a longitudinal mode of the input laser may actually be built up of contributions from multiple transverse modes excited in the resonator of the interferometer, the characteristics of the individual longitudinal mode components in each of these transverse mode are the same so the accuracy of the resulting display isn't affected. (This should not be confused with the very different situation of a laser having multiple transverse modes in its output where the frequencies, phases, amplitudes, and polarizations of the corresponding longitudinal modes in each transverse mode may differ.)

    Two practical arrangements that satisfy this condition are the (1) spherical cavity (d=2*r) and (2) confocal cavity (d=r). The confocal cavity has the larger finesse and is thus usually employed in SFPIs since the finesse is a measure of Q-factor with respect to the FSR or mode spacing, and thus higher finesse results in better resolution. A planar cavity (r is infinity) doesn't support higher order modes at all but is generally a less desirable configuration (see below).

    Note that the term "confocal" actually refers to any cavity where the focal points of the two mirrors are coincident. However, only the case where d=r is stable and thus useful for the MDI SFPI.

    The frequencies of the transverse modes of a symmetric cavity Fabry-Perot resonator are given by the following equation:

              c          1                           d
      fmn = ------ [q + ---- (1 + m + n) * cos-1(1 - ---)]
            2 * d        pi                          r


    The interferometer will be mode degenerate when there are TEM00 modes that have the same frequency as some of the transverse modes. The requirement for this to be satisfied is for the inverse cosine term in the equation above to be equal to pi divided by an integer, l. Then there will be "l" types of modes with one type - where (1+m+n) is equal to 1, modulo(l) - having the same frequencies as some TEM00 modes. When (1+m+n) is not equal to 1, modulo(l), that mode will fall in between the TEM00 modes in locations depending on (m+n)/l, modulo(l):

    While the confocal and spherical MDI configurations are the best known and most widely used, it's possible to make use of cavities having values of l other than 1 or 2 and they may be useful for certain applications. See: Variable Free Spectral Range Spherical Mirror Fabry-Perot Interferometer. Though that's for the advanced course, here are a couple of examples:

    Further investigation of these special cases is left as an exercise for the reader. :)

    For the confocal cavity, half of the transverse modes are not mode degenerate when an on-axis input beam is used as there are two types of modes depending on whether the quantity (1+m+n) is even or odd:

    This seems a bit strange that the TEM00 modes (m+n=0) have non-integer mode numbers but the equation has been confirmed from at least two different sources.

    As noted, with two sets of peaks, the FSR is effectively cut in half to c/(4*d). Rearranging the equation above with the new FSR of c/(4*d) out in front, one sees that the various transverse modes (those that differ in m+n) result in a frequency difference of c/(4*d). However, integer differences in q corresponding to the longitudinal modes, still have an FSR of c/(2*d). Where a paraxial beam (one parallel to the optical axis) enters the confocal cavity off-center, the beam path repeats itself after two traversals of the cavity (in a zigzag pattern) and the FSR is easily seen to be c/(4*d) rather than c/(2*d). However, if the beam is very well aligned and centered, the FSR will be c/(2*d) since only some symmetric modes will be excited.

    Note that when adjusting the mirror distance to be confocal, there will be many positions where the SFPI may appear to work but which aren't quite confocal. Depending on the specific distance, non-degenerate higher order modes will result in ghost peaks and/or a variation in the amplitude of the lasing modes depending on their position on the voltage ramp drive signal. The amplitude will also be lower overall. However, when the correct distance is approached, all of these ghosts will collapse into the desired high amplitude display. Don't be fooled! Thus it's best to know or determine the exact RoC for the mirrors before installing them in the SFPI so the initial distance can be set reasonably precisely.

    Planar mirrors may also be used since a true flat-flat cavity does not support stable higher order modes, degenerate or otherwise, but it is the most difficult to align and the realizable finesse is lower than for the confocal arrrangement. The "effective fineese" is also much more dependant on the alignment than with the MDI or with other non-planar configurations. Also, with optimal alignment, the incident beam is reflected directly back into the laser which may result in instability for some types of lasers. However, where the distance between the mirrors of the SFPI is adjustable (as in some general purpose instruments like the TecOptics FPI-25), there is no choice. (Intracavity etalons also usually use planar mirrors but the finesse of these does not generally need to be very high.)

    More Information on SFPI Theory and Practice

    In addition to what is present in the sections below, check out the following links:

    Constructing Inexpensive Scanning Fabry-Perot Interferometers

    I have used commercial Scanning Fabry-Perot Interferometers (SFPIs). For example, the TecOptics FPI-25 is an example of a very solidly constructed precision instruments with adjustments for just about everything. However, being so general, in some sense it is not optimal for anything! There are somewhat less flexible but easier to use SFPIs from companies like Thorlabs and Toptica Photonics. These have the advantage of being quite robust and mostly insensitive to temperature variations (with some being temperature stabilized), and are available with mirrors coated for relatively broadband reflectivity. They also have a price tag to match - those from Thorlabs start at around $3,000 not including the driver box. You don't want to ask about what the very flexible SFPIs cost. :)

    My challenge was to prove that I could construct an SFPI that would at least demonstrate the basic principles and possibly even be useful. The results are described in this and the following sections. All of mine cost me absolutely nothing (except time) but that wouldn't sound as credible as $1.00 or $2.00 or $3.00. :)

    The heart of the SFPI is its two mirrors. For longer visible wavelengths (i.e ., 600 to 700 nm), the mirrors can be the OCs salvaged from a pair of dead red (632.8 nm) HeNe laser tubes. For other wavelength ranges, mirrors from green (532 nm) DPSS lasers, green or blue ion lasers, HeCd, and other lasers may be useful. While some of these mirrors may have a relatively broad band reflectance, this cannot be counted on. More often than not, the reflectance falls off dramatically beyond 10 or 20 nm from the spec'd wavelength. And, obtaining proper single mode performance of the SFPI without great pain may require that mirrors with specific reflectances and RoCs not normally found in common lasers be used. Of course (gasp!), suitable mirrors can be also be purchased. For common wavelengths, they may be available from companies like CASIX at very reasonable prices. But in general, obtaining the optimum mirror might require ordering a set of custom mirrors. It's not the ground and polished mirror glass itself that will cost a lot of money. They can often be standard concave lenses with suitable curvature available from places like Edmund Industrial Optics or Melles Griot. It's the custom coating, which can easily exceed $1,000, and it doesn't matter that much whether the lot is 2 mirrors or 200 mirrors as what counts is the coating machine time. So, find 99 friends who want to build the same SFPI and the per-mirror cost could still be quite low. :)

    For a short RoC confocal cavity SFPI (more below), the only readily available mirrors I know of are either the misfits I'm using in my $3 SFPI for HeNe lasers (also more below) or mirrors from flowing dye lasers. Unfortunately, the latter tend to have ground, but not polished, outer surfaces. However, Since the outer surfaces aren't critical, simply using some index-matching fluid, or even common oil or water, between the ground surface and a piece of glass like a microscope slide or cover slip is know to work well enough. It's the coated mirror surface that's important.

    As far as attempting to coat your own mirrors - in two words: Forget it. :) Unless you have access to a dielectric mirror coating machine and know how to use it (and are permitted to use it!), there is no way to produce coatings that will do anything more than provide a hint of what's possible. Metal (aluminum, silver, gold) coated mirrors do not work well since their maximum reflection coefficient is around 94 to 97 percent and they have high absorption losses. Thus finesse will be poor and the photodetector signal will be very small. And except for gold, the coatings degrade (tarnish, oxidize) in air without a protective layer, with silver being the worst. For good quality dielectric mirrors, absorption losses only become a major concern for very high reflectivities (perhaps above 99.9%) and modern coatings do not degrade significantly under normal conditions as long as they are not subject to physical abuse or improper cleaning techniques.

    When specifying the mirror RoC (r) for a particular application, it usually makes sense to base it on the maximum frequency range over which there will be action, not simply on the gain bandwidth of the laser(s) being observed. Not only will this result in the best resolution, but doing otherwise may simply not be practical. For common gas lasers like the HeNe and argon ion which have longitudinal modes filling most of their gain bandwidth, (1.5 GHz and 5 GHz, respectively) there's no choice if the display is to be unambiguous. But where the modes have already been limited by an etalon or some other means, only the range of the modes that are present need to fit into the SFPI's FSR. For example:

    The other major components of the SFPI include the PieZo Transducer (PZT) to move one of the mirrors a micron or so, and a photodiode to monitor the output beam.

    High quality PZTs can be purchased at exorbitant cost. But the beeper from a digital watch or similar device will work nearly as well and has the advantage that it runs on much lower voltage than some other types. You never did like that alarm anyhow. :) But no need to discombobulate your watch as these piezo elements can be purchased from electronics distributors or surplus places for about $1.00. :) While they aren't quite as linear or have as good a frequency response as the high priced units, these deficiencies don't really matter much for an SFPI. And since they will move several microns on only 50 V, a high voltage amplifier isn't needed as with many commercial SFPIs. The 20 or 30 V p-p output of a typical function generator is quite adequate.

    . The photodiode can be almost anything since it needs neither a large area or high frequency response. I typically use a photodiode from a barcode scanner with a 10K ohm resistor load and 10:1 or 1:1 scope probe. Where more sensitivity is needed as with very high-R mirrors or low power lasers, a trans-impedance amplifier with very high gain using can be added since frequency response isn't critical. Any garbage op-amp will suffice.

    Everything else is hardware. The structure and mirror mounts are easily home-built. However, one area where it may be hard to compete with commercial SFPIs is in minimizing the effects of temperature. They typically construct the main support as a cylinder or set of rods made from Invar, a low coefficient of thermal expansion alloy. Some designs further compensate for residual effects by balancing them against those of the PZT resulting a near zero net change in FSR with respect to temperature and/or may include a heater in a closed-loop temperature stablization system. Invar stock is available or can be salvaged from various dead lasers. Some people build SFPIs by mounting the back mirror and PZT in an Invar tube, positioning the front mirror using a 5-axis lab stage, and then gluing it in place permanently when the optimal mirror spacing and alignment has been determined. But glue tends to be too permanent for my taste. :) Constructing the SFPI using Invar rods is nearly as good. But simply enclosing a non-Invar based SFPI in an insulating box will go a long way in reducing temperature effects.

    A triangle (or sawtooth) wave source (it can be a simple circuit constructed for this purpose or a general purpose function generator) and oscilloscope (preferably dual trace and/or with an X-Y display mode) will be required to view the scan but needn't be dedicated to the SFPI, so they don't count toward the cost!

    The next few sections include general descriptions and photos of several home-built SFPIs. Schematics for both a photodiode preamp and simple function generator are provided later in this chapter.

    (From: A. E . Siegman (

    When thinking about producing small and not too fast mechanical motions or pressures, consider also magnetic methods.

    After University Labs in Berkeley introduced the first really low-cost lasers in the early 1970s (priced at circa $300 each rather than the prevailing several thousand dollars and up), it also produced a really neat and equally inexpensive little scanning FP interferometer with plastic end plates and the scanning mirror driven by what was in essence a miniature loudspeaker coil.

    One of the advantages of the magnetic versus piezoelectric approach is low voltage, higher current drive circuitry, perfectly adapted to IC or semiconductor electronics. Another advantage is wider range of motion.

    Sam's $1.00 Scanning Fabry-Perot Interferometer

    This is the first of three (so far) SFPIs I've constructed, differing mostly in the mirrors and their spacing. It is non-mode-degenerate, having been built before I knew about such things. :)

    The basic design is shown in Home-Built Scanning Fabry-Perot Interferometer 1. My prototype uses the OC mirrors from a couple of dead Aerotech 1 mW HeNe laser tubes. The PZT is the beeper from some sort of musical greeting card with a 4 mm hole drilled in the center. The photodiode is from a barcode scanner. The frame and mounts are a bit different than those shown in the diagram, above. They were made from the platter clamping plates from some ancient 5-1/4" harddrives, hex spacers, and miscellaneous scrap metal. The circular plates are nice because they have predrilled holes with 6-fold symmetry thus simplifying construction. See Photo of Sam's $1.00 Scanning Fabry-Perot Interferometer. Here is a summary:

    The front mirror is removable so other reflectances or RoCs can be tried. The rear mirror is glued to the PZT. The hole was made by placing the PZT on a hard surface (e .g., an aluminum plate) and drilling through it slowly with modest pressure using a normal metal bit in a drill press. The piezo material is more of a compressed powder than a true ceramic so it's possible to grind it away (using the metal drill) with minimal chipping. Thin flexible wires were already attached but if they aren't, solder the top lead near the edge to leave room for the mirror and to minimize any change in elasticity of the top surface. Once soldered, Secure the wires mechanically with a drop of adhesive. Also note that the metallization tends to disappear with even modest heat or stress so solder quickly. Conductive paint or silver Epoxy can be used to touch up bare spots if needed but use as thin a layer as possible as it may increase stiffness and reduce response sensitivity in that area. For this reason, DO NOT coat the entire surface with adhesive of any type!

    To perform initial alignment, I used a yellow-orange HeNe laser thinking it would be easier since the mirrors are less reflective away from the 632.8 nm design wavelength. The scatter off of the mirror surfaces was used as the initial means of setting alignment, by minimizing the size of the line or blob formed by the multiple reflections. With a pair of concave mirrors, not only do they have to be aligned with respect to the input beam, they also have to be aligned with respect to each other. In other words, their optical axes must coincide which requires walking them until the scatter pattern is minimized. When misaligned, it will be a line or circle and no amount of adjustment of only one mirror may improve it. Once the initial alignment was done, the PZT could be driven and the output of the photodiode used to fine tune it. In retrospect, using the funny color HeNe laser wasn't necessary as enough red light gets through to be easily seen for alignment purposes. And the display of the modes of that multi-wavelength and multi-transverse mode laser was definitely strange.

    The preliminary results using a Melles Griot 05-LHR-911 HeNe laser were also confusing. This is a 2 mW laser using a tube with about 165 mm between mirrors, corresponding to a mode spacing of 883 MHz. The scope trace in Sam's SFPI Display of Melles Griot 05-LHR-911 HeNe Laser - Initial Attempt shows a jumbled mess due to many transverse modes being excited in the SFPI. The trace on the left should cover a span of approximately three FSRs of the SFPI - about 19.5 GHz. Three clumps that look about the same are clearly visible but the complexity isn't real. The trace on the right is an expanded region of the one on the left. A hint of the modes of the laser can be seen but only a hint. The 05-LHR-911 should have 2 or 3 longitudinal modes at most but the short cavity of the SFPI using long radius mirrors is resonating with multiple transverse modes.

    There is also some hysteresis in the PZT response. It's barely visible on the display as the pattern differs slightly on the positive and negative slopes of the triangle driving function. Using X-Y mode on the scope would show up the hysteresis more clearly. Reducing the sweep speed slightly virtually eliminates the hysteresis. (A 20 trace/second display has minimal hysteresis and is still quite usable. Of course, this wouldn't be an issue with a digital scope

    The overall linearity of the PZT is around 5 to 10 percent over a range of +/-20 V, corresponding to 5 or 6 FSRs of the SFPI. I've actually tested several PZTs (another one was from a digital clock for which the alarm was more of a nuisance than useful!). The response of one is compressed more toward the upper end of the voltage range; the other is slightly compressed at both ends. Within a single FSR, the linearity is probably better than 2 percent and a range of a single FSR provides all the information usually needed. For a system of this type where qualitative information is most important, perfect linearity, especially over multiple FSRs, really isn't a major issue in any case as long as it is known and doesn't change over time. A third PZT was quite linear but had a range of only around 1 FSR of the SFPI - probably due to the excessively thick layer of silver Epoxy I used to cover some bald spots on the piezo disk.

    To confirm that transverse modes were the cause of the complex display and to partially remedy the situation, I aligned the SFPI more carefully by adjusting the front mirror so that the 05-LHR-911 beam bounced directly back to the source with dancing interference patterns, then aligned the rear mirror for maximum amplitude of the displayed signal, and added an aperture about 0.3 mm in diameter (a pin hole in a piece of aluminum foil) inside the SFPI cavity. The aperture was mounted on a micropositioner but could be installed permanently so that doesn't blow my budget. :) The results are shown in Sam's SFPI Display of Melles Griot 05-LHR-911 HeNe Laser. The sequence of the six traces show the modes of the 05-LHR-911 cycling over time as they move under the HeNe gain curve. The horizontal scale is the same as in the jumbled mess trace, above, but the transverse modes have been almost entirely eliminated. The distance between similar peaks (2.2 boxes on the screen) is the FSR of the SFPI - about 6.5 GHz. The distance between longitudinal modes (0.3 boxes) is the 883 MHz FSR of the 05-LHR-911. The math even works. :) So, this represents success of sorts but alignment of everything is super critical and any vibrations - even the audio from a radio - create havoc with the display. There is also a quasi-periodic fluctuation in amplitude of all the displayed modes with no corresponding power fluctuations in the laser. I suspect this to be due to residual mode competition in the SFPI as the frequency of the modes changes relative to the SFPI cavity, possibly a side effect of the aperture.

    Sam's SFPI Display of a Melles Griot 05-LHR-151 HeNe Laser shows the result using the same setup for a longer laser with more closely spaced modes - 436 MHz compared to 833 MHz for the 05-LHR-911. With this higher power laser, there are still some non-TEM00 modes just visible in the display but they are fairly low level. Sam's SFPI Display of Vertically Polarized Modes of Melles Griot 05-LHR-151 HeNe Laser shows the effect of inserting a polarizing filter into the beam. Since adjacent modes tend to be of orthogonal polarization in randomly polarized HeNe lasers, every other mode on the display has disappeared.

    Finally, I tried a Spectra-Physics model 117A HeNe laser head, which when used with its mating controller is a frequency or intensity stabilized (single longitudinal mode) laser. I'm running it on an SP-248 so it's not stabilized but the modes are a bit interesting. The mode spacing is around 600 MHz which is consistent with a 2 to 3 mW HeNe laser. However, as the modes cycle, there isn't a smooth progression through the gain curve. It almost seems as though having exactly 2 modes is enhanced somehow and that it's very unlikely to see 1 or 3 modes. When 1 or 3 modes would be expected to pop up, they might appear very briefly, or be skipped entirely in favor of the 2 modes one of which is on the opposite side of the gain curve. The polarizations of the modes also appear to be of the "flipper" variety, changing suddenly rather than staying with a particular mode. I don't know if this behavior is by design. However, since orthogonally polarized modes are sensed by a pair of photodiodes in the laser head and used for stabilization, strong mode pairs could be beneficial.

    After determining experimentally that an aperture helped but didn't totally eliminate the transverse mode problem, a Post Doc in our lab wrote a simple Matlab program to calculate Hermite Gaussian transverse mode profiles given the mirror RoCs and the distance between mirrors. Plugging in the long radius SFPI cavity configuration revealed that the TEM00 and TEM10/01/11 modes have a high degree of overlap regardless of axial position. So, any aperture that suppresses them very effectively would also result in unacceptable attenuation of the TEM00 mode. So, on to plan B. :) I hope to have a compiled version of this program available in the near future as it appears to be quite useful for visualizing cavity modes in general.

    Here is a summary of the configurations I've tried so far on the $1.00 SFPI:

    Of these, the first is probably the best choice unless super high resolution is needed. All except the flat-flat required an aperture inside the SFPI cavity to suppress non-TEM00 (transverse) modes.

    Sam's $2.00 Scanning Fabry-Perot Interferometer

    Well, it wasn't actually $2.00. :) I found some small radius mirrors originally intended for a research project that is now in limbo. These should work well in a confocal configuraion in the green region of the spectrum free of those annoying transverse modes!

    The mirrors were actually Melles Griot plano concave lenses custom coated (along with a batch of microchip laser crystals) for 1,540 nm. Now, it's perhaps a not so well known fact that a dielectric mirror coated for a wavelength of X nm will also perform reasonably well at a wavelength around X/3 nm (think of a stack of 3/4 lambda layers instead of 1/4 lambda layers). The actual reflectance function will depend on the design of the original mirror (number of layers, uniformity of layer thickness, etc.) and will likely be slightly lower in maximum reflectance, but possibly not by much. So, these mirrors should work in a wavelength range centered around 513 nm (1,540/3).

    I had two types available: Those that were supposed to be 98 percent as OC mirrors and those that were supposed to be HR mirrors, both at 1,540 nm. Here are how they performed at the two green wavelengths of interest:

                             Reflectivity at       Reflectivity at
         Mirror Type       532 nm (Green DPSS)   543.5 nm (Green HeNe)
       OC (98%@1,540nm)          97.8%                    88%
       HR  (HR@1,540nm)          99.8%                    99%

    For 532 nm, neither is really ideal. The "OC" is a bit low - I would have preferred around 99% to achieve a higher finesse. However, 97.8% is still decent. The reflectance of the "HR" - which could be even higher than the measured 99.8% since the 0.02% transmission measurement was not very accurate - might be too high to get a decent signal but could result in a very high finesse. But at 543.5 nm, the "HR" mirror seems to be perfect.

    The only thing not wonderful about these mirrors is that the planar side isn't AR coated. (Since they were intended only for some tests, we saved money by not having AR coating!) But, if they are slightly tilted, hopefully, this won't be a major problem.

    There are also several radii to choose from. For the first version, I used the longest RoC which is a Melles Griot 01-LPK-01. This is a 10 mm diameter BK7 lens with a focal length of -20 mm which has a RoC of about 10.3 mm. (For BK7, the RoC of a plano-concave lens is -0.517 of the focal length.) This results in an FSR of about 7.8 GHz. Note that the FSR is c/(4*d) for the confocal cavity, one half that of the long radius or planar SFPI cavities. See the previous section. So, these will be good for all green HeNe lasers and longer cavity single mode green DPSS lasers like the C315M and C532, as well as that Far East disaster described in the section: Reconstruction of an 80 mW Green DPSSFD Laser. However, short cavity DPSS lasers including green laser pointers, the Uniphase uGreens, MCA based DPSS lasers, and possibly the Transverse TIM622 will require a shorter SFPI cavity. The other sets of mirrors go down to around a 5 mm RoC so another version may be built with a set of these.

    However, note that since the gain bandwidth of Nd:YAG and Nd:YVO4 is over 150 GHz and the SHG green conversion also doubles the frequency between modes, multimode solid state lasers may have frequencies which greatly exceed the FSR of any of these medium length SFPI cavities. Unambiguous display of their modes may require an SFPI with an FSR of more than 300 GHz - a cavity length of 0.25 mm for the confocal configuration! This is not very practical. Fortunately, what's oftem most important is to confirm single or maybe dual longitudinal mode performance so a much smaller FSR is adequate and desirable for maximum resolvance. With a bit of care in interpretation, almost any FSR will be fine for this purpose.

    The mechanical configuration is similar to the $1.00 SFPI except that the rear mirror mount can be moved along the optical axis on threaded rods to match the mirror distance to the RoC of the mirrors. A diagram along the lines of the simple design of the $1.00 SFPI is shown in Home-Built Scanning Fabry-Perot Interferometer 2. Again, mine was constructed of cast off disk drive parts and other miscellaneous junk. :) The first photodiode I used for this SFPI was a $2.00 part from Digikey - which would have been my total cost if it hadn't already been in one of my random stuff drawers. :) And, the frame is a bit shorter since the RoC of all of these mirrors is so small. Please see: Photo of Sam's $2.00 Scanning Fabry-Perot Interferometer.

    For the initial test, I am using the HR mirror set with an 05-LGR-151 green HeNe laser head. Since this is a less than 0.5 mW output laser and the sensitivity of silicon photodiodes at 543.5 nm is somewhat lower than at 632.8 nm, detection is more difficult.

    Furthermore, in order for the SFPI to be mode degenerate, the mirror spacing really has to be quite close to the RoC for the confocal configuration. Since these were originally lenses and not mirrors, the exact RoC is not really known. OK, the real story is that I didn't locate the part numbers of the lenses until after I did the initial construction and wrote this paragraph! There are many ways to determine the actual RoC of the mirrors. A collimated beam can be reflected from the mirror at a slight angle. The focal point will be at a distance of one half the RoC. Alternatively, a point source like a bare visible laser diode can be imaged back onto itself from the mirror. Then, the RoC is the distance to the mirror. However, any such measured RoC is only approximate. For the SFPI to be mode degenerate, it needs to be quite precise and this can only be determined experimentally.

    The mirror alignment itself isn't super critical. It's best to have a way of changing mirror distance without affecting alignment very much but simple three-screws adjusters work just fine. The laser used for the alignment should have a known spectrum if possible, preferably a single longitudinal mode. As the correct distance is approached, the little peaks from all the modes of the not quite confocal cavity - which may indeed be very small or undetectable - will gradually merge into one peak whose amplitude will increase and width will decrease dramatically.

    Note that the MDI doesn't eliminate higher order transverse modes. It only assures that many of them will have the same frequency as the TEM00 modes. If the distance between the mirrors isn't close to the RoC, there will be higher order modes at essentially random frequencies relative to the TEM00 modes. The result will be very low fringe contrast in the output as the PZT voltage is varied. However, as the correct distance is approached, these will approach the TEM00 modes. Visually, if the distance between the mirrors is moved slowly with the PZT around the optimal distance, the output beam from from the SFPI (going to the photodiode) will flash on and off uniformly across its entire width, while on either side there will be concentric rings of light and dark sweeping from center to edge or vice-versa. It's actually quite remarkable that varying the PZT voltage by hand (ramp turned off), the output of the SFPI can be tuned to all light or all dark very precisely when the distance is just right. Indeed, only the TEM00 mode remains! In addition, alignment of the SFPI relative to the laser is very easy. The reference I am using is to adjust the the reflection from the planar surface of the front mirror to be just below the output aperture of the laser, then adjust the position of the beam (without changing its angle) to center the reflected blob from the curved rear surface of the front mirror.

    At this point, after some fiddling, I am able to see the modes of the 05-LGR-151, though the signal is extremely low level and the finesse is poor. In addition, the modes appear to be somewhat distorted - possibly due to the distance between the mirrors not being quite correct. Switching the function generator to DC output mode and adjusting the voltage through the modes of the HeNe laser shows a very complex transverse mode pattern which is clearly not degenerate even when the mirror distance is very close to optimal. I don't know if this is due to the distance still not being perfect (commercial SFPIs are set to within a few um) or due to poor accuracy in the spherical shape of the mirrors. Focusing the beam improves the resolution and amplitude of the signal somewhat or just due to the nonuniformity of the coating which results in the reflectance decreasing from center to edge. A modest size aperture (perhaps 1 mm) will probably help to eliminate many of the higher order mode since they are quite spread out.

    Up to this point, my conclusions were mixed. Yes, the jumbled peaks were gone. And, alignment is definitely much less critical - once the distance of r is found, any two of the three rear mirror mount nuts or mirror adjusters can easily peak the output in no time flat. But, the resolution is lower than my $1.00 SFPI - between 50 and 100 MHz, compared to better than 25 MHz. Whlte the larger FSR means that the resolution will not as fine for the same finesse, another factor may be the quality of the mirrors (or lack thereof, actual specs unknown). A focusing lens (see below) and modest size intracavity aperture will help somewhat. And a photodiode preamp will help make alignment easier. As long as the reflections from the various front optics don't return to the HeNe laser, the modes are quite stable. However, very obvious instability results if a major portion of the reflected HeNe beam hits the laser's output mirror. Then, wild mode fluctuations appear in the SFPI display - some modes may momentarily double in amplitude or disappear entirely. And visible power fluctuations are also visible in the beam and interference patterns.

    The next step will be to add a proper focusing lens as shown in the $2.00 SFPI diagram (there is none in the one in the photo). Presently, the curved surface of the front mirror results in a large diverging effect on the input beam. Using a long focal length lens helps somewhat. But in a test using a short focal length positive lens mounted in a spring clothspin on a micropositioner helps even more. This cancels out the negative curvature of the front mirror and adds some additional focusing to match the TEM00 mode of the confocal cavity. The signal amplitude increases by at least a factor of 2 and the resolution also improves.

    Eventually, I will probably construct a preamp for the photodiode to provide an adjustable gain of up to 1,000 using a couple of op-amps. This will greatly ease alignment since the height of the signal on the scope on its most sensitive setting with a 10X probe now is only about 1/2 cm at best using the low power green HeNe laser. A possible design is shown in Adjustable Gain Photodiode Preamp. (Frequency compensation capacitors which may be needed for stability are not shown.) The gain is variable from 0.1 to 1,000 compared to the bare phododiode feeding a 10K ohm load. A gain of 10 would be sufficient so this should have enough headroom for other lower output power lasers and/or higher reflectance mirrors.

    However, for now, I just replaced the 10K phododiode load resistor with a 100K pot and substituted a 1X probe for the 10X probe. This resulted in more than enough sensitivity even for the low power green laser while maintaining adequate frequency response.

    Finally, I installed a 9 mm focal length focusing lens as shown in the diagram. This results in a collimated input beam coming to a focus inside the cavity (the focal length of the lenses being used for the mirrors is -20 mm).

    And then it was perfect. :) Well not quite perfect - the finesse isn't much better but it is quite stable, there is no evidence of unwanted ghost frequencies, it is easy to align, and all in all, works quite well. With careful alignment and centering of the input beam, I was even able to achieve the situation where the FSR became c/(2*d) or 14.6 GHz. In this case, every other mode display per sweep of the SFPI nearly disappeared with the remaining ones almost doubling in amplitude.

    The finesse is probably not as terrible as I'm implying. There is also some ambiguity as to whether the resolution (FSR/finesse) is defined with respect to an FSR of c/(2*d) or c/(4*d). With the normal 4 traversals in the confocal cavity, It would seem then that the resolution calculation should use c/(2*d) for the FSR, not c/(4*d). For my 99 percent mirrors, the theoretical finesse is about 300. So, 14.6 GHz divided by 300 is about 49 MHz which is certainly within a factor of 2 of what I've measured. And, as noted, it's quite possible the mirrors are actually somewhat less reflective than the 99 percent being used for the finesse calculation.

    This SFPI is now what I use for testing of DPSS green (532 nm) lasers for single frequency operation. The high reflectivity of the mirrors for 532 nm turned out to not be a problem. The ~14.6 GHz FSR is large enough to display unique modes for the C215M, C315M, C532. While the cavities of the uGreen and LWE-142 lasers are very short and have a higher FSR, it should still be possible to detect spurious non-single frequency operation since the extra modes will not be stable or have a fixed relationship to the primary mode.

    Sam's $3.00 Scanning Fabry-Perot Interferometer

    About a year after building my $2 SFPI, I came across some other short radius mirrors:

    At first I thought these were for some Spectra-Physics dye laser. But thinking about it, I'm now inclined to believe they were a HeNe laser mirror goof. The specifications called for 43 cm RoC mirrors and someone dropped a factor of 10 between design and manufacturing. How else to explain that there were literally thousands of these available surplus at one time. SP never sold that many dye lasers but production runs of thousands of HeNe laser tubes at the peak of their popularity would not have been unusual. Also, SP's dye laser pump mirrors with similarly short focal lengths tended to have the non-mirror side fine ground (not polished and AR coated as with these). Regardless of the origin, I'm not complaining. The person I got the mirrors from insists they are HeNe mirrors and will even send me a laser tube that uses them if he can find one. In principle, I suppose that is possible but it would have to be a very peculiar resonator configuration with a focal point inside. I won't hold my breath in anticipation. :)

    Installing the mirrors and slightly reworking the frame to enable a 43 mm resonator length, it was a simple matter to get this rig to work with much better finesse. That is, after I realized two things:

    1. The focusing lens from the $2.00 SFPI had too short a focal length for the much longer cavity and was smearing out and reducing the amplitude of the response.

    2. The confocal distance was indeed 43 mm and not 38 or 40 mm as I originally thought. At 38 mm, the SFPI initially appeared to work but the display wasn't stable at all, mode amplitudes varied depending on where they were on the ramp voltage, and the photodiode signal was quite weak. Once it was adjusted at 43 mm, the display looked very much like the one in a text book. :)

    The only problem with this SFPI for use with HeNe lasers is that the Free Spectral Range (FSR) for the mode degenerate confocal configuration is c/(4*d), which is only about 1.75 GHz for the 43 mm cavity. This is just barely more than the Doppler broadened gain bandwidth of the HeNe laser, about 1.5 GHz. So, there can be some confusion when interpreting lasing lines on the tails of the gain curve, though this is minor. However, a benefit is that the 1.75 GHz FSR provides nearly the largest useful resolution by almost filling the FSR with the HeNe laser modes.

    I have a set of basic parts available for building a similar SFPI. Sorry, it will cost more than $3 though. :) More information can be found at Sam's Classified Page.

    See W's Scanning Fabry-Perot Interferometer Page for an SFPI using these same mirrors (as well as others for other wavelengths). His mechanical setup uses parts that are a bit more professional and several orders of magnitude more expensive than mine though. Yet, he complains about instabilities that my resonator frames constructed from recycled harddrive parts and Home-Depot hardware don't have. :)

    Simple Driver for Scanning Fabry-Perot Interferometer

    I have also now designed a stripped down function generator especially for driving the PZT of these SFPIs. See Scanning Fabry-Perot Interferometer Driver 1. This unit generates a variable frequency triangle (approximately 5 to 200 Hz) or sawtooth (approximately 10 to 400 Hz) with an amplitude of up to at least 25 V p-p) and adjustable offset. The output may also be set to DC and adjusted over the full range using the offset control for initial setup of the SFPI. Using op-amps better than the jelly bean LM358s might increase the maximum output voltage range slightly but at these frequencies, won't make much difference in any other respect. Of course, it would be trivial to modify this circuit for a different frequency or voltage range. But, as drawn, it should cover the needs of most SFPIs using "drum head" type PZTs. In conjunction with the Adjustable Gain Photodiode Preamp the driver completely eliminates the need for anything beyond an oscilloscope and +/-15 VDC power supply.

    The $99 Scanning Fabry-Perot Interferometer

    While my $1 SFPI can be made to work, the choice in the types of mirrors that are typically available surplus or from salvage are severely limited. Alignment becomes extremely critical and an aperture is needed to suppress non-TEM00 modes. In addition, reflections back to the laser under test may be destabilizing. Though this is probably not a major issue with typical HeNe lasers or green DPSS lasers with an IR-blocking filter in their output, It could be significant for stabilized HeNe lasers and IR DPSS and other non-frequency converted lasers.)

    I lucked out for my $2 SFPI in just happening to have short radius mirrors that could be pressed into service, but most people wouldn't have this option.

    For my $3 SFPI, I do have mirrors available but there are really only useful for red HeNe lasers, for which they are nearly ideal.

    Being able to specify the mirror radius of curvature, wavelength, and reflectance, would greatly expand the possibilities and still result in an instrument for under $100. That's really not too bad considering it should have almost the same performance as a $9,999 commercial SFPI.

    (From: Christoph Bollig (

    Just some comments on the SFPI resonator options: The confocal configuration has the big advantage that it can be used at an angle or an offset! Most single-frequency lasers outputting at the fundamental (not frequency converted) don't like it if they get reflections straight back, and especially when those reflections are from a high reflectivity mirror and well aligned to go back into the laser. And that's exactly what you need to do with a plane-plane interferometer or even with most other non-confocal ones.

    With the confocal interferometer, the best choice would probably be to come in along the optical axis but with a slight offset. The back-reflection will then be at an angle. Since such an arrangement will need two round trips to reproduce, the second mirror can be HR and the "transmission" will be through the same mirror as the incoming beam, just at a different angle as shown in Confocal Scanning Fabry-Perot Interferometer. As you can see, there are no reflections back into the laser.

    Another advantage is that since the second mirror is high reflector, no hole is needed in the PZT. :)

    We have considered different options for the mirrors for use with near-IR lasers, but one of the more likely scenarios is to use a 50 mm RoC output coupler from CASIX with either 98 or 94 percent reflectivity (NDO0205, $50). see CASIX Nd Laser Optics. These are also available in other curvatures down to 25 mm). For the high-reflector on the PZT one could use one of the CASIX standard HR mirrors from the DPSS series (quite a few from CASIX Diode Pump Laser Optics Kits would do. For example, the DPO1301 or DPO1302 ($45) (or the green laser output coupler from Roithner, also 50 mm radius). Or the DPO1303 (HR at both 1,064 nm and 532 nm) which would then be useful for green DPSS lasers as well.

    The Spectra-Physics 470 SFPI

    The SP-470 Scanning Fabry-Perot Interformeter head along with the SP-476 controller provides similar capabilities to my $2 SFPI for only an additional $4,998. :) Actually, I don't know what the selling price was but these are typically $5,000 or more. The SP-470 comes in several flavors depending on the wavelength range of the mirror set (450 to 550 nm or 550 to 650 nm) and Free Spectral Range (FSR, 2 GHz or 8 GHz with 20 MHz or 40 MHz resolution). The finesse is 200 for all versions. More info may be found under Vintage Lasers and Accessories Brochures at the end of the section for Spectra-Physics in the "High Bandwidth Scanning Interferometer Brochure".

    The one I have is the SP-470-3, 550 to 650 nm with a 2 GHz FSR. This is absolutely ideal for all common visible HeNe lasers including the green HeNe at 543.5 nm. (There was no obvious reduction in resolution at 543.5 nm, though I didn't do any precies measurements. And, even for a 532 nm DPSS laser, the finesse was still at least 50.) When I first acquired this unit, the cavity length was all messed up so I had to set it for the confocal condition. This was done using a low power red (632.8 nm) HeNe laser which has only 2 or 3 longitudinal modes at most. After chasing my tail for quite awhile, I found the sweet spot. The adjustment is by turning the mirror cell at the detector end. Being recessed, a plastic "tool" was needed to get at it without fear of damaging the mirror. It's spring-loaded so should stay put, but there is no way to lock it in place. An external detector (Thorlabs DET-110 was set up beyond the end of the SFPI head, which was on a kinematic pan-tilt mount, and that was clamped down so it would not move relative to the HeNe laser. Once the confocal condition was achieved, it was relatively easy to jog the adjustment one way or the other to fine tune the cavity length. And then it really did work like the diagrams in textbooks, and almost as well as my $2 SFPI. :) OK, it is actually better in certain respects: The solid massive resonator virtually eliminates any drift due to the short term effect of temperature on the cavity length and also results in much reduced sensitivity to vibration.

    In fact, it appears as though the resolution may actually be much better than the 20 MHz listed in the specs.

    For a simple display of the modes of a HeNe laser, this high resolution really doesn't matter and may actually be a distraction. I need to find a laser that will do it justice!

    The destabilizing effect of any back-reflections from the SFPI into the laser is also very evident as random noise superimposed on the mode display. So, either an optical isolator must be used, or the beam aimed at an angle so the none of the reflection of the beam enters the laser aperture.

    The SP-470 allows for interchangeable mirrors and detector, unlike the SP-450 where everything is fixed.

    The Spectra-Physics 450 SFPI

    The SP-450 Scanning Fabry-Perot Interferometer head along with a suitable controller has similar performance to the SP-470 but is in a smaller package with a single permanently attached cable for PZT drive and photodiode output. It terminates in a strange plug that doesn't mate with the SP-476. There is no mention of that in the SP documentation I've seen. I simply cut off the plug and installed a pair of well marked coaxes for PZT and PD. The pins are as follows:

       Pin   Wire Color   Function
        A      Black      PD Anode
        B       Red       PD Cathode
        C      Bare       Shield/Ground
        D      White      PZT HV+
        E      Green      PZT HV-

    PD is simply a silicon photodiode with no other circuitry. Normally, PD Cathode would be grounded with PD Anode being the signal input. The polarity for HV has frequency increasing to the right on the display (PZT shrinks with voltage). Reverse to have wavelength increasing to the right.

    Like the SP-470, the SP-450 is also a confocal design but with an approximately 1 mm hole on the input side. But this makes it very easy to use. Just about any beam that enters the hole roughly on-axis will result in a decent display with at most fine tuning of alignment needed for it to be perfect.

    The Spectra-Physics 476 SFPI Driver

    While a low speed function generator with a maximum output of 20 to 40 V p-p will work with my PZT beeper-based SFPIs, most commercial instruments like the SP-470 described above require 100 V or more to provide enough sweep span. The SP-476 Scanning Fabry-Perot Interferometer Driver is basically a dedicated high voltage function generator designed specifically for this purpose. It generates a variable frequency sawtooth ramp with selectable and variable amplitude and offset. The maximum output is switch selectable between 300 and 1,000 V (though the actual peak output seems to be a bit less than these values). A photodiode preamp with 5 gain ranges, as well as a temperature controller (for SFPI setups with thermal control capability) are also included.

    Note that the SP-481 Dye Laser Etalon Controller has most of the functions of an SP-476 and some additional ones like off (just use the centering control to set level) and a slow speed set of ranges. It has a bunch more stuff as well for the dye laser application but that doesn't interfere with the SFPI operation. The only thing it seems to lack is a selection for 300 V or 1,000 V maximum output - it's always 1,000 V (or perhaps a bit more). But since there is a knob to adjust the amplitude from 0 to maximum anyhow, that's no great loss.

    Both the SP-476 and SP-481 show up on eBay quite frequently, generally going for less than $50. Although they are quite old, except for the power transformer which is custom (and identical for both models), the circuitry uses common readily available components should repair be needed. I am not aware of a schematic for either model, but they aren't very complex and the only things known to be problematic (by design) are the stack of three, 500 V, 10 uF electrolytic capacitors in the high voltage power supply. That rating of 500 V is marginal since the total voltage across them is very near 1,500 V - or perhaps even a bit more depending on the AC line voltage. Although there are equalizing resistors across the caps, that's still running too close to their rated voltage for my tastes! Replacing one of the capacitors with a pair of 20 uF, 450 V parts in series with equalizing resistors selected to redistribute the voltage would be a good idea. This was done on unit I've seen,

    A description and photo of the SP-476 can be found under Vintage Lasers and Accessories Brochures at the end of the section for Spectra-Physics on page 5 of the "High Bandwidth Scanning Interferometer Brochure".

  • Back to Laser Instruments and Applications Sub-Table of Contents.


    Basic Description

    While there are many ways of determining the wavelengths produced by a laser or other light source, the simplest one beyond the use of calibrated eyeballs is probably a monochromator. It's possible to construct one from inexpensive parts but they also show up surplus by themselves or as part of other optical devices like spectrophotometers, DNA analyzers, fluorescence spectrometers, and other lab equipment with even more obscure names.

    A monochromator is an instrument which accepts a light source as its input and can select not quite a single wavelength, but a narrow band of wavelengths. A monochromator is probably the simplest device for determining wavelength of a laser or other light source where standard calibrated eyeballs aren't sufficient. (The longer harder to pronounce term "monochronometer" is also commonly used but it refers to the same type of device.)

    There are many types of monochromators but here we only describe one of the simplest, consisting of the following:

    The arrangement above using a planar diffraction grating is acceptable if the input light source is well collimated and aligned with the monochromator's optical axis. But, making the diffraction grating slightly concave with its two focal points at the slits makes the system less sensitive to the orientation or divergence of the input beam and provides better selectivity since it is essentially imaging the light at the entrance slit into the exit slit. Alternatively, spherical lenses or mirrors can be used with a planar diffraction grating to achieve the same effect.

    Due to the way a diffraction grating selects wavelength, if the linear travel of the lead screw is converted into rotation of the diffraction grating by a lever of the proper length, the result is a linear relationship between the "nut" location on the lead screw and wavelength. As long as the pitch (lines/mm) of the diffraction grating is known accurately, the relationship will be exact. Thus, a simple multiturn precision dial can be used to read off wavelength. Or, for an automated instrument, a stepper motor will have a constant nm/step size.

    A fully optical monochromator - with no electronic detector - is perfectly adequate and may actually be preferred for measuring laser wavelengths in the visible range at least since almost any laser is powerful enough to result in a beam at the output of the monochromator to be easily seen. However, for measuring the spectrum of something like a glow discharge (as in the bore of a HeNe laser tube) or UV or IR lasers, a high sensitivity detector is essential. Spectra for varioue elements and compounds can be easily found by searching the Web. The NIST Atomic Spectra Database has an applet which will generate a table or plot of more spectral lines than you could ever want.

    CAUTION: When exploring the interior of a monochromator, DO NOT touch the surface of the diffraction grating. Cleaning a grating without damage is difficult at best, and may be impossible for some types of gratings

    The following sections describe some typical monochromators.

    Intruments SA Model H2O 1200 Monochromator

    An example of a simple monochromator is the Instruments SA H20 1200VIS, designed to span the visible spectrum with either manual or motorized control. A slightly battle weary sample of this unit is shown in Instruments SA H20 1200VIS Monochromator. A diagram of its organization is shown in Basic Monochromator Opto-Mechanical Layout and the actual underside mechanism in Instruments SA H20 1200VIS Monochromator Lead Screw Lever System.

    The two mirrors allow the input and output beams to be co-linear but have no other effect. A variety of input and exit slits permit the resolution and sensitivity to be easily changed. All inner surfaces of the monochromator as well as some additional "Absorbers" are coated with a super flat black material to absorb as much stray light as possible. This includes both unavoidable scatter as well as the zeroth and higher order diffracted beams (not shown) from the grating.

    In the diagram, a hypothetical light source consisting of red and green lines is shown, perhaps from a very strange Ar/Kr ion laser. The green beam is diffracted less than the red beam and is thus not passed through the exit slit.

    Assuming the mechanical design of the monochromator is correct, only two adjustments are needed for calibration: The angle of the diffraction grating with respect to the lever, and the dial setting with respect to the lead screw.

    When I found the 1200VIS, both of these were far off. Simply adjusting the dial to coincide with a 632.8 nm red HeNe laser resulted in a green 532 nm DPSS laser pointer reading 529 nm. This indicated that the lead screw wasn't moving the lever enough over that range. To remedy this, I slightly loosened the set screw locking the lever to the diffraction grating shaft, but still tight enough so that normal dial twiddling wouldn't affect the relationship. Then, using the 632.8 nm and 532 nm lasers as references, the diffraction grating was rotated incrementally with respect to the lever until the difference between the readings was exactly 100.8 nm. It would have been even better to use more extreme wavelengths like a 457 nm DPSS or argon ion laser and 647 krypton ion laser, but these will have to do for now. Checking some other known wavelengths including 543.5, 594.1 and 611.9 nm (green, yellow, and orange HeNe lasers), as well as a 640 nm (errant laser line being produced by the red HeNe laser) showed them to be quite accurate.

    For determining laser wavelengths, a simple white card is all that is needed on the output. However, in its original application of detecting spectral signatures of plasma flames and such, a sensitive photodiode or PhotoMultiplier Tube (PMT) detector would be mounted beyond the exit slit.

    The overall performance (including wavelength precision and repeatability) using the dial, is now better than 0.5 nm, limited by very noticeable backlash in the multiturn dial mechanism. Originally, the 1200VIS also had a stepper motor (which I removed), PMT detector, and controller with data acquisition system (all whereabouts unknown). That was probably much more accurate but for my intended uses, this will be fine.

    Verity Instruments Model EP200 Monochromator/Detector

    These devices have been turning up on eBay lately in a variety of flavors, both manual and motorized. The latter is really only more useful if one of the mating controllers (which I haven't seen on eBay) is also acquired. I do not know if the motorized version has a wavelength readout and can be used manually as well. The unit I have includes a micrometer adjustable monochromator with a PhotoMultiplier Tube (PMT) detector, its high voltage power supply, and preamp, all built into a case about 7.5" x 7.2" x 2.6" inches. It runs on +/-15 VDC.

    As its name implies, the optical output of the EP200's monochromator is sent to a detector inside the unit which produces a voltage between 0 and +10 V proportional to light intensity at the selected wavelength. There is no exit port for light. The case is very well sealed against stray light - the light goes in but it never comes out. :)

    An annotated photo can be found in Verity EP200 Monochromator/Detector Organization. In addition to the parts being labeled, the beam paths for the a sample input, zeroth order (reflected input), and first order (useful) spectral lines are shown. The input here may be from the bore discharge of a HeNe laser tube. The wavelength micrometer is set for 587.6 nm in the photo thus selecting the intense yellow (helium) line which passes through the exit slit to the PMT. Of all the other lines shown (there are many more in the spectra of He and Ne), only the green one even makes it to the vicinity of the exit slit, but it's off to one side. Note that only the central ray for the incoming beam and each of its spectral components has been drawn on the photo. However, for a diffuse source like a glow discharge (as in this example), light bulb, or even an LED, the internal beam will expand to fill the large concave holographic diffraction grating (which provides high light gathering power). The bounce mirrors must also be larger than what might be expected due to the size of the internal beams. Only with a collimated laser, would the actual beam paths closely resemble the narrow ones shown.

    This model uses an actual machine shop type micrometer assembly to select wavelength. While not as convenient as a direct reading multidigit dial, rotation selectivity is better since there are only 25 nm per revolution compared to 100 nm for the typical dial. And, there is no backlash in the readout so the precision is better. However, when using narrow slits, the wobble in the micrometer becomes significant. The motorized version with its long shaft may be even better but it's useless without the matching controller because there is no readout.

    The sensitivity using the PMT is truly amazing. With the HV nearly as low (close to 0 V) as it can be, -200 VDC, and the gain turned nearly all the way down (10 percent), simply placing a small neon lamp power indicator near the entrance slit overloaded the preamp. Numerous lines in the neon lamp spectrum could be easily found. Since the PMT provides most of the amplification, the preamp really isn't that sensitive in the grand scheme of things. I measured 10 V out for 3.75 uA in at the 100 percent gain setting. Thus, replacing the PMT with a photodiode would only be useful for high intensity sources or low power lasers aimed directly into the entrance slit. But that would be such a waste of the EP200 since one of its main benefits is having the super high sensitivity detector built-in. In fact, when using the EP200 with coherent sources like lasers, there may be a small amount of ripple in the peak response versus wavelength and orientation of the instrument, presumably due to interference effects similar to visible speckle. This is not present with gas discharge sources. The EP200 is also polarization sensitive with a difference in response of about 2:1 for s and p polarized light. Thus, using the EP200's output to monitor the mode sweep of a random polarized HeNe laser may result in excessive amplitude fluctuations. But why would anyone want to do that with a monochromator?!

    Detailed information on this series of instruments can be found at Verity Instruments Monochromators. A description and photo of the interior is there as well as connector pinouts. One thing I did determine that isn't on the Web site is that there is a small slide switch on the PMT HV PCB inside the unit to select internal (adjustment pot) or external PMT high voltage control on pin 1 of the DB9 (+2 to +10 VDC for -200 to -1,000 VDC). The Web site simply mentions that internal or external HV control is selected at the time the order is placed. Some units (don't know if they would be newer or older than the ones I've seen) may only have a jumper. The lid can be removed without much risk of contamination as the box is not sealed. Just don't touch the diffraction grating as it cannot be cleaned.

    A sticker under the micrometer cover as well as another one inside the unit details the function of the 4 position DIP switch, which is to control the PMT preamp bandwidth as follows (0=Off):

         Position     Preamp
           4321      Bandwidth
           0000      530    Hz
           0001        5.3  Hz
           0010        0.53 Hz
           0100        0.24 Hz
           0110        0.17 Hz
           1000        0.11 Hz
           1010        0.09 Hz
           1100        0.08 Hz
           1110        0.07 Hz

    For manual control, only the first or second setting would probably be useful. Otherwise, the response speed is so slow that spectral features would be missed.

    The pinouts for the DB9 connector are available on the Verity Web site but here is a summary with the voltage polarities explicitly noted:

      Pin  Function
       1   HV Programming (optional, +2 to +10 V)
       2   -15 VDC (225 mA, polarity protected)
       3   +15 VDC (50 mA, polarity protected)
       4   Remote HV Monitor (-2 to -10 VDC)
       5   Signal Output (0 to +10 V)
       6   DC Offset (Zero voltage)
       7   Circuit Ground (Power and signal return)
       8   NC
       9   Circuit Ground (Power and signal return)

    I think the HV will actually go down (up?) to 0 V but probably isn't very useful much closer to 0 V than about -200 VDC. Note: Some documentation I've seen shows the HV Programming input being -2 to -10 V but all the units I have work fine with +2 to +10 V. I do not know for sure what the DC offset pin is used for. It is connected to the wiper of the ZERO pot and produces a small DC voltage (less tha 1 VDC) that varies with the pot. But, it may also be an input to allow the controller to set the DC offset remotely.

    The next test was to look at the spectral lines of the discharge in the bore of a 1 mW HeNe laser tube. Placing the bare tube next to the EP200 entrance slit and approximately level with it resulted in a large response. (HV of -300 VDC and gain of 50 percent.) However, this particular EP200 came with 500 um slits, too wide for my taste. :) While the strong lines could be seen, weaker ones adjacent to them were buried. :) So I modified the slits by using 5 minute Epoxy to glue a piece of a single edge razor blade to each, positioned to reduce the slit width to between 100 and 150 um - about as narrow as could be done by eye. (See below for more details on modifying the slit width.) This worked great in improving the resolution and allowed weak lines adjacent to strong ones separated by well under 1 nm to be resolved. However, since each slit was narrowed from one side only (that was enough of a pain in itself!), the wavelength calibration shifted by about 1 nm. (I must have guessed wrong since if they had been narrowed from the proper side relative to each-other, the wavelength calibration shouldn't have changed.) To remedy this, the micrometer mounting plate screws were loosened just enough to allow the micrometer to be nudged by about 1/1000th of an inch using the 585.25 nm and 587.56 nm yellow lines of the HeNe laser tube bore discharge as references, and confirmed with the 632.8 nm red lasing line.

    Those 585.25 nm and 587.56 nm lines are significant in that they are from neon and helium, respectively, and if reasonably similar in amplitude, the ratio of helium to neon is correct inside the tube. On the tube I tested, the intensity of the He line was about double the Ne line, indicating that the He:Ne ratio was high. That's better than the other way around. :) This thing makes such determination so easy. :)

    I've built a DC power supply and detector meter box (from junk parts of course!) to drive the EP200 heads conveniently. By default, its analog meter shows the detector output. However, by pressing a button, it will show the HV, which is adjusted via a 10-turn lockable knob if the EP200 is set up for external HV programming.

    I also constructed my own fiberoptic cable adapter (these are also available from Verity) which positions the tip of an SMA fiber connector at the entrance slit. The other end of the fiber would then have a focusing lens that can be positioned conveniently near the spectral source. Even though the amount of light coupled through the fiber into the monochronometer is generally quite small for anything but a laser, with the high sensitivity of the EP200, there is easily enough to take readings.

    A second EP200 I acquired had a bad photomultiplier tube but I replaced the original (Hamamatsu R928HA Hamamatsu R928 Datasheet) with an RCA 931A I had laying around. That seems to work OK, certainly well enough for my needs - it's way too sensitive! There are probably many other compatible PMTs. As long as the PMT has the same side-input, pinout, and fits the socket and housing, that's probably good enough for non-critical uses of the EP200.

    On a third EP200, the grating had fallen out of its mount. How the set screw loosened up will probably remain a mystery. Although there is a long scratch on the grating (possibly from the trauma, possibly from bouncing around during shipping, or possibly it was there even when new). Since the scratch just happens to be perpendicular to the rulings, it really doesn't cause any degradation in performance of any consequence. This unit worked fine after reinstalling the grating and calibration.

    On a forth EP200, the PMT had actually cracked - the main cover had a major ding in exactly the wrong place. Although no internal damage was visible, this must have whacked the PMT. It's otherwise in good condition awaiting a transplant.

    CAUTION: Do NOT attempt to measure the output of even a very low power laser by aiming it into the entrance slit. That will completely overload the system and may damage the photomultiplier tube. For a typical 1 mW laser, just arranging the beam to hit a white card positioned at 45 degrees near the entrance slit will provide more than enough signal with the PMT HV near the lower end of its useful range (say -250 to -300 VDC).

    CAUTION: Do NOT attempt to adjust calibration unless you have a source of a known unambiguous wavelength to use as a reference! It's way too easy to shift it too far to get back easily without one. A low power HeNe laser or green DPSS module shining on a white card would be suitable, but NOT a red pointer or other diode laser unless its peak wavelength is known exactly.

    If anyone has proper narrow slits or anything else related to these monochromators that they don't need, please contact me via the Sci.Electronics.Repair FAQ Email Links Page.

    Initial Testing and Adjustment of an EP200

    Checking one of these units for basic functionality is quite easy, requiring only a spectral test source like a neon lamp, HeNe laser tube, or other gas discharge lamp. Given the extremely high sensitivity of the EP200, using the light shining through the window from an outdoor high intensity sodium or mercury vapor street lamp may even be possible.

    Additional equipment that will be needed are regulated +15 VDC and -15 VDC power supplies (50 and 225 mA maximum, respectively), a 10K ohm pot and 5K ohm resistor to build a circuit for setting the high voltage (HV) if using external HV Programming, and a DC voltmeter capable of reading 10 V full scale. A flashlight may be useful as a quick test for confirming that the photomultiplier tube (PMT) detector is responding to light.

    Referring to the pinout for the DB9 interface connector, above, wire up the 15 VDC power supplies, Signal output, and HV programming (just in case). Circuit Ground is the common for everything including all voltage measurements. The COM test point is connected to Circuit Ground. The 5K resistor goes to +15 VDC and then to the top (clockwise end) of the pot, the wiper goes to the HV Programming input, and the bottom (counterclockwise end) goes to Circuit Ground. The value of the resistor and pot aren't critical as long as their ratio is 1:2 so that the HV Programming input is 0 to +10 VDC. Anything from 1K to 50K should be fine for the pot. For a permanent setup, a 10 turn pot may be desirable as the gain is quite sensitive to HV.

    There is no need to remove the large cover which encloses the optics and electronics of the EP200 except to flip the PMT HV selector switch if needed, or a major problem is found. For all tests, this cover should be in place with the screws tightly secured. Ample details on what's in there can be found on the Verity Web site or from the photo Verity EP200 Monochromator/Detector Organization.

    However, we all know that curiosity will get the better of you, so as long as it's open, check that nothing has fallen out, that the diffraction grating is secure in its clamped mount, it rotates freely against spring tension, and that the metal shroud surrounding the grating glass itself is pressed in as far as it will go. DO NOT touch the surface of the grating as it can't be cleaned without degrading its performance!!! DO NOT attempt to remove or adjust the grating in its mount - that affects focus and precise wavelength calibration (beyond what is discussed below). It's not supposed to be all the way in. If for some obscure reason it must be removed, use a depth gauge or other instrument to determine exactly how far in it should go and note which side is up (so that the blaze angle is correct when reinstalled). If there are any serious dents in the cover, confirm that there is no corresponding internal damage.

    CAUTION: On all the EP200s I've tested, simply removing and replacing the cover may alter wavelength calibration by a fraction of a nm. I assume that any slight change in stress on the baseplate deforms it enough to shift the wavelength peak. So, expect to have to do the basic wavelength calibration described below if you do go inside.

    Now that that's out of the way, block the entrance slit with a piece of black tape and double check your wiring before proceeding. It doesn't matter if the micrometer compartment cover plate (two thumbscrews) is installed for any of these tests.

    1. With your HV Programming pot fully counterclockwise (0 V), apply power and confirm that +15 VDC and -15 VDC are present at the proper DB9 connector pins.

    2. Connect your DC voltmeter or a multimeter between the HV monitor test point or pin 4 on the DB9 and Circuit Ground or COM.

      • If there is between -2 VDC and -10 VDC present, the internal HV pot is in control. Set it for -3.0 VDC corresponding to -300 VDC, which is close to the minimum useful HV.

      • If there is no voltage present, adjust your HV Programming pot to confirm that it is in control. If so, set it for about -3.0 VDC output on the HV test point (-300 VDC to the PMT). Note: According to some documentation I've seen, the HV Programming input should be negative. However, all the units I have work fine with positive HV Programming input. But if for some reason yours doesn't, flip the polarity and try again.

      If it is not possible to obtain any high voltage or the full -10 VDC (-1000 VDC to the PMT) on the HV test point with either the EP200 pot or external HV control, the HV power supply (a potted brick, an EMCO model 6858) or associated circuitry may be defective, or there may be a short in the PMT or its wiring.

      To change from internal to external HV programming or vice-versa, remove the main cover on the EP200 by taking out all the screws around its periphery and locate the HV select slide switch near the edge of the electronics PCB between the PMT housing and slit mounting plate. Flip it to the opposite position (toward the control/connector panel to select the EP200 HV pot).

      However, I highly recommend using external HV control as it is more flexible and convenient allowing for very quick and easy control of PMT gain, and won't wear out the internal pot! This is very likely the default for most of these units.

    3. Once the presence of HV has been confirmed and it is set for about -300 VDC, connect the Signal test point or Signal Output (pin 5 on the DB9) to your voltmeter. The reading could be anywhere from negative to off scale at this point.

    4. Set the 4 DIP switches to the OFF position to select maximum signal bandwidth.

    5. Press and hold the "GAIN" pushbutton and adjust the GAIN pot for approximately +5 VDC (on the Signal test point or pin 5 of the DB9) which corresponds to 50 percent gain. (The useful range is from 0.1 to 10.)

    6. With no light input, it should be possible to set the ZERO pot to produce near 0 VDC on the meter, though it may be quite touchy and like to go way negative. Keep it just positive if it won't cooperate by going to exactly 0 VDC. If the Signal Output remains pegged beyond +10 V and won't settle down in positive territory, try turning down the HV to 0 VDC. If the meter can now be zeroed but pegs to over +10 V with almost any HV, the PMT is probably defective. Else, it may be a circuit problem. (Some dark current which varies with HV may be normal but with -200 to -300 VDC, it should be negligible.)

    7. Set the wavelength micrometer for approximately 600 nm. (If the micrometer doesn't turn easily, check the black locking ring near its base, counterclockwise a fraction of a turn to unlock.) Each turn of the micrometer corresponds to 25 nm with each division being 1 nm. Multiply the direct reading by 100 to obtain the actual wavelength.

    8. Shine a flashlight (if available) head-on into the entrance slit. With a broadband source, there should be a response on the meter regardless of wavelength. However, some LED flashlights may not he broadband. So, if using an LED flashlight, rotate the micrometer through the visible range and look for a response. If this test is successful, the EP200 is likely fully functional but may need to be calibrated for wavelength.

    Before proceeding with wavelength checks or calibration, the EP200 main cover should be securely screwed down including the locking screw of the input slit since a slight shift in wavelength may occur when this is done. The EP200 should be placed on a solid surface so it can't wobble as any position or orientation change may result in a variation in the signal output and confusion as to where a peak is located.

    1. Set up your spectral test source in front of the entrance slit, or aim the monochromator to the source. the f/3.5 acceptance of the system is low enough that precise alignment isn't needed but arranging it to be fairly close to in-line with the slit will be best.

      CAUTION: DO NOT shine a laser beam directly into the entrance slit. Almost any laser - even a half dead laser pointer - is way too powerful to be used directly. However, it is safe to shine a low power laser on a white card placed near the entrance slit. More on this below.

    2. Refer to the appropriate spectral charts to determine what wavelength emission lines should be present with your source. For example, three relevant lines for the bore discharge of a HeNe laser are: 585.25 nm (neon yellow), 587.56 nm (helium yellow), and 640.2 nm (neon red). The normal HeNe red 632.8 nm is also present, but quite weak. There are dozens of other lines present but the three cited are particularly strong. If these can be located exactly where they should be, you're done. If they are there but not exactly where they should be, you're almost done. :) When turning the micrometer, avoid applying side-pressure as this will result in a wavelength shift and hysteresis.

      Note that with the typical 500 um slits, it is difficult to resolve the 585.25 nm and 587.56 nm lines as their spacing is less than the spec'd resolution of the EP200. But it's no problem with 200 um or narrower slits.

    Other lines that can easily be checked in a HeNe laser tube bore discharge include: 447.1 nm, 471.3 nm, 492.2 nm, 501.6 nm, 587.6 nm, and 667.8 nm from helium, and 540.1 nm, 585.2 nm, 588.2 nm, 603.0 nm, 607.4 nm, 616.4 nm, 621.7 nm, 626.6 nm, 633.4 nm, 638.3 nm, 640.2 nm, 650.6 nm, 659.9 nm, 692.9 nm, and 703.2 nm from neon. Note that the lasing wavelength of 632.8 nm is not among these medium to strong lines.

    If you're not using a HeNe laser tube but a source like a gas discharge spectral lamp containing some other gas(es), spectra for varioue elements and compounds can be easily found by searching the Web. The NIST Atomic Spectra Database has an applet which will generate a table or plot of more spectral lines than you could ever want.

    If the spectral lines located above are at the proper locations on the micrometer or within the uncertainly due to what minimal backlash there is, you're done. Otherwise, adjustment of the micrometer assembly will be required. This is best done with a HeNe laser or other source with a single spectral line. Otherwise, it may be difficult to know which line you're seeing. The use of such a laser is assumed below. Note that a red diode laser pointer is NOT suitable as its wavelength is not known precisely, nor is the line very narrow. However, a green DPSS laser pointer at 532 nm is perfectly fine.

    1. Set up a low power HeNe laser with its beam shining on a white card placed at a 45 degree angle next to the entrance slit, approximately centered and level with it. CAUTION: Do NOT shine the laser into the entrance slit as it will be way too powerful. This would grossly overload the system and may damage the PMT.

    2. Attempt to locate the 632.8 nm wavelength (assuming a red HeNe laser) by adjusting the micrometer. Unless someone has totally disassembled the EP200 at some point, it should appear somewhere within the adjustment range of the micrometer, probably quite close to where it should be. A HV setting of -300 V and gain setting of 5 V (50 percent) should result in adequate response for a 1 or 2 mW laser. You can try increasing the HV slightly if the response is very weak so that the reading reaches at least half scale.

    3. If the wavelength location is too low, the micrometer must be moved toward the entrance slit side of the unit. If too high, it needs to move away. The return spring on the grating tends to push it away but with the cover in place and pressing on the seals, some persuasion may be needed.

    The following three steps can probably be skipped if the wavelength is within a few nm of the correct location.

    1. Use a right-angle hex wrench to loosen both of the micrometer plate mounting screws inside the micrometer compartment just so they are barely touching the plate. The return spring will move the micrometer assembly away from the entrance slit side of the EP200 to its stop.

    2. Set the micrometer to 632.8 nm exactly.

    3. Use a narrow screwdriver or other suitable tool as a lever vertically between the micrometer mounting plate and EP200 main cover. Slowly move the micrometer assembly toward the entrance slit side of the unit (pull the tool toward the control/connector panel) while monitoring the Signal Output test point or connector pin. When the position coincides with 632.8 nm, the reading will jump. With care, it should be possible to lever the plate back and forth very slowly around this point stopping exactly at peak output so the mounting screws can be tightened. 1/1000th of an inch movement corresponds to 1 nm. With care, a precision of 1/10,000th of an inch is possible. This sounds more daunting than it really is.

    1. Check the peak setting. If the wavelength is only slightly off, it may be possible to loosen the micrometer mounting screws so they are just snug enough to hold the plate in position. Then, gently nudge the plate in the appropriate direction to center the peak at the 632.8 nm setting.

    2. If possible, check some of the lines in the HeNe tube discharge spectrum to confirm calibration. If fine adjustment is needed, it's better to use one of those because there are negligible interference effects to confuse the peak location, as there are with the coherent laser output, which may be somewhat touchy and dependent on input position and orientation.

    3. If another laser is available with a much different wavelength (like 532 nm), it would be worth checking that it too is now lined up correctly with the correct micrometer setting. Should this for some reason be off by more than a fraction of a nm, the relation of the grating shaft to its lever arm has changed. Adjustment of that is a more involved process reserved for the advanced course. :) But, no one should have touched and unless the unit was terribly abused, it shouldn't have changed on its own.

    This description probably makes the procedure sound like it will take all day. Wavelength calibration should require only a few minutes unless you are an absolute perfectionist, in which case it will take forever. :)

    Narrowing the Slits in an EP200

    For end-point detection in whatever processes these instruments normally monitor, the most common 500 um slit is perfectly adequate. But for looking at closely spaced spectral lines, a narrower slit is almost essential. Although the slits width isn't adjustable on the EP200, modifying the slits to be 100 to 200 um is relatively easy. The first procedure is reversible:

    1. Remove both slit assemblies (slit plate in plastic holder). The entrance slit is locked into position by the cover screw next to it. The slit assembly can then be pulled out. The exit slit is under the circular cover plate held in place by 4 small screws on the bottom of the EP200. Once the cover plate is removed, the slit assembly can be pulled out. The entrance and exit slit assemblies are identical.

    2. Gently confirm that the thin metal slit plate itself is secure in each plastic holder. On some units, I've found that the glue has weakened and almost any pressure results in the slit plate popping out. If loose, fallen out, or in doubt, use a tiny dab of 5 minute Epoxy to secure it. Make sure it is parallel to the edge with the shiny side should be down

    3. Prepare a sacrificial single edge razor blade. It doesn't have to be new but the edge should be free of any nicks or dings. Use a pair of pliers to break off pieces that will fit in the hole in the plastic behind the slit.

    4. Narrowing the slits from both sides is not essential but is desirable to avoid a wavelength shift depending on where an small diameter input source is located - above, below, or centered on the slit. This is because the blades on each side will be at the different distances from the diffraction grating. This may result in an error of a fraction of a nm depending on source location, though resolution will not be affected significantly. But, to prevent a fixed wavelength calibration shift if doing only one-side surgery, the entrance slit should be narrowed from the opposite side as the exit slit.

    5. Put a tiny dab of 5 minute Epoxy in the recess near the edge on the appropriate side and use tweezers to place the bit of razor blade in position. I just eyeballed the slit width but using something more sophisticated to measure it is permissible. :)

    6. Check the status of each slit as the adhesive cures to make sure it hasn't shifted position.

    7. Reinstall the slits and check calibration. Since the slits are much narrower than before, increasing the HV and/or gain may be necessary. If the calibration did change significantly, use the procedure in the previous section to correct it. With the narrower slits, calibration error will be more noticeable.

    Where it is known that going back to the wider slit will never be desired, then the following may result in better performance:

    1. Carefully pop the original metal slit insert off of the plastic holder.

    2. Use a sharp pair of scissors to split the slit into two parts without harming the slit edges.

    3. File each side to the two halves can be mounted at the appropriate closer distance.

    4. Use 5 minute Epoxy to reattach them to the plastic holder confirming the correct spacing and adjusting as needed before the adhesive sets.

    The focus is more critical with narrower slits. If the diffraction grating's position in its mounting clamp isn't exactly right, resolution will suffer. If it's never been touched, then don't touch it now as it's unlikely to have moved on its own, and wavelength calibration may be affected by position. But, if you've been fiddling with the grating, now is the time to adjust focus using a diverging beam (laser or LED) as the input. Since this has to be done with the cover removed, the source will need to be bright with gain set low enough so the ambient light doesn't overwhelm the system.

    Monolight Model 6100 Scanning Monochromator

    These consist of a "head" unit which is a compact monochromator where the diffraction grating rotates continuously on a motor shaft. With a suitable controller, an optical spectrum over a wide range (e.g., from 300 to 1,100 nm) can be acquired in about 85 ms. Thus one application is in a fast, though not particularly high resolution, optical spectrum analyzer. (In that may be the designation printed on the cover.) The resolution is about 1.4 nm for my particular unit.

    Some general info used to be found at Macam Photonics: Monolight Monochromators. But it may be that these are no longer current products. (And it may be that Gamma Scientific and/or EG&G, now part of Perkin Elmer, sold these systems under their name.)

    Simply applying 15 VAC to the power jack will make the motor spin, though the stability may not be that great. Although motor speed is regulated based on a voltage-to-frequency function from the 36 pulse/rotation optical encoder disc, it's likely intended to be more precisely phase locked to a crystal reference by the controller. The 6100 has a built-in trans-impedance (current to voltage) preamp for a photodiode. If displaying the spectrum of a laser, the photodiode can be almost anything as long as it's relatively small area (low capacitance). I used one from a barcode scanner for testing, just positioning it near the output slit. The 6100 provides a trigger signal that can sync an oscilloscope which can then be used to display the spectrum in leu of the controller and data acquisition system. Although a digital storage scope is desirable, my Tek 465B worked just fine in showing the 3 lines of my funny yellow-orange-orange PMS/REO LHYR-0100M HeNe laser head. The main problem was excessive sensitivity. My photodiode detector had no gain control, being, well, just a photodiode. Perhaps the intended detectors have an adjustment. I had to use neutral density filters to reduce the intensity to a level that didn't saturate the preamp.

    If anyone has more specific information including schematics for the 6105 head unit, please contact me via the Sci.Electronics.Repair FAQ Email Links Page. I had to add a pot to adjust the pullup resistance on the optical encoder on my sample as the signal was about half the amplitude it should have been leading to some peculiar behavior. Although it appears as though this pullup is a "select on final test" resistor, being in a two terminal header rather than being soldered into the PCB, I assume that somehow the output has gone down due to a weak LED or other problem in the opto-detector reading the disc pulses by reflection, similar to the reel rotation sensors in some VCRs. It is probably a standard 3 terminal device (LED and photodiode) which could be replaced but I can't read the part number without disassembling the unit.

  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Optical Wavelength Meters

    Principles of Operation While instruments like monochromators and optical spectrum analyzers are capable of determining the wavelength of light sources from light bulbs to lasers, their accuracy depends on the precision of multiple mechanical parts and the quality of the initial calibration. This is because they use what might be termed an indirect method of analysis - typically a diffraction grating moved by a precision mechanism. If there is any real-time reference, it is likely only at a single wavelength so there could be significant error at wavelengths not close to it.

    Where the source to be measured is broadband or has multiple spectral lines, such techniques are generally the easiest and fastest (but see below). However, where a single wavelength CW laser's output needs to be determined very precisely, alternative methods are generally used. (Wavelength meters capable of reading pulsed lasers also exist but that's for the advanced course!) Instruments of this type may have an accuracy and resolution that is orders of magnitude higher than a monochromator or optical spectrum analyzer.

    The following description is for one common approach and the one used in the Burleigh WA-20 a typical older model dating to the early 1980s.

    The basic wavelength meter or "wavemeter" compares the unknown input with a reference laser by counting fringes for both sources simultaneously in a Michelson interferometer where the path length difference is varied periodically by a motor-driven mechanism. The unknown wavelength (or frequency) is then related to the reference by the ratio of the number of fringes for each during a fixed period chosen to be near where the path length difference is small (to minimize the effects of the coherence length of the unknown laser). Where the desired display is in wavelength (e.g., nm), the reference wavelength is divided by N/No (where N is the number of fringes for the unknown and No is the number of fringes for the reference). Where the desired display is in frequency (cm-1), the reference frequency is multiplied by N/No. (Frequency here is the meaning used by spectroscopy-types: 1 cm/wavelength, or wave number.) The division and multiplication can be easily accomplished with digital counters and simple control logic, similar to that in any vanilla-flavored electronic counter/timer. Phase-Locked Loops (PLLs) will generally be used for both the reference and unknown detector signals to multiply the fringe counts and thus the resolution.

    A red HeNe laser is generally used for the reference laser. Even if not stabilized, its wavelength is known to better than 1 part in 106 and is 632.816 nm (at 1 atm in air). A stabilized HeNe laser locked to the gain curve can be a couple orders of magnitude better and an iodine line stabilized HeNe laser, even better. Another source of error is the change in the refractive index of air over the typical wavelength range of at least 400 to 1,000 nm. For this reason, for better accuracy, some wavelength meters put the interferometer optics in a vacuum chamber (less than 10 Torr). However, simply providing a lookup table for wavelength correction would be nearly as effective and much less of an implementation issue, though the actual pressure and temperature have to be taken into consideration.

    The performance of this fundamentally simple device is quite amazing. The resolution and accuracy of the Burleigh WA-20, which is one of the earliest commercial wavemeters, is better than 1 part in 106 (less than 0.001 nm or 1 picometer over the measurement range of 400 to 1,000 nm!). No routine calibration is required. While degradation in alignment is possible, the effect will be to increase the power level needed to take a reading but will not noticeably effect the resolution and accuracy. As long as the instrument is happy with the signal levels, the resulting display should be accurate. The most common problem may be a bad belt between the motor and interferometer drive!

    In fact, the modern replacements for the WA-20 is the Burleigh/EXFO WA-1500. (Go to EXFO, "Products and Solutions", "Spectral Test Equipment", "WA-1500 and WA-1000 Wavemeter".) The WA-1500 is virtually identical mechanically to the WA-20 but incorporates a frequency stabilized HeNe laser for the reference and keeps the interferometer at atmospheric pressure instead of in a vacuum. Software correction for the non-linearity of the index of refraction of air is then used with inputs from pressure and temperature sensors. With its stabilized reference laser, the accuracy of the WA-1500 is better than that of the WA-20 - +/-0.2 picometer compared to +/-1 pm for the WA-20. (The WA-1000 uses a non-stabilized HeNe laser like the WA-20 and has similar accuracy.)

    It should be noted that this implementation of a wavemeter is a subset of a more general technique called Fourier Transformer Spectroscopy which is capable of dealing with arbitrary spectra. (See, for example: World of Physics: Fourier Transform Spectrometer.) Rather than simply counting fringes, the Fourier transform is taken of the fringe waveform during one or more scans of the path length difference. For a single spectral peak as with a CW single frequency laser, the FT is a single peak. For a source with multiple peaks, the fringe pattern becomes visually complex, but the Fourier Transform will be the desired spectrum. This approach is also used in some wavemeters that can deal with multi-line laser input. For example, the WA-650 is an add-on that converts the WA-1500 or WA-1000 into an optical spectrum analyzer by Fourier Transform processing of the fringe pattern.

    In fact, it should be possible to process signals from the back of almost any wavemeter which has a built-in reference laser to use it as an optical spectrum analyzer. The interference signal for the unknown source, the interference signal for the reference laser, and a scan position sync pulse are required. This would be very simple if the scan was linear. But with wavemeters using a motor-driven scan like the WA-20 or WA-2100, the speed and thus fringe frequency isn't perfectly constant and this would totally mess up the FFT. It should be possible to correct it as long as a reference laser signal is available. The details are left as an exercise for the student. In fact, this would make a nice term project in DSP. :)

    But, the beauty of the basic single wavelength wavemeter is at least in part due to the simplicity in terms of its principles of operation, mechanical construction, and electronics.

    While a Scanning Fabry-Perot Interferometer (SFPI) may have better resolution, it typically doesn't have very good accuracy or stability with respect to absolute wavelength or frequency unless additional techniques are used, adding to complexity.

    While minor enhancements like the use of a voice coil magnetic drive instead of a motor can improve the speed and reduce the size of the Michelson interferometer-based wavemeter, higher performance instruments may use something called a Fitzeau interferometer with no moving parts. Multiple wedged etalons generate fringe patterns which depend on the source wavelength. These are captured via CCD arrays and analyzed in software. These instruments can deal with pulsed lasers and have much faster dipslay rates (100s of Hz or more compared to a few Hz for motor driven interferometers) and even more immune to alignment problems. Tuneup of a Burleigh WA-20 Wavemeter The Burleigh WA-20 is a typical older wavelength meter that uses a motor-driven moving interferometer mirror and fringe counting to determine wavelength (in um) or inverse frequency (in cm-1) of CW lasers between 0.4 and 1.0 um for the visible, which may be extended to 4.0 um with the IR option (which substitutes a different beamsplitter and detector). This model dates from the early 1980s, though the specific unit I'm working on has a manufacturing date of 1995. There was also a WA-10, with the only difference being that while the WA-20 maintains the entire interferometer inside a chamber that can be evacuated to below 10 Torr to for better accuracy, the WA-10 simply has a dust cover. The reason that a vacuum is beneficial is that there is a small non-linear depedence of the index of refraction of air on wavelength so a measurement of a laser with a wavelength far away from the 633 nm reference might see an error of as much as 3 parts in 106 in air.

    A red HeNe laser (polarized but not stabilized) is used as both the wavelength reference, and to provide a "tracer" beam to facilitate alignment of the unknown laser to the input of the WA-20. It passes through the same interferometer optics, but more-or-less in reverse. Thus, alignment of the reference laser beam is sufficient to guarantee alignment of the entire system.

    Neither the WA-10 or WA-20 are manufactured or supported now, but the modern replacements, the WA-1500 (with stabilized HeNe laser reference) and WA-1000 (without) are substantially similar in design, though they both operate without a vacuum, but have pressure and temperature sensors using software correction for the non-linear index of refraction of air. With its stabilized reference laser, the WA-1500 has somewhat better accuracy than the WA-20 while the WA-1000 is similar. They both have better sensitivity (20 uW instead of 100 uW). For specifications, Go to EXFO, "Products and Solutions", "Spectral Test Equipment", "WA-1500 and WA-1000 Wavemeter".

    The WA-20 I had on loan for testing and adjustment was so misligned when I received it that the tracer beam was partially cut off and less than 1/10th the intensity it should have been, and the "Ref Error" light was flashing due to low signal level. Yet, despite these problems, it was still able to measure the wavelength of an external red HeNe laser to the expected accuracy, though higher than spec'd power was required. That is, after the drive belt which had fallen off was put back in place. :)

    There is an alignment procedure in the user manual that is strightforeward, if somewhat tedious. It uses the reference laser entirely to align the mirrors and beamsplitter in relation to the input aperture. Once this is done, the unknown laser input is also automagically aligned since it uses the same optics. Once this procedure was complete, the system was able to read the red HeNe laser as well as a highly attenuated C315M green (532 nm) laser at power levels below the spec'd minimum of 100 uW.

    Additional items that still require attention are obtaining a replacement drive belt and replacing the O-ring in the motor vacuum seal feed-through since it's leaking at too high a rate. However, except for being run at 1 atm instead of a vacuum and accepting the slight reduction in accuracy, it's now in good shape.

    And, the Power indicator uses a strange 60 V, 20 mA incandescent lamp, and of course is likely to be burnt out on a well-used WA-20. I replaced it with a high brightness LED, soldered to the slide contacts of the original lamp along with a 1N4148 across it for reverse polarity protection. An additional 3.9K, 2 W resistor and 1N4007 diode were added in series with the original 3.9K resistor in series with the one already there. An alternative that wastes less power would be to tap off one of the 5 VDC or 12 VDC supplies but this makes it easy to restore the original arrangement if desired.

  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Ring Laser Gyros

    Basic Description and Operation

    (Includes contributions from Richard Graham of the Ring Laser Gyro Group at Canterbury University.)

    Mechanical gyroscopes measure rotation by measuring forces on a rotating mass which has been machined and spun at high precision. Having moving parts they are often clunky, bulky and distinctly 'low-tech'. They often take a long time to 'spin up' and stabilize.

    In principle the ring laser gyroscope can replace these with a fully optical system using counter-rotating laser beams, photodetectors, and digital electronics with no moving parts larger than photons and electrons.

    In practice, it isn't so easy.

    In its simplest form, the ring laser gyro (RLG) consists of a solid triangular block of glass with a hold drilled out parallel to each edge. Mirrors are added at each corner, a laser gain media such as Helium-Neon is added and a stable laser cavity is established. The gain media is chosen to allow two counter-rotating laser beams to be established - one clockwise (CW) and the other counter-clockwise (CCW).

    At some point two of the output beams are mixed together and (due to the slightly different optical frequencies) a beat frequency in the audio band will result and this can be measured to very high accuracy with a photosensor and appropriate electronics.

    The frequency measured is known as the Sagnac frequency. It is proportional to the rotational velocity, operating wavelength and ratio of cavity area to perimeter.

    A complete 3-axis inertial platform would require 3 RLGs mounted at 90 degrees to each-other. The entire affair can be fabricated inside a solid glass block!

    However, there are problems with this simplistic implementation.

    Fabricating a large monolithic slab of low thermal expansion glass is extremely difficult. As a result, very large cavities have been constructed in a discrete fashion, i.e., from individual pipes mounted on a concrete base. But it is difficult to maintain mechanical stability of the device.

    For the most part, these difficulties have been overcome to a degree sufficient to allow for navigation of aircraft, spacecraft and submarines. In such applications RLGs are increasingly being used in place of mechanical gyroscopes.

    Photos of an RLG laser assembly from a commercial inertial platform can be found at Flavio's Ring Laser Gyro Page. This is a triangular ring cavity dual discharge HeNe laser with part of the beam path being external to the block. One or more of the mirror mounts include coils, presumably to do the dithering. (Hopefully, there will eventually be descriptions there as well once we figure out what's going on.)

    There is some interesting information on RLGs at the Canterbury Ring Laser Projects Page.

    (From: A. E . Siegman (

    If you go to the Laser Gyros Directory, you'll find photographs of an early square He-Ne ring laser gyro built by Sperry and some early designs for Honeywell's monolithic ring laser gyros.

    The Sperry gyro couldn't actually be rotated in the lab - kind of hard to spin a one ton or thereabouts optical table. So they relied on the Earth's rotation, or at least the vector component of it perpendicular to the table at Sperry's latitude, to test their system.

    I recall a conference talk on their work in which the speaker noted that, given the backscattering and lock-up problems associated with a ring laser at this low rotation rate, their primary conclusion was that as best they could tell the Earth was still rotating, but at a highly uncertain rate.

    Home-Built Ring Laser Gyro?

    So you want to build one? Good luck! :-)

    (From: Douglas P. McNutt (

    The mechanical precision is the hard part and that's what makes it virtually impossible for an amateur to construct a ring laser gyro. The two opposite traveling waves have to have extremely high spectral purity which translates to high quality, high reflectance flats at the corners. Not a home job.

    It might be easier to build a fiber gyro in which the light passes many times around an effective ring through a wound fiber.

    (From: Christopher R. Carlen (

    The mechanical part is horrendous. We have an open cavity HeNe at my school's lab, and it is a challenge to keep lasing on a heavy damped breadboard with the mirrors mounted on a thick dovetail rail, bolted to the breadboard.

    Then you complicate that by going from a straight, two-mirror cavity to a three or four mirror cavity ring configuration, and then spin it real fast. Can you say "centrifugal force?"

    A fiber loop isn't quite the same as a ring laser, because the ring laser actually has the laser gain medium in the ring. As opposed to having the beam directed into a ring. The gain medium in the ring cavity ensures a standing wave is set up in the cavity, which would not be so for the fiber loop.

    Of interest for the future of laser gyros are the new photorefractive polymer devices that exhibit the property of two-beam coupling. This device allows coherent transfer of energy from one beam to another, when the beams are intersected in the material. This can be used to assemble a ring resonant cavity, pumped from the outside by a laser. This can be done with a small diode laser resulting in an assembly much smaller and easier to keep still while spinning than a gas laser ring cavity.

    Photorefractive oscillators using inorganic PR crystals have been studied for some time. The first announcement of a resonant cavity using a PR polymer has just occurred in the past few weeks (March, 1998).

    (From: Douglas Dwyer (

    If you are trying to make a laser gyro as a home project you've got a lifetime project.

    I think the ring laser is often carved out of a solid block for stability , a major problem with both ring lasers and fibre gyros is locking of the two phases - when rotated the phase relationship between the two paths sticks until a certain rotation rate is reached at which point the two paths unlock and it starts to work properly The solution to this could be to deliberately modulate the phase of the light with pseudo random noise and demodulate at the phase detector. Also as stated the fibre gyro is less attractive because of the inherent greater spectral width of the laser.

    I wonder if one could bake a Mossbau gyro. I once saw turntable rotation detected by the relativistic effects on the gamma radiation and absorption. That could be easier.

  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Fourier Optics

    Introduction to Fourier Optics

    The Fourier Transform (FT) of a signal - be it one dimensional such as audio or RF, or multidimensional such as an image (picture) - is a powerful tool for the analysis and processing of information. In a nutshell, the FT provides information on the frequency content of the signal. The signal and its FT form what are known as a 'transform pair'. The FT is a completely reversible operation so if the FT of the signal is completely known, the signal is also completely determined.

    Some applications for the Fourier transforms include:

    Refer to any book on signal processing for more details Fourier analysis and applications including all of the exciting equations!

    The usual modern way of performing the Fourier transformer operation is to digitize the data and use a special optimized computer algorithm called the 'Fast Fourier Transform' or FFT. However, even the most efficient variation of this approach is highly computationally intensive - especially when large multidimensional arrays like high resolution images are involved. To achieve adequate performance, digital signal processing accelerator cards, multiprocessors, or even supercomputers may be needed!

    Enter Fourier optics.

    It turns out that under certain conditions, a simple convex lens will perform the Fourier transform operation on a two dimensional (2D) image totally in *real time*. The theoretical implications of this statement are profound since real-time here means literally at the speed of light. In practice, it takes great effort and expense to make it work well. Many factors can degrade the contrast, resolution, and signal-to-noise ratio. Extremely high quality and expensive optics, precision positioning, and immaculate cleanliness are generally essential to produce a useful system. However, to demonstrate the basic principles of Fourier optics, all that is required is a common HeNe laser and some relatively simple low cost optics.

    Basic Setup for Simple Fourier Optics Experiments

    You don't even want to think about what a high quality Fourier optics setup for serious research would cost. However, for demonstrating the fundamental principles, it is possible to get away with much less. The necessary components are shown below:
       +-------+     Spatial Filter  Input          Fourier Transform        Output
       | Laser |===>()===---:---===()::():::><:::():::><:::():::><:::():::><:::()
       +-------+    FL     PH      CL  TR        TL        TP       ITL        OP
                     |<-f1->|<-f2->|   |<-- f -->|<-- f -->|<-- f -->|<-- f -->|
    A laser with a long coherence length is required. A diode laser will probably not work well. Therefore, this is likely to be a HeNe type. A medium power laser (i.e ., 10 mW) will make for a brighter display but a 1 mW should work just fine. CAUTION: Take appropriate precautions especially with a higher power laser. However, once the beam has been collimated to a large diameter, the hazards are reduced.

    Ideally, you have a nice optical bench to mount all these components. Otherwise, you will have to improvise. The first three items (the spatial filter components) really do need to be accurately and stably positioned. See the section: Laser Beam Cleanup - the Spatial Filter.

    Laboratory quality lenses for Fourier optics research cost thousands of dollars each. However, you can demonstrate the basic principles and do some very interesting experiments with inexpensive optics.

    Comments on Fourier Optics

    (From: EandorY (

    I just finished a class in this, using "Linear Systems, Fourier Transforms, and Optics", by Gaskill (Wiley).

    A coherent source yields a Fourier transform of the electric field, including the phase factors. An incoherent source will perform essentially the same effects on the radiance, rather than the field. A coherent source is used to develop the concepts, and so most of the books show the experimental verifications of spatial imaging with coherent sources.

    A negative lens will give a virtual image. If you want to perform spatial filtering, I think you're forced to use a positive lens. You also perform the inverse transform with another positive lens. You should therefore be able to confirm basic spatial filtering concepts with a hobbyists' telescope.

    Gaskill talks about a few special configurations, but the easiest to get to is to locate a laser to one side of the lens, place the transparency at the front focal plane, and find the Fourier transform plane at the point where the point source (a laser) comes to focus. To make things really simple, put the laser twice the focal distance away from the lens, the image at the focal distance, and find the FT at twice the focal distance on the far side of the lens. An alternative is to take a laser, collimate the light to obtain plane wave illumination, place the image anywhere between the source and the lens, and find the FT plane at the focal distance on the other side of the lens. It is the focal point of the light source that determines the position of the FT plane.

    Like I say, I just took the class, am still shell-shocked, and haven't had a chance to absorb or experiment with these techniques, so I could be misunderstanding the text. (From: Norman Axelrod ( Yes, you need a laser. HeNe works, but not a diode (the laser needs to have good coherence). Focus the laser through a pinhole (focusing lens and pinhole combination is called a spatial filter). then re-collimate the light with a lens. Place the image or aperture 1 focal length from the collimating lens, then you can either use a bare screen placed at distance away, or a second collimating lens. This is necessary to get the far-field pattern.

    (From: Brian Rich (

    A really cool book about this that I have a copy of but may be out of print is "Laser Art and Optical Transforms" by T. Kallard. Look for it at a good university library.

    (From: Norman Axelrod (

    There is another way to phrase what is happening that might make it more intuitive for folks with more of an optics background.

    First, the light used should be parallel and coherent.

    The light transmitted through the transparency (or light reflected from a 2-dimensional image) is diffracted by the transmission and phase changes provided by the image. As is done in elementary physics, a lens (here, a high quality lens) is used to take the light that is diffracted at different angles and focus them at a distance of one focal length from the lens (just like a burning lens, except you use parallel coherent light coming into the initial transparency and you have more than one beam at the burning distance).

    The key physical point is that the Fraunhofer diffraction pattern of an object is the Fourier transform of that object. This is true in the sense that the amplitude and the phase of the radiation at any point in the diffraction pattern are the amplitude and phase at the corresponding point in the Fourier transform.

    For simple examples:

    Arrays of identical apertures provide diffraction patterns that are the product of the intensity patterns from the individual apertures and the intensity patterns from the geometry of the array. If the array is random, you get the diffraction pattern of the individual apertures. Young (of Young's Double Slit) used this for one of the earlier measurements on the diameter of blood cells. One of the more amazing things (at least to me) that you can do with this is to take remove the horizontal OR vertical lines from an image of a wire screen with crossing vertical AND horizontal lines. By a simple modification of the light in the transform or diffraction pattern plane, you can produce an image that ONLY has either vertical or horizontal lines! The Fourier transform or diffraction pattern from a wire screen (like a screen from nylon stockings or from on a screen door - - but with tighter geometry) with periodic holes on a square grid consists of bright regions on a similar square grid. If you take an opaque screen and put a long narrow opening to allow ONLY the light from near the x-axis to get through, the resulting image has only vertical lines! This is called the Abbe-Porter experiment (and is discussed in Goodman's book). We have patents on this in which we used a simple opaque cross (in the transform plane) to eliminate perpendicular lines in an image and re-image only non-rectangular features. The perpendicular lines (lined up with the two axes of the cross) are effectively eliminated, but circular features and irregular features are imaged just fine! My favorite book on this remains Optical Physics by Lipson & Lipson (Cambridge Univ Press).

    (From: Tom Sutherland (

    Please allow me to recommend Professor Goodman's excellent and recently updated text "Fourier Optics". If I had my (last edition) copy in front of me I'd give you a better answer, however I do recall that the exact fourier transform of a pattern illuminated by a coherent plane wave is produced at the back focal plane of a lens if the pattern is located at the front focal plane of the lens. The intensity (but not the phase) of the fourier transform is produced if the pattern is located anywhere else in front of the lens (but of course there are some questions of scaling). (From: Robert Alcock ( Have a look at the book "Introduction to Fourier Optics" by J.W. Goodman. McGraw-Hill Book Company 1968. The first few chapters set the theoretical framework for the book by explaining 1D and 2D fourier transforms and scalar diffraction theory. I think that the chapters that you may find particularly interesting are:

    It's a fantastic book that should answer all your questions.

    (From: Herman de Jong (

    Let me explain the optical Fourier Transform by lenses with an example: Suppose for simplification we essentially look at a two dimensional system: we use cylinder lenses and slit object.

    When you use a broad laser beam and eliminate a slit (a pulse function), it will have a near field and a far-field pattern that is not exactly the same. The far-field pattern is a utopia but you get very close to the utopia the further away you put your screen. The intensity pattern is a squared sinc function (the sinc function is the FT of the pulse function) that scales with distance. We conclude the infinity pattern to be the squared of the FT of the slit and the associated E -field is actually the FT. If you use a cylindrical lens to image the slit on a screen you also get an FT provided you collect all relevant light from the slit onto your lens and the lens is perfect. It scales with the ratio of object an image distances It so happens that the FT of the FT the original but for a scaling factor and a minus sign in the inverse FT. I'm not sure how but in otical intensity FT's it makes no difference probably because of the squared of E -field that eliminates the minus sign.

    It gets much more difficult to grasp with 3D and rotationally symmetrical optics, objects and images. You wouldn't want to know and I wouldn't be able to answer many questions.

    (From: James A. Carter III (

    It is possible to form the Fourier transform by placing the transparency in a convergent-cone optical field formed by a single laser. This technique is used when one wishes to scale the transform to be optimally sampled by a detector with fixed spatial sampling. Changing the location of the transparency with respect to the focus of the cone (i.e ., changing the quadratic phase of the optical filed) will change the scale of the transforms as it maps spatial frequency (sometimes called the "plane wave spectrum") to spatial coordinates. Actually, no lens is required at all if you have a large enough lab and can invoke the "far field" condition. The "Fraunhofer" condition uses the quadratic phase of the lens to negate the second order term in the scalar diffraction integral using denoted as "Fresnel" diffraction. The far field condition puts the observation plane far enough away from the transparency plane to make it essentially a constant term in the integral and again you have a 2-D Fourier transform.

    The lens can be thought of as a way to image the far field (ideally at infinity) to the back focal plane. If the transparency is not at the front focal plane, then the transform field (amplitude and phase) at the focal plane will have a quadratic phase term. The quadratic phase is irrelevant if the field is detected (with detector or film) because then all phase information is lost. If the field is recorded with a reference phase (i.e ., a hologram), or is filtered for subsequently inversing the transform, then the quadratic phase should be corrected. The simplistic way to do this is to use a plane wave illumination (collimated source) and place the transparency at the front focal plane. Using your imagination and knowing the symmetry of the Fourier transform should justify this rational.

    The field at the transform plane contains only the information that is collected and sampled by the lens. Thus, the ability to sample higher spatial frequencies depends on the collection angle (numerical aperture) of the lens. Some feel that the illumination beam must be spatially filtered to produce a uniform distribution. This is no more the case than saying that every Fast Fourier Transform should just be zero padded. Hamming, Hanning and other windowing algorithms are used to suppress the side-lobes produced by the finite sample extent. The Gaussian distribution of the laser can actually improve the fidelity of the transform and eliminate "ringing." The quality of the lens in terms of wavefront aberrations is important, but no more important than the wavefront quality of the beam. These phase aberrations may effect the point spread function of the system (seen when no transparency is present) and it is the point spread function that convolves with the transform and limits fidelity.

    The text by Jack Gaskill and Joe Goodman are excellent for details. Another excellent source is the "(The New) Physical Optics Notebook: Tutorials in Fourier Optics" by Reynolds, DeVelis, Parrent, Thompson. This is available from Optical Engineering Press (SPIE). The "old" version of this was used in my training at the U. of Rochester when I took physical optics from one its early authors (Brian J. Thompson).

    Many interesting things can be done with this simple engine.

    (From: Jeff Hunt (

    I'm a grad student at the Optical Sciences Center at the University of Arizona, and I think that Jack Gaskill's book on the subject is quite good. Just like Gaskill says, it covers what Goodman's text does, but it explains things in a way that is easier to understand (Goodman is the authority on the subject, from what I understand.)

    (From: DeVon Griffin (

    Having done Gaskill ten years ago, I would say that the main drawback of the book is his notation. The m double-hat triple prime sort of thing makes trying to pick it back up after not having looked at it for awhile a daunting task.

  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Barcode (UPC) Scanners

    Introduction to Barcode Scanners

    The use of the Universal Product Code (UPC) has revolutionized grocery/supermarket and other retail store checkout and inventory control as well as being applied to other numerous and varied applications including package routing and tracking, and even tagging of wild animals and an aborted attempt to use similar codes printed in your weekly TV section to program your VCR with a hand-held barcode wand!

    Some would argue that the use of such technology in supermarkets at least, has dehumanized the buying experience and stacked the deck in favor of the merchant since prices tend to no longer be printed on each item and the checkout process is now so fast that it is virtually impossible to catch mistakes should they occur. Since the price-to-item relationship is stored in a computer somewhere, it is indeed possible for there to be errors - but in reality, these are generally rare.

    Space and other factors prevent me from going into the details of the Universal Product Code itself but here are some Web sites that have info and many links to barcode manufacturers, barcode specifications, barcode generating software, and other information that may be useful:

    The quick summary is that the pattern of black lines familiar on virtually all products nowadays - the UPC code - has been carefully designed to be easily decoded when scanned in either direction, at an arbitrary angle, and with variable speed. There are actually many other barcodes besides the UPC, used for inventory control, tracking, and other diverse applications. (If you should need to stay in a hospital, you will be given a barcode!)

    The UPC consists of 12 total digits. The first digit is the type of product (0 is for groceries, 3 is for drugs, etc.), the next 5 digits on the left half are the manufacturer code, the first 5 digits of the right half are the product code, and the last one is a modulo check digit. Each digit as its name implies can have a value from 0 to 9, encoded as a set of 4 alternating bars and spaces, each of which may have a width of 1, 2, 3, or 4 units called "modules". The total width of each digit is defined to be 7 which allows for 20 unique codes - 10 used for the left 6 digits the other 10 for the right 6 digits. The left six digits are coded with odd parity; the right six digits with even parity. Additional details can be found at the first Web site, above.

    Anatomy of a Barcode Scanner

    For the purposes of the discussion below, we restrict our attention to the type of equipment found at your local supermarket - the barcode scanner that is mounted under or beside the conveyer counter (and may include an electronic scale but that is another story). While details vary, the basic architecture of these devices tend to be very similar. Once you are familiar with one model, parts identification and the optical path of any other one will be almost immediately obvious. Hand-held scanners may not even use a laser but a linear array of LEDs. Large industrial barcode scanners may contain a much more powerful laser and somewhat different optical path. Some of the newest barcode technology does away with the laser scanner altogether and uses a 2-D video camera (CMOS or CCD) based imaging system and high speed DSP (Digital Signal Processor) instead. This eliminates most of the complex and costly optical and mechanical components making for a compact robust system. But currently, the traditional electro-mechanical laser scanner is still most common.

    The basic principle is to use a collimated laser beam, rotating multifaceted mirror, several stationary mirrors, and other optics, to generate a scan pattern above or beside the scanner which will intercept the UPC code printed on the item to be scanned in almost any orientation. While the scan may appear to consist of multiple lines or a continuous pattern, it is in reality a single rapidly moving spot.

    Looking through the glass of the scanner, it may appear that all sorts of stuff is arranged at random. However, this is not the case. :) Refer to Optical Path of Typical Checkout Barcode Scanner as you read the description below (which also includes some comments on potentially useful parts that may be obtained from these units):

    The outgoing beam is set up to be a small spot in the active area above or beside the scanner - the scanned item volume. However, the return from the UPC printed on the item is in general not well focused but is a diffuse reflection. Thus, as noted, all the mirrors have to be large to capture as much of this as possible to feed to the photodetector. The return path is the same as the outgoing path until the objective combo lens. This focuses the return beam onto the photodetector: See the document: Sam's Gadget FAQ for more on salvaging parts from barcode scanners.

    Apparent Brightness and Safety of Barcode Scanners

    There really aren't too many safety issues with respect to these devices even though they contain a Class IIIa (1 to 3 mW) laser and the beam may appear to be quite bright. (Note that barcode scanners systems are listed as Class II laser devices since access to the laser and optics requires some disassembly.)

    Metrologic Model MH290 Hand-Held Barcode Scanner

    This hand-held HeNe laser based barcode scanner apparently was the source of the power supply described in the section: HeNe Inverter Power Supply Using PWM Controller IC (IC-HI1). The entire HeNe laser (tube and power supply) is about 1"x1.5"x5" and weighs only about 3-1/2 ounces!

    (From: Art Allen, KY1K (

    The unit I have which uses a power supply 100 percent identical to the schematic and PCB layout of IC-HI1 is a Metrologic Model MH290. It is labeled with a 1990 date of manufacture and says 12 VDC at 550 mA on the scanner unit itself. The wall wart that runs the system is rated at 12 VDC at 1 A.

    The MH290 is a hand-held unit with a trigger, you pull the trigger when you are ready to scan and the laser starts scanning for 4 or 5 seconds and then shuts down. To attempt a second scan, you have to pull the trigger again. Inside the hand unit there is the receiver, a second PCB to support the receive electronics and the spinning mirrors (driven by a small 15 degree per step stepper motor). The MH290 is smart enough to know when the laser is on, and the error is produced if it doesn't come on OR if it stays on longer than it should.

    The MH290 connects to another unit via a 9 pin RS232 type connector, the other unit has the EEPROM and related components for decoding and interfacing to the computer itself. The MH290 hand held scanner does not connect directly to the computer and all power sent to the MH290 comes from this other box.

  • Back to Laser Instruments and Applications Sub-Table of Contents.

    CD, DVD, and Other Optical Disc/k Systems

    There are two consumer oriented applications for lasers that are by far dominant, at least in terms of the number of units produced. These would be laser printers and related equipment, and CD, DVD, and Blu-ray and HD DVD players and drives. Laser printers aren't really very interesting from a technology perspective - an IR laser diode with fancy focusing and scanning optics. (I suppose laser pointers are also something that should be included as being a common consumer laser but it's not clear how many of these are actually used for their intended purpose!)

    Optical storage, of which CD, DVD, MiniDisc, and LaserDisc are a subset, are all based on very similar technology requiring extreme precision to be able to read (and perhaps write) micro-size features on a spinning disc/k. The first optical drives were developed in the 1970s using HeNe lasers. LaserDisc players were the first consumer electronics to benefit. Early ones used HeNe lasers but even more modern LaserDisc players seemed to simply substitute an IR laser diode for the HeNe laser while retaining much of the other optics without significant miniaturization. However, LaserDisc players were never the same sort of mass produced product as the CD player and were more directed to high-end and specialized markets like interactive education and training since the disc format allowed rapid access to video snippets or up to 54,000 individual video frames on a LaserDisc. With the introduction of the personal computer around the same time, the LaserDisc was an ideal video storage peripheral, unsurpassed until the advent of the DVD. (And some people would claim still superior.)

    For more information on optical storage technology, see the Notes on the Troubleshooting of Compact Disc Players and CDROM Drives. In addition to descriptions of how the technology works, there are photos and diagrams of optical pickups ranging from one in an HeNe laser-based LaserDisc player prototype through modern DVD drives. The size difference is dramatic with the typical DVD pickup being roughly 1/1000th the volume of that LaserDisc pickup. Yet, it must perform all the same functions.

  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Laser Printers and Similar Equipment

    Introduction to Laser Printers

    All modern laser printers use IR diode lasers of 5 to 30 mW maximum output. Their wavelength is generally around 780 nm (like those of CD and many other optical disc/k systems).

    Very old laser printers used helium-neon lasers but these are even rarer than HeNe laser based LaserDisc players. However, if you do find one, there will likely also be an Acousto-Optic Modulator (AOM) and driver since directly controlling HeNe lasers at high speed isn't feasible - don't neglect these very desirable components!

    And, of course, those large graphic arts machines may have large HeNe lasers and even air-cooled argon ion lasers though newer ones will use Diode Pumped Solid State Frequency Doubled (DPSSFD) green lasers.

    Anatomy of the Optical System of a Laser Printer

    The optical path from laser to photosensitive drum is in the order listed below: The laser and optics components in laser Fax machines are similar but in addition, there will be the cold cathode fluorescent lamp, imaging lens, and CCD array of the input section. In principle, this could also be a laser scanner with virtually identical construction to that of the printer but I don't know if this is ever done in practice.

    See the document: Notes on the Troubleshooting and Repair of Printers and Photocopiers for information on how the image exposure and fixing portions of this equipment works as well as warnings and precautions with respect to the hazards of toner dust. See the document: Sam's Gadget FAQ for more on salvaging parts from deceased equipment.

  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Laser Light Shows Lasers

    Some Basic Info on Light Show Lasers

    For more information on lasers suitable for light show and related multimedia entertainment applications, see the Chapter: Argon/Krypton Ion Lasers. For more information on all aspects of laser light shows, check out the Web site.

    (Portions from: Erik Huber (

    I worked in a big disco as LJ - Did a lot of raves and such stuff. I also DJ a little just for fun. The laser power you need depends on the room you have. If you want to scan pictures you need more power. If you just use rays, you won't need so much.

    The prices for such lasers look like these:

    WARNING: Be aware that the maximum laser power level for the human eye is about (2.5 mW)/(cm2). Never look into the beam!

    (From: Steve Roberts (

    If you wish to scan graphics on clouds, it takes from 10 to 20 watts of well collimated argon light to do so, and the problem is only people within about a 10 degree cone around the laser site from where it hits the cloud will see the graphics. Everybody else at best sees a faint flash from within the cloud, and in most places in the US the conditions for doing it will only be right a few days a year. It's also not a good surface for images, any thing more then a simple logo or spirograph pattern is unlikely to be recognizable. Scrolling text didn't work. How do I know, I was the one running the spirograph generator as a guest at the laser site.

    Safety of Laser Shows

    (From: L. Michael Roberts (

    In the USA, laser shows in clubs/bars/parks are regulated by the CDRH (Center for Devices and Radiological Health, a division of the FDA). Audience scanning is NOT permitted in the USA while it is common in the rest of the world. A large scanned effect spreads the laser power over a wide area and usually has some motion to it (such as the sine waves used to make rippling sheets of light). This means that the energy density and the exposure times are low.

    If the laser beams are not scanning directly on the audience (dancers) then the effects are probably safe. If the system uses scanned beam effects, then it is probably following the rules of it's jurisdiction and is probably safe.

    Having done a few of those shows overseas, it's not just moving fast, but that's part of it. In fact, moving too fast can in some cases brighten the beam to exceed the MPE (Maximum Permissable Exposure) because of the dynamic characteristics of the scanners. It's the dwell time on each point of the image as the scanners are tracing it out, it has to be carefully measured for each animation or effect with a scope, fast photodiode, and a laser power meter. Each image has to be carefully designed using the show software to avoid sharp corners and other hotspots. Just scanning it fast is not enough - you will note that only very large scans flow over the audience. There is what is referred to as the zero line, well above the audiences head. As the images dip below the zero line, they are reduced in brightness by the hardware and by the show programmer. A scan fail system is also usually in use that will cut off the system should a scanner fail, and this has to happen fast if the MPE is not to be exceeded, in a fraction of a millisecond, so very careful engineering has to go in this.

    Please folks, just because you saw a beam scanned over the audience in a club and you have a laser, don't try it at home without getting the equipment to make the measurements and calculate the MPEs. It is not possible to determine if a effect is eye-safe by eyeball alone. The European clubs pay between $50,000 and $100,000 for these systems so a lot of time and money is spent on doing the safety analysis when programming a show. There are permits and licenses involved as well. Each frame of the show - and there is usually 6 to 15 frames per second - must be checked and carefully designed when doing this. The show must be checked for each facility it is ran in as well. You really need to take a class in how to do it safely. Such classes are offered at ILDA and ELA meetings and by safety inspectors/laser providers in Europe.

    Please note that this is not normally legal in the US as we have lower MPEs that make it ineffective when done anyways. It was suspended in much of Europe recently for a review of the power levels in use, new standards were implemented with tighter controls and it is again legal in parts of Europe. It is also legal in Canada, but again, measurements have to be made.

    If you're gonna do a show and you don't know what your doing, the basic guidelines for where the beam may go are a minimum of 3 meters up from the highest point in the audience and a two meter horizontal separation from the audience to any beam. In the USA, a CDRH Variance is required for any public show above 4.95 mW, and the penalties are draconian for failure to obtain them. The MPE in the US is about 2.3 mW per square centimeter per second for visible lasers.

    Single or Multiple Lasers for Color Presentation?

    For multiple color presentations, it's possible to use either a single laser that produces more than one wavelength, or two or more lasers combined into a single beam using dichroic mirrors. The only commercially available multiple wavelength solution is to use a 'white light' mixed-gas argon/krypton ion laser. The advantage is that it's possible to get a reasonable mix of colors and relative intensities, and the beams are already aligned to each-other. The disadvantages include large power requirements, high cost, and possibly relatively short life. The wavelength balance also changes with age, use, and output power. However, there is really is no viable alternative at present to obtain suitable red, green, and blue wavelengths from a single laser. Solid state RGB lasers are never built as single lasers, but either multiple lasers, or one or more lasers with some other wavelength conversion scheme added on. These are very complex and expensive but will no doubt improve and come down in price as demand increases.

    Where a non-gas laser solution is desired, what is often done is to combine green (532 nm) and blue (473 nm) DPSS lasers with a red (635 or 650 nm) diode laser, though red DPSS lasers (656.5, 660, or 671 nm) are now becoming available. However, this is 3 separate lasers whose beams need to be matched in size and divergence, and possibly polarization, and then combined into one.

    (From: Wes.)

    For our 1.4 to 1.5 Watt RGB system, we use the following:

    This combination makes a nice white balance. You could use 50% less red if you have 635 nm red. If you have 671 nm from a red DPSS laser, might as well forget it (would require way too much power since very very low visibility of that wavelength). FWIW, I think 650 nm makes better colors than 635 nm.

    (From: John R. (

    In my opinion, I would rather have a single mixed-gas 'white light' laser to avoid the hassles of beam collimation of two independent lasers. This is especially true if you do shows on the road where everything is jostled around. You may get better life with a red-only krypton tube, but you are almost always fiddling with near- and far-field collimation to keep your PCAOM output efficient across the entire spectrum.

    The color balance in a single mixed-gas laser will slowly change over time, but it is easy to make software color palette corrections on the white-light balance in a few minutes. (At least until the red output drops too much.)

    As for tube lifetime, I think it is function of art, science, tube current, luck, and the phase of the moon when the tube was installed. I know one laserist who only got 600 hours on a tube. I know another that has lasted for many years.

    About the Schneider High Power DPSS RGB Laser/Projector

    This laser has been reported on in various laser trade publications and discussed on the USENET newsgroup alt.lasers. Such systems represent the future direction of technology for RGB laser show and laser TV equipment due to their higher efficiency and more robust construction. Cost is still a problem though. :)

    (From: Patrick Murphy (

    The Schneider solid-state RGB laser does exist and is in use for laser shows, including the Hershey Park outdoor show in the U.S. There are two main versions of the laser. One is just the laser for light-show type applications. The other is the laser plus a video projection head (scanning mirror type) to create infinite-focus, wide color-gamut video. I saw both versions doing a combined show (video + laser graphics + laser beams), a few weeks ago at the Schneider factory in Germany.

    The following information relates just to the light-show model, imaginatively called "Showlaser" .

    The original $160,000 price mentioned elsewhere was an estimate; the actual U.S. price will be somewhat lower than this ($120K? $140K?). This is still a lot, but not quite as much as the estimate. Schneider realizes the price is high for the laser light show market and will be seeing if it is possible to lower it.

    The useful output power is 13 watts of modulated white-light from the end of a fiber (e .g., into your scanners). The colors are nicely spread -- red at 628 nm, green at 532 nm, blue at 446 nm -- so you get very dramatic violet and purple. (In video applications, there is no speckle, skin colors are normal, and saturated colors are quite striking.)

    The input power is 220 VAC at 3,000 W (e .g., about the same as two hair dryers). It has its own internal chiller, which you fill every few months with a gallon of distilled water. So in this sense it is "air-cooled", as you don't have to hook up an external chiller.

    Because everything -- laser head, modulators, chiller, power supply -- is built into one unit, the Showlaser weighs 660 pounds. This is roughly the same as all the parts of a medium- or large-frame ion laser together. The unit is compact and is on casters so the weight is not quite as bad as it could be.

    The working part of the laser is manufactured by Jenoptik (it says so it in a big decal on the Showlaser's side). The working principle is described in this paper: RGB Lasers for Laser Projection Displays. Here is the abstract:

    "JENOPTIK Laser, Optik, Systeme GmbH has developed the first industrial all-solid-state Red-Green-Blue laser system for large image projection systems. Compact in design (0.75 m 3 , 180 kg, 3 kW power consumption), the system consists of a modelocked oscillator amplifier subsystem with 7 ps pulse duration and 85 MHz pulse repetition frequency, an optical parametric oscillator (OPO), and several non-linear stages to generate radiation at 628 nm, 532 nm and 446 nm with an average output power above 18 W. Each of the three colors is modulated with the video signal in a contrast ratio of 1000:1 and coupled into a common low order multi mode fiber. The system architecture relies on efficiently manufacturable components. With the help of FEM analysis, new engineering design principles and subsequent climatic and mechanical tests, a length stability below 50 um and an angle stability below 10 uR have been achieved. The design includes efficient laser diodes with integrated thermo-electric cooler and a life time above 10,000 hours. The stability of the output power is better than +/-2% in a temperature range from 5°C to 40°C. The system operates reliably for more than 10,000 hours under field conditions. The design is based (among others) on work by Laser-Display-Technologie KG and the University of Kaiserslautern."

    The working part contains numerous optical components on a breadboard. Although it looks like a nightmare to align, everything is actually controlled by a computer. Once it is factory-set, in theory you never need concern yourself with what is inside. Schneider says the laser will last 10,000 hours before the diodes need replacing.

    "AVI-Imagineering With Lasers" is the U.S. distributor. They've received the one for Hershey Park, with more on order. So far, the Hershey Park laser has traveled well for AVI. It was trucked five times and four times there were no problems at all when the laser was turned on. The fifth time there was a power loss which may or may not have been due to traveling. (The cause is still being studied.) Since the solid-state laser is much newer than decades-old ion technology, I think people should expect a few "teething pains" to be worked out.

    Schneider also makes high-end TVs sold in Europe. I have been through the factory (same place as the laser division) and it is an amazing place, with raw materials such as plastics and electronic components coming in one end, and consumer boxed TVs coming out the other. Schneider also recently bought a majority interest in "tarm", the well-known German laser show company. So Schneider does things on a big scale, they know what they are doing in laser, and they want to do it at a consumer level.

    Obviously, it's pretty amazing for an RGB laser to get 13 watts of modulated light from a standard 220 VAC dryer-type outlet, with only occasional water top-offs, and a 10,000 hour claimed life. On the downside is the weight and the natural bugs that come with development of any new technology. The price is the biggest obstacle at this moment. With luck that may be coming down to a more affordable level, as volume, development, technology etc. improve.

    Inexpensive Combining of Argon Ion and HeNe Laser Beams

    Also see the section: Combining Light from Multiple Lasers.

    (From: John R (

    White-light color control with a red HeNe and multiline argon ion laser and be done without a PCAOM, but you may not like the answer. It is much cheaper than the PCAOM method, but still involves lots of work and moderate costs. Of course, if you are a laser hobbyist, nothing is cheap, especially if you want laser beams other than 632.8 nm red!

    For a minimum white light color control system:

    1. You need a multiline argon ion laser with at least the 488 (cyan-blue) and 514.5 (green) lines.

    2. You will need three separate dichroic filters. (Edmund Scientific and others sell these).

    3. One dichro is used to split the multiline Argon beam into a transmitted blue line and a reflected green line at 90 degrees. This gives you the isolated blue and green beams.

    4. Once you have the separate blue/greens, you need some method of intensity color. Three possibilities are single-channel AOMs, blanking scanners, or simple beam shutters.

    5. Once you get the blue/green beams through intensity controllers, they must be recombined using another dichro.

    6. Using a third dichro, the Argon beams and then super-imposed onto the red HeNe laser beam. (Of course, you should have some type of intensity controller for the red HeNe beam as well.)
    Thus, the final "white light" beam is made up resultant actions of three dichros and three intensity controllers. If you have some type of analog controller for each R/G/B color, you can blend them produce an incredible amount of colors.

    I once built one of these "RGB color boxes" using an argon and HeNe laser. It worked quite well, but there was the major hassle of alignment of multiple dichros, other mirrors, and three AOMS. A significant portion of the Argon power may be lost because it has to pass through three dichros.

    As for costs, if you can get surplus AOMs, dichros, and make your own mirror mounts, maybe $200 to $400 - if you're lucky!

    Unfortunately, there is no simple or cheap way of doing it.

    And, if you are thinking about mixing yellow and orange HeNe's with argons and red HeNe's, I seriously doubt you will achieve the performance (and ultimate cost) of even a used PCAOM.


    1. Both yellow and orange HeNe's only give a few milliwatts. They will easily be over-powered by the argon laser not only in terms of actual milliwatts, but in apparent visual brightness to your eyes.

    2. Unless you are just shining the independent laser beams onto the same spot on the wall, accurate near- and far-field collimation of a multiline argon with three yellow, orange, and red HeNe's is almost impossible.
    You will need some lots of custom dichros to combine the beams and numerous beam leveling mirrors to achieve it. Lots of dichros and lots of mirrors translates into "lots of losses" and a bitch to establish and maintain collimation. Three dichro color systems are still lots of work. In this case, you would have a FIVE-color dichro system.

    You may also run into problems as each independent laser has its one beam diameter, divergence, and spatial TEM characteristics. So if you could collimate them, the resultant "white light" beam will have lots of color fringes.

    Of course, it is your time, money, and effort, therefore, I wish you good success. But using a higher power red HeNe and then blending it with the multiline argon is still the better approach.

    For more information, try Laser FX. Their Website author also has an excellent handbook on lasers and laser shows. There are a couple of chapters devoted to RGB color control in lasers, including HeNe/Argon methods. If you are serious about making white light beams (and learning about lasers and shows), this is the book to have!

    Also, other ideas. Neos Technology has a 4-channel PCAOM crystal for $680 and driver for $600. If you are a hobbyist, this is not cheap. However, if you can get a PCAOM system, it is vastly superior to the RGB/dichro color method.

    (From: L. Michael Roberts (

    To combine the two lasers your best and lowest cost solution would be a dichotic. Firstly you need to have a set of two FS mirrors on optics mounts (E.g., Newport MMI or RMSM OM3/4) to level and steer the beam. Purchase a cyan or red dichro (from Edmund or PPS); mount it on another optics mount. With a cyan dichro, you shine the argon through the dichro (which transmits green/blue wavelengths). Set the dichro in the beam at 45 degrees at the point where the ar and HeNe beams are made to cross at a right angle.

    Careful adjustment of the steering mirror pair on each laser will allow you to produce two beams that are level relative to each other (and the baseplate of your projector) and cross at right angles. Set the dichro in the position where the beams cross at a 45 degree angle relative to the Ar beam (with the 45 degree angle such that the HeNe beam is reflected away from the Ar source).

    Adjust the beams until the HeNe and argon beams overlay each other on the dichro (near field adjustment). Now look at the resultant beam at some distance or on the projection surface. Adjust the dichro so that the two spots overlap (far field adjustment).

    Adjusting the dichro will cause some change in the position of the Ar and HeNe beams so you then re-adjust the near field (laser steering mirrors to overlap the beams on the dichro); then the far field (dichro to overlap beams on the screen). 2-4 adjustments going back and forth form near to far field may be required, but in the end you will have the two beams exactly overlaid on each other. To the eye, the beam will appear a pinkish white - colour balance can be adjusted by varying the brightness of the Ar laser.

    A cyan dichro is recommended as it reflects red and you want to conserve red photons. You will note that some of the argon beam is deflected in the direction the HeNe would have been going if not reflected. This is due to beam splitting at the surface of the dichro. If you use a red dichro, those would be red photons you would be throwing away.

    You can now place a PCAOM (from NEOS or MVM) in the combined beam. Make sure the polarization of the HeNe is vertical (check the ar while you are at it - they are usually polarized vertically but poor alignment could have you a bit off) and that the PCAOM cell is correctly oriented. Varying the control voltages to the PCAOM will allow you to have additive (RGB) colour control. You can get 16.7 million colours or more depending on the PCAOM and the system used to control it.

    Dichroic Mirrors for Separating Multiline Beams

    Dichroic (dielectric) mirrors can be used to split a multiline laser beam into two or more sets of separate lines. They enable the construction of simpler, smaller, and more efficient systems compared to dispersive techniques like prisms or gratings. But good quality dichros are not cheap.

    (From: Steve Roberts" (

    There are 3 quality sources of laser show dichros that I have used:

    For pricing, you're looking at $20 to $50 a square inch, depending on quality, and whether a precut size is available. Some may charge a cutting fee or a little more for the AR coated units. Keep in mind you need to know if you want CMY or RGB and 0 or 45 degree incidence, as most folks will stock the whole set of combinations. Be clear - specify that you want "transmit blue reflect green at normal incidence" Or "pass blue/green combine red at 45 degrees". Most people don't think about it, but "pass deep blue and violet" for a argon laser turns out to be a nice dichro to have.

    Prisms are generally only useful for separating one line, and for laser display purposes, you need all the power you can get, so you want all the blue or all the green lines, etc. They are also a pain in the neck as dispersion versus angle is constant, and a dichro can be tilted off axis quite a bit and still have throughput. Many traditional laser projectors for planetariums did just that, have a prism and a color selection galvo, but this takes up several feet of space to do and is difficult to support from a control systems point of view and to align. With a prism, you're wasting from 60 to 85% of your light at any one time, as you're only using one line.

    Also beware that Edmund Scientific's dichros are more or less coated for TV/spotlight applications and thus leak some blue or green that a laser show dichro wouldn't. This spoils the effect of clean contrasting colors, so you need a dichro designed for laser display. Edmund's dichros are great with a tungsten source however.

    When you order, ask for backside AR coats on your dichros if available. Otherwise you'd have 8 to 10% Losses from the Fresnel losses.

    Visibility of High Power Laser Beams

    The following applies to the visibility of the beam itself (i.e ., Star Wars Light Saber style), not to its appearance then it strikes a surface.

    (From: L. Michael Roberts (

    To create visible beams in *total* darkness you can get away with as little as 100 mW. For beam effects in a club or other venue with some ambient lighting, 1 watt is about the minimum you need to make visible beam effects. Outdoors you will need 5-6 watts to make visible beams (again depending n ambient lighting conditions).

    In all cases, a scattering medium (smoke or dust) is required to deflect the light towards the observer's eyes. In clean, clear air in winter, I have seen the beams from a 20 watt argon look lamer than the beams from a 1 watt indoors with a good haze.

    (From: Steve Roberts (

    In a dark room with average dust levels and high humidity you can start to see the forward scattering of an HeNe beam at about 1 mW! 30 to 40 mW of argon makes an OK side view beam in a dim room, but its not exactly a Star Trek photon torpedo kind of glow. It helps if the argon is configured multiline and is doing more green then blue, as the eye peaks in the green. To see the beam in a well lit room requires smoke of some form.

    Most laser light show types don't like the common aquafog, it irritates your lungs after constant exposure, so we use hazers indoors. A hazer works by making very tiny particles of medical grade oil. These are small enough to be flushed out of your lungs by normal breathing and if properly set up, are odorless and OSHA approved. Fog machines for the most part are crackers, they work by incomplete combustion of glycols (aquafog) or burning of oil in air. Hazers fragment the oil in CO2 and thus are almost odorless. Plans for a homemade hazer of sorts that uses air are at LaserFX on the "Backstage" pages. It has a slight odor but is not that bad to be around, and mind you I have asthma! I have done indoor shows for 1,200 people using 60 mW and a cracker. I have also done shows indoors for 100 people with a 5 mW hene, it depends on ambient lighting and air circulation/humidity.

    It is a minimum of about 5 watts of argon light for a decent outdoor smokeless beam show, with 20 watts being more typical.

    (From: Steve Quest (

    Visible wavelength lasers are more visible in 'plain air' if the angle of incidence is low (you're close to the same angle of the beam) and if the power is greater than about 5 watts. I perform an outdoor laser show using a 30 to 57 (max) watt YAG (frequency doubled to 532 nm) which is plainly visible in mostly clear air (no need to smoke, or fog the air). When I want to do beam effects with a 5 watt argon/krypton white-light laser, I have to fog the air up.

    Plain outdoor air has enough particulate matter to scatter a laser beam so long as it is above 25 or so watts, thus making the beam visible. Of course, the more power, the brighter the beam looks, but CDRH has limits, and that limit is .9725 mw/cm2 at 750 feet, so the days of power beam shows going all the way to outer space and beyond is over :-(.

    I use a Laserscope laser, which is FDA (Food and Drug Administration) approved, and am following CDRH (Center for Devices and Radiological Health) guidelines, receive FAA (Federal Aviation Administration) approval and air clearance before every show, and make sure that NOTAM (NOtice To AirMen) are issued to pilots flying in the area of my shows, giving exact details as to what is going on. Pilots love the shows, and air traffic routes planes WAY out of their flightpaths to fly near the beam shows to get the best seats in the house. :) However, I have to beam-off when they get too close, then they return to their flightpath, and I can resume the show.

    I used to be able to sparkle off the new moon with my YAG at full power and full convergence. It takes some doing but you can see the sparkle from the Sea of Tranquillity with the naked eye off the corner cube reflector, aka: retroreflector left there in 1969 by the astronauts.

    (From: Sam.)

    WARNING: Shooting a laser into the sky is irresponsible and highly illegal without prior approval from the proper agencies. Airline pilots do not appreciate being blinded!

    Here are some additional comments on the effects of viewing direction on apparent brightness:

    (From: Johannes Swartling (

    What you see is light that has been scattered by the small particles in the fog or smoke. This kind of scattering is called Mie scattering, and occurs when the size of the particles is comparable to or a little smaller than the wavelength of the light. In Mie theory, there is something called a scattering profile - i.e ., the probability that the light will scatter in a certain direction.

    Now, in the case of very small particles, such as molecules, this scattering profile is isotropic. That means that the light will scatter in all directions with equal probability. This special case is called Rayleigh scattering, and can be seen from pure air if you have a strong enough laser, such as an Ar-ion laser. When the particles get larger, however, the light will tend to scatter more and more in the forward direction. That is what you see from the smoke. When you look along the beam in the direction where it comes from, you see a lot of light that has been scattered just a little bit off the direction of the beam. When you look along the beam away from the laser, there's a lot less light that has been scattered backwards.

    (From: Pissavin (

    One interesting phenomenon; Depending on whether dust or smoke is used, there is an asymmetry: With smoke, if you put your head near the laser and look down the beam, you see almost nothing. Now, look toward the laser (BUT NOT DIRECTLY INTO THE BEAM!) and you see a clear beam. Then replace the smoke with dust and the effect will be reversed.

    (From: NeoLASE (

    Large particles like dust have more back scattering centers while small particles like smoke and haze have more forward scattering centers. Mie scattering effects, and all that stuff, I've heard/read of but I haven't studied in detail. Used a lot in laser particle size analysis.

    Limitations of Lasers for Large Scale Shows

    (From: Dean Glassburn (

    For the most power available, usually a krypton ion laser running red only and an argon ion laer for the blue and green is combined. The krypton red wavelength (647.1 nm) is not the best for color combination for true RGB mixing but it is about all that is available with adequate power. Remember, even if the argon were to produce 20 watts evenly split between green and blue, and 10 watts of red from the krypton, a total wattage of only 30 watts is available for the entire picture area. This really isn't that much for a large scale presentation and is why Vegas uses RGB light bulb boards as well and stadiums use Jumbotrons or Diamond visions, not lasers. The total light available is 1,000's of times brighter, and even with coarse resolution, the distance from the screen blends the image. Raster scanning with a laser is very inefficient, but with vector scanning and raster some unique effects can be created. Better yet use 10,000 watt lamps, one for each color via the proper filtering and use light valves to control the each device for each color. Like a projection TV except on a huge scale. And cost is always a factor.

    Use of Pulsed Laser for Laser Shows?

    (From: Steve Roberts (

    How well this works depends on the pulse rate and pulse width of your laser and how fast you are scanning, and how much you like dots and dashes in your image. It also depends on how you are shaping your image - i.e ,, some non-galvo imaging systems use pulsed YAGs for projection video.

    However if you are talking about an AO Q-switched YAG at a high rep rate, you can do, say, 10 to 12K galvo graphics. It just shimmers a lot and has faint spots that wander through the image. The real killer is that the divergence of pulsed YAG lasers of any significant power is extremely high and when the divergence magnitude starts to catch up with the resolution of the points in the image, you get a blob. When it catches up with the scan angle, you get a bigger blob. This happens at say a couple of hundred feet from the laser.

    I have witnessed this as a member of the crew on a show using a Q-switched YAG for beam effects. The company owner wanted to try scanning images on a building some distance away to see how his collimator worked. Up close it wasn't bad. But, more then a hundred feet or so from the laser, it was "The Green Blob".

    Holographic Laser Show Images?

    Being able to project a 3-D image hundreds of feet into open space is pure science fiction - there is no current technology and even basic theory that would make this possible without some medium to act as a screen. However, some pretty vivid illusions that may give the impression of such a display do exist and you may experience one at your next large scale laser show:

    (From: L. Michael Roberts (

    The most common way of creating this illusion is to use a scrim or a water screen. The scrim is a thin fabric screen, like mosquito netting, that is often dyed black or dark grey. It is rolled/lowered/flown into place while the audience is looking at something else, then used for laser graphics projections. Using typical modern 30K PCAOM projectors, flicker free images can be projected onto the scrim. While most of the laser beam goes through the scrim, enough of the laser is intercepted and reflected by the threads in the scrim to form an image.

    The water screen is a similar concept except that it uses a thin film of water droplets sprayed into the air as a projection surface. Both there techniques allow one to create the look of an image suspended in mid-air - especially if the audience is fixed in relation to the projection surface.

    There is a beam interference technique in the early stages of development but it isn't likely to ever result in a large scale display out in open air. It was pioneered by Dr. Elizabeth Downing. The image is generated inside a specially doped glass cube using scanned IR lasers. At present. the display s barely 2" on a side. For details see 3D Laser Based Volumetric Display.

    Laser Show on a Shoe String

    A low cost way of getting into laser shows is described at's Low Budget Laser Graphics System which includes information on suggested lasers, galvos, modifications to a sound card to pass DC, and the computer system and software. Much more info is of course available on the Web Site.

    (From: Gronk (

    I am fairly new to lasers (been studying and researching on internet for about 1.5 years now, especially Sam's Laser FAQ) and decided a few months ago to do my own laser show for our New Millennium eve party. We had about 30 or 35 people in attendance, and a musical show that lasted about 40 minutes. The equipment consisted of a home built Lissujous pattern generator (not the spinning motor kind) with laser modulation driving a GAL-2, a 1 watt stereo audio amp with raw audio from the show music driving a GAL-2, and 2 lumia wheels with 3 lasers shining through them.

    All this was projected on a silver screen (plastic tarp) suspended about 15 feet above the audience (no audience scanning done of course) . The lasers were all laser pointer types with the batteries removed and wires attached, and all connected to a home built laser power control station which controlled power to each individual laser. Fog beam effects were accomplished by spraying 'fog-in-a-can' at the beams. It turned out great, with all who attended enjoying it very much (granted, most of them had never seen a 'real' commercial laser show).

    It was a really fun project and will be done again at years end this year! I would encourage anyone who might be thinking of doing this to go for it! It was not really expensive, and was worth every penny for the all around experience. I also included my son (who was way better than me at operating the pattern generator) in the show, so he got a real kick out of it too. Highly recommended!

    (From: John Craker (

    I built a basic laser show from a dead (semi dead?) LaserDisc player. When hooked up to my home stereo, it displays lovely (and useless) Lissujous patterns on my ceiling.

    I basically robbed a section of the chassis that housed the HeNe laser and another section that had two deflection mirrors. Pointed the output of the laser into the mirrors. I hooked up the coil of each mirror to each channel of my stereo. With the difference in the stereo signal, you have each mirror oscillating at a slightly different rate, and since one mirror deflects in the 'Y' axis, and the other in the 'X', you get this great ever changing display. Size is pretty much adjusted via the volume. :)

    (From: Sam.)

    Based on a photo that John sent me as well as the sample in Optical Pickup from HeNe Laser-Based LaserDisc Player, the deflector from this LaserDisc player would appear to be virtually identical to what Meredith Instruments used to sell as GAL-2 (I don't think they have them anymore). I wonder if that's where they got them. In the LD player, the galvos were used for fine tracking and tangential (timebase) correction. I also have seen similar deflectors in other somewhat newer Laserdisc player optical pickups. However, if an IR rather than a HeNe laser was used (as would be the case with anything after about 1983), the mirrors will likely not be highly reflecting at visible wavelength! Along with a HeNe laser or laser pointer, and low power audio amp, you're in the instant light show business. Well, at least for those boring Lissujous patterns! :) The GAL-2 is sensitive enough to be driven by a personal stereo but the 4 ohm input impedance may overload its output if it is designed for 32 ohm headphones.

    Note that while the GAL-2 and the similar laserdisc deflector appear superficially similar to a pair of loudspeaker voice coil/magnet assemblies, the pole pieces of their magnets are on either side of each coil rather than within and surrounding them as in a true loudspeaker. Thus, the coil, and thus mirror, pivots from side-to-side as expected and desired rather than moving in and out.

    Building a Beam Table

    If you are conducting high-precision scientific experiments, or doing holography, you will need one of the BIG (4 x 8 foot (1.2 x 2.4 m approximately), vibration isolated optical tables like the ones available from Melles Griot, Newport, and others. You will also need a large wallet, not to mention a solid foundation and space to locate it!

    If you are a starving laserist who wants to make something for mounting lasers, a few optical components and scanners, you can semi-DIY for a reasonable cost.

    Go to your local machine ship and have them order a sheet of 3/8" (9.5 mm) T6016 aluminum large enough to mount your laser(s) with space left over. I would suggest 5' (1.52 m) by 18" (0.46 m) for medium frame lasers, longer if you want to build a beam table. Now have the machine shop drill and tap 1/4-20 holes on a 1" grid (M6-1.0 holes on 25 mm grid for metric).

    To save money, do not have the entire plate drilled and tapped. Leave the area where you intend to mount the laser(s) blank - you could have the shop put in the mounting holes for the laser(s) or you could do it yourself. The area where you intend to mount the electronics can be drilled and tapped on a 2" (50 mm) grid. You will need some mounting holes in that area, but unusable holes under transformers and PSUs just cost money. The area at the output of the laser(s) should be drilled and tapped with the full 1" (25 mm) grid as this is where you need the most flexibility for mounting optics.

    When the plate is done, have them chamfer the edges and send it out for black matt anodizing. 4 or 5 years ago, I could have one of these made up for around $250 CAD - YMMV

    One last tip: When choosing the mounting position of the laser, make sure the output beam will fall between two lines of holes, and parallel to the holes, in the grid to allow the most flexibility on mounting items on either side of the beam.

    Galvo Type Deflectors for Laser Light Shows

    May I suggest what I suggest to all beginners in Laser Shows?

    1. Buy a copy of L. Michael Roberts' (no relation) book on Laser F/X, its well worth the money and will save you much reinventing of the wheel.

    2. Save up and buy decent scanners, much fun can be had with the slower stuff, such as G330s, but in the long run you will find yourself needing to acquire faster scanners anyway and will be setting yourself back financially and time-wise with the slower units.

    Acceptable galvos for beginners:

    Don't bother with galvos like CECs - they are designed for exposing beams in small chart recorders using a ultraviolet arc source, they are referred to as "pen" galvos, and thats what they are, about the size and shape of a ink pin, with a small mirror about .5 mm across. They are thus too small to make a XY mirror pair, especially since the external magnet needed is huge.

    Dye Laser for Red through Yellow Wavelengths?

    Green and Blue are generally produced by either a multiline argon ion laser (though a DPSS laser is often replacing the power hungry ion laser for green at least). However, getting high power red requires either a krypton ion (or mixed gas) laser or very expensive DPSS laser. Even the largest HeNe laser (SP-125, multimode if one exists) won't break the 200 mW barrier and it's very difficult and costly to get decent beam quality from a red diode laser. Orange and yellow are at least as much of a problem. So, what about pumping a dye laser with an argon ion laser?:

    (From: Steve Roberts (

    As an example, a Coherent 930 medical system uses a modified I90 tube with a CR599 three mirror dye head. Threshold for the dye from the factory docs with fresh R6G, fresh optics, and a good tweak, is 1.5 watts all lines from the argon, lasing at a few milliwatts tuned at 640.2 nm. Note that the power is only about 40 mW at 2.5 watts pump, reaching a max of 3.2 watts with 9 watts pump. The specially selected MRA tube with extreme multimode optics reached 12 watts when new at 40 amps, but was designed to only sustain these powers for 30 seconds or so at a time. The opthalmologist or dermatologist who would use one of these needed about 0.7 watts of treatment power.

    2.5 watts pump is about 24 amps down the tube.

    If you moved the unit around without draining, the dye reservoir vents are set up in such a way that you would leak dye solution into the PSU. There is no drain on the unit, you'd have to suck it out. The dye solution is not just methanol, it has some nasty additives to quench triplet states that prevent the dye from lasing, the dye pressure is about 40 to 150 psi adjustable and it squirts across a air gap.

    I still have the R6G stains on the garage floor from scrapping a aurora dye a few years ago.

    Now if you have room for a second three-phase laser (in addition to your green/blue argon ion) laser at your rave and a large box truck with lift-gate, don't mind a 400 pound 6 foot long 18" wide console on wheels (build the beam table on top of it!) and like cleaning liquid cancer off your optics while ruining a change of clothes every time you open it up, then this is the laser for you. Splitting it into boxes would cost a lot as the linear PSU is spread out all over the thing. If you run it at 2 watts of tunable red through yellow it would be a hell of a show, especially if the stepper controller on the tuner was rewired. If the tuner is removed, it would lase broad-band by a few nanometers at the peak of the dye.

    By the way, the blue pump beam is nearly totally adsorbed if the thing is tuned to rock, and fluctuates and sputters like a lumia on the wall of the dye head.

  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Laser Based Systems for 2-D and 3-D Display

    Whatever Happened to Laser TV?

    I am sure everyone has heard of the predictions that there would be mural (or stadium) sized TV screens using lasers instead of the other silly technologies like LCDs and light valves. This was 10, 20 years ago. Where are they? The idea is simple: Replace the three electron guns in the color CRT with red, green, and blue lasers and raster scan a TV picture onto your favorite screen, barn, or mountain-side. :-)

    There are now companies marketing (or at least seriously demonstrating) laser based TV displays. The most recent versions use a single multi-color diode pumped solid state laser. One such unit has an optical output of about 13 W. To put this in perspective: The visible output of a 250 W incandescent bulb is about 13 W. So, that's a lot of light for a small screen but isn't going to compete in a theater setting. And, you don't want to ask about the cost! :) But, see the section: About the Schneider High Power DPSS RGB Laser/Projector for info on one such unit.

    Using laser diodes directly rather than solid state lasers has some fundamental problems. The first has to do with color. Untill recently, you could have any color of laser diode you want as long as it is red. :) While moderate power (perhaps up to 500 mW or 1 W) red laser diodes have been around for awhile, laser diodes with an actual blue wavelength (430 to 445 nm as opposed to deep violet - around 400 nm) are just becoming available as costly engineering samples with all sorts of strings attached and they have power outputs of only a few 10s of mW at most (see: Availability of Green, Blue, and Violet Laser Diodes). Of course, even 445 nm is more violet than blue, 460 would be better, but it's a start. Green laser diodes aren't even on the horizon in an commercial form and those tested in the lab have had very limited life and may have operated only at cryogenic temperatures. Unfortunately, even if high power RGB laser diodes could be purchased for $10, due to the fact that they would operate with multiple spatial modes along one axis, generating a tightly collimated beam suitable for direct scanning would be very complex and expensive, if not outright impossible. Better go to plan B. :)

    However, there is what might be described as a hybrid technology that still use lasers for the light sources but with a MEMS (Micro ElectroMechanical System) for the modulation. The Grating Light Valve (GLV) is a 1 dimensional array of MEMS-controlled diffraction gratings. See Silicon Light Machines Products and Technology. A typical system for TV or computer display would utilize 3 GLVs (one for each primary color). Each GLV would have enough channels for a vertical or horizontal line of the display and conventional (low speed) mechanical deflection such as a galvo would be used for the other axis. Such systems have been demonstrated and the GLV already has a track record in the printing industry where it is used to expose master printing plates a swath at a time using a high power IR laser diode line source. While there are no fundamental technical problems with this approach and it is certainly much simpler in some ways than direct scanned laser TV, there is still the not so minor issue of low cost high power lasers. But at least, multimode diodes can be used so when high power blue and green laser diodes are available, we'll be all set. :)

    (From: James A. Carter III (

    Just to let folks know where this Laser TV thing has been.

    In the 1920's, a company in England, Scophony Labs (I think that's right) patented a method for using Bragg diffraction on tanks of water (that's right H20) to display TV signals using white light (thermal) sources. They had to use BIG beams because they didn't have lasers. BIG beams mean low modulation rates due to acoustic transit time. Their idea was to scan the spot so that the acoustic pulse was stationary on the screen. I believe that they didn't use galvonometric scanners for the horizontal scan, instead they put mirrors on motor shafts (similar to what some cinemagraphic projectors used at the time). The scan rate and magnification were selected so that the scan velocity vector was equal and opposite to the image of the acoustic velocity vector. This may have been an idea way ahead of its time.

    Just ten years ago, I helped design the optics of a system that does display not only NTSC images but scan to HDTV as well. This is not a cheap system and is certainly is not suitable for avionics; although the Air Force (through TRW) did buy many systems. It used an air bearing motor to drive a many faceted polygonal mirror scanner for the horizontal scan and used a "galvo" scanner for the vertical. The AO modulators had enough band-width (at least 500 times what you get from PCAOMs) to project NTSC images in a flying spot mode. That is the scanner was going much to slow to give the Scophony condition. When we ramped the system (it was a closed loop continuous multiscan projector) to 1280 by 1024 sources, the scan was fast enough that we achieved the Scophony condition and realized over 35 MHz of video bandwidth per channel. This is somewhat inadequate for computer CAD graphics but was quite acceptable at the time. The display was dazzling, to say the least. Per laser color for each red, green and blue channel with red at a deep and rich 635 nm (dye laser pumped by the otherwise useless cyan lines), and the argon lines for green and blue. We used a 10 watt argon from Spectra-Physics to be the photon engine (SP was an investor here). One of these went to the NAB show and displayed our beloved President Ron.

    Unfortunately, the lasers were not reliable enough, to expensive to repair and replace, and more light is always better. Further, the big guys (TRW and SP) started to bicker and the company went under. The last time I saw one of these systems was at SP Corporate in San Jose. I was there to install a 25 watt laser, but that's another story.

    Current commercial work centers on dumping the high speed scanner and using an AO cell to modulate the whole line at one time. Bragg cell technology can give the Time-Bandwidth Product (TBP) required which is certainly over 1000 and closer to 2000. Unfortunately, acoustic attenuation (Beer's law in time and space) and the non-uniformity of the laser source (typically Gaussian) require losses to make a nice uniform display. Even with HIGH power pulsed lasers (repping at the horizontal line rate or at a multiple), the display can lack luster.

    As always, more photons... more photons...

    (From: Tony Clynick (

    I am pleased to tell you that laser video projection is still very active in the UK. Based on the original laser video projector (LVP) made by Dwight-Cavendish in the early 1980's, the projector now made by the team at LCI (Laser Creations International in London) has been installed at several permanent sites in theme parks since 1994, mostly in East Asia, and has been used for dozens of temporary shows world-wide since 1987. Most applications are in exhibitions, outdoor shows and theme parks.

    The LCI-LVP uses SP white-light lasers with special optics to provide good flesh-tones so the need for dye lasers is eliminated. A polygon scanner (GEC Marconi - thanks Alan) provides the line scanning, at rates of up to 36kHz. AO modulation and Scophony balance provides video bandwidth up to 30MHz, so HDTV (1250/50 and 1125/60), as well as PAL/NTSC/SECAM are available in the LCI-LVP. Output on screen of a peak-white modulated raster of over 15 watts has been achieved. The largest image projected so far was 50 metres wide. The collimated scanned beam provides an infinite depth-of-field, which was put to good use last year at the Singapore National Day on a giant 35m x 28m high-gain screen laid over the slanted stadium seating. The difference in projection distance between the top and bottom of the screen was nearly 100 metres, so the LVP was the only machine capable of a focussed image over the whole screen. All LVP's supplied so far by LCI are also capable of vector scanning using the waste AO beam.

    (From: Chris Cebelenski).

    I know of one experimental project that uses an array of galvo's to project a raster image at 1/2 normal NTSC refresh rate (15 fps). The cost of this endevour so far has been, well, let's just say it's been expensive. :-)

    Currently it's configured like this:

    There are several problems with this:

    1. Size. Stadium sized projections are fine, but it doesn't work too well in a dome. U2 would love it. (Search for: "Popmart tour" on altavista).

    2. Cost. Enough said!

    3. Power loss. Even the 5 W laser can get dim. With some mods it could work with multiple large-frame lasers, but then there's #2 again.

    4. Unlike most laser systems, it works best when projected against a BLACK background. White backgrounds have much better reflectivity, but the image really doesn't look right due to bleed and scatter.

    5. Max and min sizes - make it too large and it breaks up and the scanners can't keep up. Too small and the resolution of the scanners isn't good enough to provide a clear image and cross-talk is rampant.

    (From: Steve Roberts (

    Two years ago I was at a Laser-FX conference in Canada, we had the chance to watch (I have it on tape) a Russian made scan system with no moving parts, all acousto-optic and almost totally analog driven, that produced sharp clean monochrome images without flicker the size of a billboard using a 6 watt 532 nm YAG . The marketing person explained that RGB existed in the lab and was not far away. I believe the company name was Lasys Technologies. Scan head and laser was about the size of a PC/AT case and sat on a tripod, and was easily handled with low weight. Ran off 220 VAC three-phase, but I was told 220 single-phase would not be a problem. Further details can be obtained from: L. Michael Roberts ( who was the organizer of the conference.

    (From: L. Michael Roberts" (

    Some of the newer laser based video projectors (e.g., the Samsung unit) use a white light laser (Ar/Kr) as the source - 3.5 to 10 watts depending on the image size and brightness desired. The beam is split into it's prime component colours, modulated, recombined and then scanned.

    Many of the older units used a tandem laser pair - an Argon and a red-only krypton. Some units even use three lasers - an argon with blue optics, and argon with green optics and a red-only krypton. This takes a LOT of water and power to operate.

    There is presently a lot of work being done on producing compact diode pumped YAG based red and blue lasers. Laser Power showed prototypes of these lasers at the ILDA meeting in Amsterdam last November. This would allow people to build a fairly powerful (2 watts input approximately) laser based video projector that is air-cooled and can run on 115 VAC.

    (From: Sam.)

    Here is a link to an article about a system that may be commercially viable in the near future. It uses second and third harmonic generation to produce green (532 nm, 13 W) and blue (447 nm, 7 W) output, respectively, from a pair of Nd:YVO4 diode pumped solid state lasers along with a diode pumped optical parametric oscillator to generate the red (628 nm, 10 W) beam.

    The company claims their market advantage to include higher resolution (1,600 x 1,200) and better contrast ratio (1,700:1) than competing non-laser based technologies. They also cite lower maintenance than arc lamp based systems. However, the cost is also much higher at present and I question the brightness of 3,000 lumens at the screen (this is about equivalent to the total light output of a pair of 100 W incandescent bulbs) so it may still be inadequate for theater-size applications.

    And, here's a description with photos of a laser TV system built back in 1985 (along with some other related laser display gadgetry):

    And some comments from Doug:

    (From: Doug Dulmage.)

    One thing that is nice about TV using lasers is the use of a true red "gun". I've built 3 or 4 different versions of laser video projectors using argon and krypton lasers and the first thing you notice when you put a standard color bar signal up is that it looks "different". The reason is that in normal television there really is no such thing as a red phosphor. They are actually closer to orange than red, but by color mixing and a little fooling of the brain, you see red from the orange phosphor. So when you finally do see a video display that comes from a fairly dark red line (like the 650 of the krypton), things that normally look really bland like browns, violets, and other colors that depend on red, look stunning. It makes normal television look much more like film that video. Oddly enough, a couple of commercial laser video companies went to great lengths to produce the orange line instead of the red from a krypton by using argon pumped dye lasers to produce the orange. I could never, ever figure out why go to such trouble except that they were so anal about trying to follow NTSC standards for color that they ignored the benefit of having a true red. I had a little secret method for curing those situations where the client would complain about the color and I could give them orange back without the use of the dye laser, but normally once they saw real red, they wouldn't let you touch it. It makes sense, most color CCD camera (at least with three CCD's) use color dividing prisms that cutoff into the red more than orange.

    Laser Based 3-D Displays

    Displays capable of providing information about the three-dimensional aspects of a scene can be divided into two classes:

    There have been a number of volumetric (not true holographic) displays developed over the years using rotating mirrors, disks, LED arrays, disks inside cathode ray tubes, etc. These are all scanned in such a way as to cover a true volume of space at a rapid enough rate (at least that is the objective) to produce the illusion of a solid 3-D volume floating in space. The scanning source can be a laser, electron beam, or the projected output of another 2-D display like a CRT or LCD panel.

    Currently, there are technical issues to be resolved with respect to the bandwidth of the channel to get the information into the display (Gigabytes/second are required for adequate refresh rates). But more fundamentally, these techniques are incapable by their design of rendering solid shaded surface views. The volumetric display is one of 'look through' or 'structured fog'. However, such a technique in a practical application could be extremely useful.

    With technologies as yet unavailable, one could conceive of a 'selective activation' display where points in 3-space are rendered opaque or emissive by intersecting Laser beams or something like that. There has been progress in this area with emissive displays - intersecting laser beams resulting in the production of colored points of light. However, all these technologies suffer at present from serious resolution and bandwidth limitations - not likely to be solved for decades at least. (See below.)

    A true holographic display would be capable of an ***arbitrary*** viewing mode including the display of solid surfaces with shading which would be viewable with correct perspective and shading from a range of angles. I do not know of any actual examples of such technology at present. An emissive volumetric display like the one described below cannot implement hidden surface removal - essential for life-like rendition. While wire-frames and look-through displays have many uses, they aren't likely to be of much value for a boob-tube replacement! :)

    A brief description of some of the alternatives can be found at: Pangolin's Laser Show Guide - Making 3D, floating images. Additional details on one of these, the spinning helix approach, can be found at: Technical Description of a 3D Volumetric Display System.

    Also see the sections starting with: Introduction to Holography

    (From: L. Michael Roberts (

    Already in the works! A "Three-Colour, Solid-State, Three-Dimensional Display based on two-step, two-frequency upconversion in rare earth doped heavy metal fluoride glass is described. The device employs infrared laser beams that intersect inside a transparent volume of active optical material to address red, green, and blue voxels via sequential two-step resonant absorption. Three-dimensional wire-frame images, surface areas, and solids are drawn by scanning the point of intersection of the lasers around inside the material. The prototype device is driven with laser diodes, uses conventional focusing optics and mechanical scanners, and is bright enough to be seen in ambient room lighting conditions.

    The full article is available on-line at 3D Laser Based Volumetric Display.

    (From: Michiel Roos (

    That's a block of (expensive) glass with some lights in it? Last thing I heard, they'd only got a low resolution. But a couple of years ago I was at a Philips trade show. There was a true (?!?) 3D laserTV system. In a room, a music video was shown. There were a number of layers displayed in air (fog?) so you'd get a 3D view. Nice thing was that you could walk right through the image and still see it. But I've never heard of it again. Anybody knows if they're working on this now?

    3-D Laser Engraving Inside a Glass Block

    Examples of art pieces made under computer control of a pulsed laser focused inside a glass block can be found at 3D Laser Art Co.. They have a basic explanation of the process but no specifics and no mention of the type of laser that is used.

    (From: Steve Roberts (

    Engraving inside a block of glass is a pretty easy thing to do if you have a high power pulsed YAG laser. I've seen problems in labs with cheap glass lenses developing spectacular defects in the middle of the glass, so a variable focus lens, some galvanometer scanners for positioning, and a monster pulsed YAG - plus some decent software and you should be able to carve in flint or lead glass.

    It's all too easy to create microcracks on the insides of the cheap lenses.

    (From: David Toebaert (

    The December 1999 issue of 'Laser und Optoelektronik' has a beautiful picture on the cover of a piece of lead crystal with the Dresdner Frauenkirche inside, 3-D engraved using Nd:YLF (Q-switched AND mode locked) lasers. It was developed by the Fraunhofer Institut fur Werkstoff- und Strahltechnik.

    (From: A. E . Siegman (

    The basic process is a.k.a. "bulk (or internal) optical damage" produced by a focused laser beam. The basic effects were observed with the very earliest ruby and other pulsed lasers in the early 1960s, very often unintentionally and to the detriment of expensive optical components including sometimes the laser rods themselves. This led to a whole field of "laser damage" studies, including a series of NIST-sponsored symposia and other publications over the next several decades, and quite a lot of early work in the Soviet Union also..

    The physical process involves a complex mixture of photoionization, multiphoton ionization, melting, vaporization, and various stimulated scattering processes, leading to bubble formation, track formation, and "micro-explosions" occurring at either the focal spot or at various intrinsic defects inside the material. The exact details of what happens depend on the wavelength, intensity, and pulse duration of the laser pulse and the physical characteristics of the material.

    There are a number of small firms in the U.S. and elsewhere who will write the kind of decorative cubes you saw in the gift show, in glass or plastic cubes, using computer-controlled pulsed YAG or other lasers. They will also fabricate inexpensive customized versions as mementos for going-away parties, bowling trophies, and so forth.

    There is also a recent (late 1990s) patent by a British guy on a subsurface marking apparatus of this sort which has been used by a major distillery for writing subsurface serial numbers into the bottoms of zillions of Scotch whiskey bottles. I'll not provide a citation because IMHO given the prior art and state of knowledge of these effects the patent should never have been issued.

    (From: "Beric" (

    Its actually British Technology, but as usual developed overseas. The patent is owned by United Distillers. They are micro cracks, that are laser written into the glass.

  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Introduction to Holography

    What is Holography?

    Holography represents a class of techniques which capture 3-D information about a scene as an interference pattern on or in an extremely high resolution 2-D film. When the film is developed and viewed under the right conditions (some require a laser for viewing while others can use a suitable white light source), the result is a recreation in every detail of the original including the ability to move your viewpoint and look around objects, proper hidden surface removal (solid objects appear solid), shadows and highlights, and so forth. In principle, the hologram is optically indistinguishable from the original. A normal photo of a hologram would look the same as a photo of the scene itself.

    However, in so far as the technology exists today, holography is NOT what is often depicted in Sci-Fi and other movies and TV shows. Some of this deficiency is due to fundamental principles of what holography is and how it works while much of it is due to the inadequacy of present technology:

    (Portions from: Rick Poulin (

    While holography is really still in it's infancy it already has many other fascinating applications. Just a few of these include:

    Description of Holography Technique

    While there are significant differences in the details of the process needed to produce those little logos compared to large white light holograms used for marketing or 3-D volume images for medical diagnosis, the basic techniques are similar and can be summarized very briefly. The following is the sort of holography setup that is within the capabilities of a determined amateur:

    (From: Joshua Halpern (

    As far as a stable optical table goes:

    1. You could buy one on eBay. You need to find one near you otherwise the moving costs will kill you. Otherwise you need to hire riggers. That costs between $500 to $1,000

    2. There are two simple ways to build one. They start with the same base, blocks of stiff styrofoam. The simplest thing then is to put a block of granite or marble on top. Of course, you now have the problem of attaching anything to the marble, but the table will be vibration isolated

    3. Otherwise get a thick piece of magnetic steel and put that on top. you can then use magnetic bases. You buy the bases from a machine shop supply place - not an optical supply house which would charge 2 to 3 times the as much.

    4. For even better isolation you put a sandbox on top of the styrofoam, fill it with sand (there are grades, you want a grade with fine round kernels). Put a sheet of plastic down before you float the steel/stone on the sand and seal the plastic to the sides of the sandbox to avoid having sand everywhere.

    Note that all of these options involve moving something very heavy. It will cost you money, but unless you have experience in doing such things pay it. The money is a lot cheaper than the medical costs.

    Basic Amateur Holography Setup

    See the section: Holographic Information Resources for alternatives - this is just one option.

    (From: Brian Hogan (

    I haven't made holograms for a long time, but I started from the ground up. If you've got $3K to play with, you can really start off very well. But if you want to save money, you can build a complete setup for less than $1,000. It may be far more advanced than what you may have intended, but you'll be able to create pretty professional holograms.

    The best bet is to get a 5-10 mW HeNe surplus laser for about $200 to $300 dollars. This type of laser should have a coherence length of at least 6" or so. You'll also need some holographic film (I used to use Kodak stuff many years ago -- don't know if they still make it but it was relatively sensitive and easy to use). Next, you'll need to build a stable table. In a pinch, a heavy wooden plank, slab of marble, etc., laid on a few partially inflated inner tubes will probably be enough. I strongly recommend against a sandbox as it's more of a pain in the ass to keep things clean and to prevent optics from constantly shifting as you move things in the sand. Set the table up on the lowest floor, preferably on a concrete foundation, to minimize vibrations. Then you'll need to get some redirection mirrors and expanding lenses. Finally, you'll need the chemicals to develop the exposed film.

    From complete scratch, you are looking at an investment of about $350 to make a simple hologram.

    Here are more detailed suggestions:

    1. Ditch the sandbox idea. While it does work, it's a pain to keep sand from getting on all of the optics. Also, the light color of the sand means that you'll often have to mask out stray reflections. Lenses and mirrors have a tendency to shift when you move things around. I strongly recommend that you build a solid, rigid table and place it on inner tubes. For my setup, I made 6 columns out of cinder blocks about 3 ft high. I then put down on top of the columns a 2 inch thick pine plank measuring 4'x8'. I drilled six 4" holes in the plank spaced evenly out and then placed 6 forklift inner tubes centered around these holes. (The holes in the plank allowed for inflating the inner tubes later on from the bottom to adjust the air cushioning.) On top of the inner tubes I built a box out of wood measuring 4'x8'x4" inside dimensions. Into the box I poured about 11 cubic feet of Redimix concrete, using chicken wire and rebar for strengthening. The top was smoothed. After five days of curing, I glued a 1/16" thick sheet of steel (4'x8') to the top of the concrete. I painted the steel and the sides flat black. This was definitely a very heavy, solid table that was not really intended to be moved (except with dynamite!) Anyway, this might be more than what you'd like, but the table performed exceptionally well. The height was such that it made for comfortable working. The size meant I could do many intricate setups with multiple beams. The steel top meant I could use magnetic mounts for the optics. Total cost was less than $200 bucks in 1986.

    2. Get good optics. I got most of mine through Edmund Scientific. They're a bit expensive, though. All mirrors should be first surface (aluminum on the front surface, not on the back). I recommend getting several mirrors of about 2"x2". You'll also need to get two or three in the 4"x6" range and higher. You can never have too many mirrors. The lenses you'll need should mostly be concave. Look for the largest diameter, shortest NEGATIVE focal lengths you can find. These lenses will expand beams, which is generally what you'll be doing in holography. I would try to get an assortment of -6 to -20 mm double concave lenses at least 10 mm in diameter. If you don't use plate film, get some clear glass plates about 4"x6" to sandwich the film. I built a special jig that would clamp the film between the glass plates. Be creative, but try to make the clamp jig as small as possible -- you don't want it to interfere with any laser beams coming from behind the film to illuminate the object to be holographed. Also in the optics category, you'll need to get at least 1 variable beamsplitter mirror.

    3. Make or buy good optics mounts. You can go out and purchase optics mounts, but talk about EXPENSIVE. My table had a steel top, so I built magnetic mounts. The base of the mount was nothing more than a doghnut magnet (3 and 5" in diameter). I solidly epoxied 3/8" steel rods to these magnets vertically. Most were about 18" tall but some measured as much as 36" tall for overhead illumination shots. The optics themselves were glued to masonite pieces (with holes for the lenses). I used laboratory stand clamps to hold the optics in place. They clamp to the optic mount rods and can swivel the optics 360 degrees. Everything was painted flat black to reduce reflections. I built about 16 mounts in all. Like mirrors, you can never have too many.

    4. Get the most powerful laser you can afford. I did most of my holography with an 8 mW He-Ne laser that I purchased as surplus from Meredith Instruments. The more power means shorter exposure times and better results. You must get a TEM00 mode, single wavelength laser. I never tried a diode laser, but I don't recommend them because the beam is not round like a TEM00 laser. A good surplus HeNe laser will cost at least $300, but it's the most important part.

    5. Get the right film. Holography requires high resolution, special film for the purpose. I'm not sure Kodak is still in the holographic film business, but I had very good success with their film. I also used Agfa holographic film with pretty good results. Check around on the internet for sources. There are other types of media (e .g., dichromatic emulsions), but try films first. For processing, I used Kodak D-19 developer and Kodak fixers. I used a bleach mixture I made myself out of sulfuric acid (look in plumbing section of home center for drain cleaner -- very dangerous stuff!) and potassium dichromate. There are many other formulas out there so check around on websites. Processing must be done under clean conditions in a dark room. You can use a dim green safelight so that it won't exposure the red light sensitive film. (Also see below. --- Sam.)

    6. Though this is a long description, it should give you some ideas. There are many books out there that should give you much more information. The setup I described will cost somewhere around $1000. Once you've had some success with making basic holograms, you'll probably invest in specialty optics and other stuff to make more advanced holograms. With my setup, I was able to do practically anything anybody else could do with equipment costing many times as much as what my stuff cost. The key is to be creative not only with the actual holograms themselves but also with the equipment you use.
    Good luck and have fun.

    (From: Rick Poulin (

    I used to be a holographic experimenter and got my supplies from Agfa but sadly they got out of the business and left many people scrambling for a new cheap source. If you want to pay through the nose, Edmund Scientific or MWK Laser Prodcuts are the high water marks for pricing.

    If you want cheap film or glass plates there is a source in Russia called Red Star. Go to the Royal Holographic Art Gallery Film Page for the North American dealer in British Columbia, Canada.

    (From: Jens Kilian (

    The difficulty of making holograms is *much* overrated. If you're not going for commercial quality or for fancy stuff (image plane, rainbow etc.), a simple Denisyuk (reflection) hologram can be made with *very* little equipment (laser, lens, plate, chemicals).

    With the right plate exposure time is in the seconds, not hours range; and the vibration problem can be reduced with a robust setup like this:

          Laser =====> -------------\ Front-surface mirror
                                    | \
                                 ======= Plate
                                 | XXX | Object + support

    I've been to a workshop (see below) which was held in a public building next to one of the main thoroughfares in Stuttgart, where *everybody* produced near perfect holograms, even the guy, not me :-), who carried out a developed plate from the darkroom into near full sunlight.

    The workshop was run by: Junker Holografie. We used HRT plates. Clickety click... *darn*, HRT has shut down (HRT Holographic Recording Technologies GmbH).

    (From: Fleetie (

    Well, I ended up paying a lot of money for front-surface mirrors and an AR-coated beamsplitter, and such like when I briefly (!!!) took it up, but the plain fact is if you just want your first hologram, and you have the film and developing chemicals, you just need:

    Just put the lens right by the laser, get the beam nice and wide. Place the (ideally glassy or transparent or translucent) objects somewhere in the diverged beam. Put the film down-beam somewhere, so that the objects are between the laser and the film. You may find that the objects cast a shadow on the film; as long as a significant part of the film is not in shadow, it should be ok.

    Unless something moves really grossly, or you severely under- or over- expose, you'll get at least some kind of a transmission hologram out of it. (It won't be optimally efficient, but you really should see SOMETHING.)

    Even if something moves (but not TOO much), you'll often end up with a hologram that looks kind of stripy; the more movement, the more stripes.

    To view the hologram, just leave everything set up the way it was, remove the object(s), put the hologram back in the film holder in the SAME orientation in which it was exposed, let the laser illuminate it, look THROUGH the hologram towards the laser at the place where the objects were. You should see a holographic image of the objects. (I used to cut a little corner off the rectangle of film at the top right, to help get the film orientation the same when I wanted to view it. Then there are only 2 ways to orient the film, rather than 8. Just remember whether you had the emulsion side of the film facing towards or away from the laser. (Put a corner of the film between your lips; the emulsion side will feel sticky.))

    (This is all in my limited experience; standard disclaimers apply.)

    After that, you may want to try a 2-beam setup, with a reference beam shining directly onto the film, and another beam illuminating the object(s) but not the film. Then you can play with the relative brightnesses of the beams, and get better interference, and therefore a brighter hologram.

    It gets harder when you want to produce reflection holograms, which can be viewed in white light. You need more power, really, to get your exposure times down.

    Have fun anyway if you decide to go for it.

    Complete Holography Kits for Education

    Several companies provide all the equipment and materials needed to get started in holography. One example can be found at the Arbor Scientific Holography Page. Their prices may not be the best on individual pieces but the convenience of one-stop shopping may outweigh the additional cost (except probably for the laser especially if you opt to use a cheap laser pointer for this!). Also check the various companies listed in the section: New, Surplus, Walk-In, Mail Order, Kits/Plans (Commercial).

    The following is from a posting to the USENET newsgroup alt.lasers in early 1999. I have no direct knowledge of the contents or quality of these kits or whether they are still available.

    (From: Steve McGrew (

    I've just received and tested the first shipment of a new holography kit for education. It includes a HeNe laser, an optical breadboard, adjustable mounts, dielectric mirrors, and a detailed, understandable manual in good English (I helped with the translation). The manual details a series of experiments and explanations that will lead a student through all the basics of optics up through 3D holography. The kit and experiments are designed for a college-level optics course, but would be suitable as well for science enrichment at the high school level. The kits are made in China under the supervision of a university optics professor. Each kit fits neatly into an aluminum suitcase. If you were to buy all the parts for the kit in the U.S., they would cost somewhere in the range of $1,500.

    My cost is $525 plus shipping; I'll provide these kits to any bona fide school for my cost plus 10%, and will provide advice as needed to teachers and students. (Price subject to change, so please ask for confirmation of current price.)

    Holography Using Cheap Diode Lasers

    If you ask most laser 'experts' about the possibility of using a laser pointer or inexpensive diode laser module for making holograms, the typical response will be to forget it - the coherence length is only a few mm and therefore inadequate. This apparently isn't the case. The coherence length for a typical laser pointer or diode laser module may actually be more like 200 mm (10 inches) - comparable to that of an HeNe laser and, with care, will remain stable for long enough to make an exposure. While it may be unreasonable to expect any old $8.95 laser pointer to produce the same quality results as a $500 HeNe laser, surprisingly good holograms can be obtained on a budget. And, it would appear, that in some cases, they can actually be superior.

    While I don't know how to select a laser diode to guarantee an adequate coherence length, it certainly must be a single spatial (transverse) mode type which is usually the case for lower power diodes but those above 50 to 100 mW are generally multimode. So, forget about trying to using a 1 W laser diode of any wavelength for interferometry or holography. However, single spatial mode doesn't guarantee that the diode operates with a single longitudinal mode or has the needed stability for these applications. And, any particular diode may operate with the desired mode structure only over a range of current/output power and/or when maintained within a particular temperature range.

    For for information on laser pointer holography, see:

    Also see the section: Holographic Information Resources.

    (From: Frank DeFreitas (

    I had my fingers crossed tighter than ever for this one -- moving up to 35 mW of power for holography using a diode source. It worked!

    The module used contained the Hitachi 35 mW, 658 nm diode, along with AR-coated anamorphic prisms (optional) and high-grade collimating optics. The measured optical output after collimating optics is 27 mW and total cost for putting the whole thing together was about $50 to $60.

    This little baby exceeds the performance of any HeNe in its power range, including the $5,000 Spectra-Physics at 25 mW.

    Those diodes are real little buggers once they're set up with an interferometer. Very strange behavior (at least strange after working with gas lasers for so many years) - and in a good way.

    In any case, this baby is ROCK solid. The final test which put us over the top was so incredible that I thought there was something wrong with the set-up. I would tap on the table just to make sure. It's almost as if a fringe-locker was in place. Even with the best HeNe that I've had here (Spectra-Physics 124B Stabilite) there would ALWAYS be some "drift" or what I call "float". (Float is the feeling that fringes are not entirely still -- it's not something that shows up very clearly to the eye. It's more of a "feeling" when testing). The fringes with the new diode are locked so tight it's almost like watching a still photograph.

    As far as the coherence length is concerned, I measured (using a Science and Mechanics PhotoMeter placed in the fringes) out to 14 feet without any change. As you may know, this amount of coherence would require a rather expensive etalon on any lab laser. Up until this point, we were only capable of recording a few inches using diode lasers.

    This diode created two very bright test holograms that exhibited depth all the way back with the object(s) (1. ocean coral, 2. angel statue with wings). For a special twist, I used an initial set-up for a 30 x 40 cm hologram and then just shot two 4 x 5s with the set-up as-is. Even though the size of the holograms are 4 x 5, they will give you an indication of what a 30 x 40 cm hologram would turn out like -- since your beam spread, exposure, etc. are calibrated for that size.

    For a complete report, along with photos of the module, the holograms, the visible beam in my lab and a interesting size comparison to a Spectra-Physics 124B HeNe laser go to the Our Own 25 mW Laser Page. (There are also other reports preceeding this one which may be accessed at the Holoworld site.) D and S Lasers is a spinoff of Holoworld offering plans, a kit, as well as an assembled 25+ mW diode laser system with long coherence length suitable for holography.

    As for using green laser pointers, realize that these are based on an entirely different technology than laser diodes in red pointers. Green pointers are Diode Pumped Solid State (DPSS) frequency doubled lasers. To be useful for holography, a laser has to have a decent coherence length. For a short cavity laser like a that in a laser diode (a fraction of a mm) or green laser pointer (2 to 10 mm typical), this implies single longitudinal (and of course single transverse) mode operation. Some red diodes do this under some conditions (by controlling diode current and diode temperature). Depending on the specific configuration of the laser cavity in a green laser pointer, some may also operate single mode. Maybe. But, stabilizing them without major modifications may be difficult. The CASIX DPM crystals generally do not operate single mode but may do so at times depending on pump power and pump beam alignment. A discrete cavity pointer laser will likely operate single mode up to a modest power level and then switch to multimode. Many or most green pointers are now quasi-CW and/or Q-switched which further complicates matters.

    (From: Colin K. (

    Laser diodes do work. I would not say they work well. At least the APC style most amateur holographers use. There needs to be a method of locking the frequency to single mode. If you only need 5 mW then Integraf has a very reliable diode for $35 with a coherence length of more than 6 ft. I run one from two D-cell batteries and have made more than 30 holograms with it with no failures. As the red diodes increase in power it becomes increasingly hard to get the line to stabilize. I have a TEC based laser with the Panasonic 50 mW diode and I have had much difficulty keeping it in a single mode. When I can the coherence length is quite long. More than 12 feet.

    The 35 mW laser Frank sells from the Holoworld site (APC with Mitsubishi Diode) makes a good hologram most of the time but it will run in multiline mode at random times.

    The best laser I have found in red is the Analog Technologies TLM-S1 Tunable Laser Module but it's not cheap (don't ask!). There is also a less expensive non-tunable laser that will be available for about $800 very soon. I am hoping to test a sample with a 50 mW diode in a few days. The 25 mW has extremely long coherence lengths.

    (From: Tony (

    I thought that laser diodes would be unsuitable for holography due to their supposedly very short coherence length until 1999, when I read of holograms being made using laser pointers. I didn't believe it, but thought it wouldn't hurt to try. I bought a laser pointer (the bullet style with light feedback regulation), broke it open and fixed the diode and board to an adjustable mount, powering it from 3 AA cells. It worked first time, producing brighter holograms that were ever possible with my old 1 mW He-Ne. Having only a small table I've never been able to confirm the long coherence lengths quoted by some but I have found reflections from objects at the back of the table, giving a coherence length (taking into account the path difference there must have been) of at least 50 cm. I tried a few pointers and found only the cheap no-regulator types with only a resistor and diode don't work. One thing to remember is they do need to warm up just like a gas laser so don't expect to click the power on and off for an exposure - it's still best to use a shutter. Set up an interferometer to check the warmup time as well as you table's stability. The simplest way (assuming you've already built a vibration damping table) to make a transmission hologram with a diode laser is: Remove the collimating lens from the pointer, this produces a 'stripe' of light which can be used instead of a beam expander. Screen off the edges of the stripe next to the laser until only your objects and reflector are illuminated. With the laser at the left centre for example, you would place your object below centre of the right side and your reflector for the reference beam above centre on the right. Arrange your plate at the bottom of the table, the fun part being to keep it out of the direct beam while facing the reflected light from your object and being fully illuminated by the reference beam at the correct angle. You'll have to use some white card in the plate holder to try and balance the light from the object and reference beams. All this is much easier with more mirrors of course but for a zero-budget experiment it does work. You can make a partial reflector for the reference beam by painting a piece of 6 mm glass black on one side and roughly control the intensity by moving it nearer or further from the plate or film.

    Holographic Video Displays

    To create a useful holographic display of a moving scene requires an almost unbelievably large amount of data processing and throughput. Suppose you just wanted to produce a holomovie of a 50 x 50 x 50 cm volume using a 50 x 50 cm display device. Given that your typical holographic film must have a resolution on the order of a wavelength of the light used to create/reconstruct the hologram - 1,000 line pairs/mm or better - this would mean that some sort of spatial light modulator (e.g., LCD) would be needed with a similar resolution to reproduce moving images. That implies over 1.25x1012 or 1.25 Terapixels! And you thought high resolution laptop screens were expensive! To make things easier, we'll assume 1 bit per pixel for the interference pattern, resulting in 100 Gbytes per frame! To provide smooth motion, one needs a minimum of 24 to 30 fps so you are looking at 2.4 Terabytes/second. Now, granted, various compression techniques (e.g., MPEG-26 by then) can be used to reduce this by perhaps a factor of 10 to 100 or more (and no doubt such processing will be much more advanced once this sort of folly becomes at all practical) but that is still 24 Gbytes/second through the communications channel. Hmmm, that doesn't look quite as impossible! This doesn't take into account the need for color but at least the laser(s) will probably be the least of your problems in bringing such technology to market!

    Such a display is simple in principle:

    I was actually discussing stuff like this (in a former life) in the early 1980s realizing that either a dedicated special purpose computer or something as yet non-existent would be needed to achieve any sort of througput.

    That is still the case.

    However, for stationary images (e .g., medical visualization where one wants to view anatomy from various angles with proper perspective, etc.), the speed may not matter as much as long as writing doesn't take more than a few seconds.

    So let's see.... For a 10 cm x 10 cm SLM, resolution order of a wavelength of visible light, that's only about 50 billion pixels. Not your ordinary CRT electron gun - more like a scanning electron microscope. A few 10s of Giga bytes per second (for a 1 second refresh rate) is the same order of magnitude as the internal memory busses on some of the latest microprocessors, so no big deal. :) Of course, then multiply that annoying frame rate thing. ;-)

    A search of a patent database at using keywords like "Three Dimensional Display" and "Holographic" should turn up a variety of interesting, though probably for the most part unrealistic (as yet) approaches to this problem.

    (From: Steve Roberts (

    The problem is twofold, resolution and bandwidth. Resolution, because a hologram needs far more sensors per mm then available CCDs can provide, and bandwidth because only a dedicated direct array of fiber optic lines could handle the bandwidth. Your not going to see the scene shot with actual lasers. A computer and two or more cameras will be used, to synthesize the data. Experimental small scale displays have been made at low resolution, but the Cray computer they used to do the calculations is not something I'd have room for in my living room. Laser beams loose coherence after a short distance, so the guys at Monday Night Football aren't going to go blind, as lasers will not be used to gather the images. Maybe at the end of my lifetime in 30 years, but not any time soon.

    (From: James Hunter Heinlen (

    There have been a few made. Right now, the only applications that can afford such tech is very high end medical, and government, mostly military, but I believe the DoE has one in their nuclear power simulation program. At any rate, they are fiendishly expensive, and the one I saw (when I was still doing consulting in the explosives industry) used a couple of Cray YM/P-2E's (when they were new) as signal processors, plus other computers to do the modeling, run the simulation, and produce a real time data stream to use as a signal to be processed. It was considered the low end of the tech, and produced a dim (but beautifully detailed) 3D moving image of whatever you wanted in real time. They were using it to display the progression of a shock-wave through multiple layers of (non-ideal, realistic) rock in fine detail. We had to turn off the lights to see the display. The 'monitor' looked like a plexiglass fish tank. If you want more info, there was a couple of good articles about the displays in Government Computer News when they first started making this type of system.

    (From: Ted (email address N/A).)

    There have been a few attempts to display true interference pattern holograms created by lasers on very high resolution LCD displays. I was at a digital imaging conference and they had one there. The screen itself, I think, had about 50,000 x 50,000 pixels. The actual holograms were scanned by a drum scanner at 90K x 90K pixels each and displayed at 1:10 (or something like that) on the screen, which was about 17"! The hologram was very bright, more brilliant than most I've seen on film. The spokesman said each hologram file took well over 100 MB.

    Note our eye process signals at about 27 fps, so about 30 fps is needed. At 30 fps, a one-second holographic animation of such would be 3 GB! An hour would be 180 GB+. Clearly, even true hologram motion, is still a long way. Artificial interference holograms created by computers would require even more storage and processing power. But, at the rate things are going in the computer industry, it is highly feasible in 10-20 years this could become a reality.

    (From: Andre de Guerin (

    There is a new type of liquid crystal display that generates a hologram directly by producing the interference patterns on the surface of the LCD then illuminating it with visible light.

    The display this produces is a moving 3-D hologram in real time.

    One slight problem... The LCD density is something ridiculous like 3,800 x 3,800 pixels with a pixel size of 10 um x 10 um. There would be major problems with mass producing this sort of display, given that standard 1280 x 1024 laptop screens 1/10th the size have problems with dead pixels.

    (From: Sam.)

    Actually, there are bit more than one slight problem, not the least of which is that the resolution cited is at best marginal and feeding it with data must be a real treat, bandwidth and processing-wise! :) However, dead pixels, at least, would not be a major problem, just adding a bit to the background noise since localized defects in a hologram do not appear localized in the 3-D reconstruction.

    Suitable Lasers for Holography

    Here is a summary of the types of lasers with a long enough coherence length to be used to make holograms of macro-objects (more than a fraction of a millimeter in depth). See the relevant chapter on each type for more information.

    Helium-Neon (HeNe) lasers:

    Most polarized HeNe lasers can be used to make decent holograms if allowed to warm up for a half hour or more. A spatial filter will probably be needed (or at least highly desirable) to clean up the beam. The best will be large frame lasers like the SP-124 and SP-127, but internal mirror lasers in enclosed laser heads should be nearly as good with adequate warmup. HeNe lasers produce at most 40 or 50 mW at 632.8 nm (red-orange).

    Argon or krypton ion lasers:

    These have relatively short coherence length unless fitted with an etalon for single frequency operation. However, even a single wavelength without etalon argon or krypton ion laser may be acceptable for some holography work. The most useful (highest power) lines would be 488 nn (blue) and 514.5 nm (green).

    Diode lasers:

    I don't have specific recommendations but some should be excellent if temperature and current controller. Check on the holography Web sites or ask on the holography forums, below.

    Diode Pumped Solid State (DPSS) lasers:

    These must be single frequency to be useful for general holography. This is partially due to the typically short cavities of (non-ring) DPSS lasers, and the wide gain bandwidth of most SS lasing materials. DPSS lasers are mostly 532 nm (green). But there are some blue ones at 457 nm or 473 nm. Specific DPSS lasers I know to be excellent for holography are:

    I don't know of any new inexpensive DPSS lasers that are consistently acceptable for holography.

    Pulsed Solid State (PSS) lasers:

    Holograms can be made with some PSS lasers. While cavity lengths for PSS lasers are usually relatively long (at least compared to most DPSS lasers), the wide gain bandwidth of the SS lasing medium will mean many longitudinal modes are able to coexist unless specific means like an etalon are included inside the cavity. I don't know of any specific commercial PSS lasers to recommend, but with minor modifications, the widely available small rangefinder laser, SSY1, has been used successfully for holograms. See the section: Using SSY1 to Make Holograms.

    Holographic Information Resources

    See the chapter: Laser Information Resources, specifically:

    There is a weekly holography show on-line at Holotalk which has feature stories and special guests by hosted by The Internet Webseum of Holography. You may need special speech/video plugins for you browser to take advantage of this Web page.

    Monitoring the Wavelength Stability of a Laser Diode

    While some laser diodes are particularly good for use in holography and interferometry due to their natural tendency to operate in single spatial and longitudinal mode, many others can be convinced to behave by a combination of current and temperature tuning. However, some means is needed to check for mode hopping and multimode operation. This can be done with fancy and expensive instrumentation this is normally out of reach for even the well equipped holographer. There are low cost alternatives which provide some of the same information.

    (From: Jonathan Head (

    Here's my problem - laser diode frequency stability. It used to be the holographer's biggest issue was vibration stability. Now it's frequency stability, at least if you use a laser diode. And given the huge advantage of coherence length, robustness, and lower cost over HeNe lasers, who wouldn't?

    I'm building a heat sink for it. A TEC maybe later, right now I'm going low-tech. I believe I can keep the temperature quite low (close to 0 C) and stable enough to shoot between mode hopping episodes, with the design (by Colin Kaminski) I have. I have run numerous monitoring tests with my interferometer and audio detector (solar cell/amp/headphones) which, believe it or not, (at first I didn't) can actually (and cheaply) detect mode hopping via the amplitude shifts in the beam. They are audible clicks, which turn into static when the diode starts into multi-mode operation between mode hop free temperature zones. The beam quiets down when solidly in multi-mode, then the static returns followed by dwindling clicks, as it transits to the next temperature zone.

    You can therefore easily detect mode hopping *without* an interferometer at a cost of only about 8% of the total beam diverted to the solar cell using a glass plate beamsplitter. A single beam will do. I've placed the BS before the shutter and can use it to time my hologram exposures. But I digress. Although since I haven't found this in your FAQ I thought I'd mention it.

    For some time I've been running tests with an interferometer in conjunction with the audio set-up mentioned above. The correlation between the two (audio and visual) is interesting and useful. Primarily I'm testing methods to control the temperature of the LD, and monitor its mode hopping and linewidth behavior, without the benefit of expensive instruments. (I think holographers have enough expenses just from the film, plates, optics, and time away from family.)

    The first report I saw of the possibilities came from Tom Burgess, who posted on Frank DeFreitas' holography forum that he noted clicks, and rasps, in the beam that he thought might be mode hopping since they were accompanied by jerks in the pattern, and fringe washouts, respectively. This turns out to be the case.

    It helps to have an interferometer set up simultaneously to observe beam activity, but it isn't strictly necessary (and quite impossible if you are shooting a hologram).

    It's been previously shown that there is a correlation between noise in the total intensity of the beam, and mode hopping. The solar cell will pick this up as output intensity fluctuations directly caused by the laser switching between wavelengths. A "click" is heard for each discrete mode hop, and sometimes the mode hopping is quite rapid, which results in a "static" like sound of various tempos and sound levels.

    In addition to a small silicon solar cell, all you need is an amplifier equipped with phono jacks, a short RCA cable and headphones. A photodiode would work also. Using a plain piece of glass and perhaps a transfer mirror or two, divert part of the beam directly to the solar cell, which is connected directly to the amp inputs. Listen via headphones or you may get feedback interference from external speakers. I'd also recommend diverting the beam before the shutter, so that you can monitor the beam before an exposure. Once you've established that a background hum can be interrupted by blocking the solar cell with your hand, you can then attempt to "listen" to the beam for various manifestations of mode hopping activity.

    This is a practical means, especially for holographers on a budget, to determine suitable windows of opportunity in which to make their exposures. The wavelength stability of laser diodes depends on temperature and injection current, among other things, and unless these two factors are strictly controlled there will always be a chance for mode hopping to ruin an otherwise good hologram.

    The absence of audible indications (clicks and/or static sounds) will not, however, guarantee that the LD is operating in single mode, or at least with a narrow enough linewidth, to make a good hologram. This is because there are also times during multimode operation when no mode hopping occurs, and/or the intensity fluctuations are out of range to pick up. I've found this occurs as the diode moves through a zone of instability, of which there are many, determined by the particular combinations of case temperature and current. The audible indications occur as the LD enters and exits an unstable zone. In the middle of the unstable zone, it is often quiet (even while fringes are completely washed out). Therefore, one can determine where the laser is operating fairly easily, by simply monitoring the situation.

    This should save a significant amount of wasted film for those holographers using a bare-bones laser diode. For anyone using a TEC this is a way to find the zones of stability, and establish favorable set points.

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    Laser Communications

    Basic Description

    The term 'laser communication' can mean many things but generally refers to the transmission of information via a laser beam in free-space or a fiber-optic cable. A laser communications system must then consist of:

    Amateur Laser Communications

    For more information and discussions on amateur laser communications, join the Laser Reflector. It is run by ham radio operators who do long distance free-space communications. One is working on laser EME (Earth-Moon-Earth), and another is into non-line-of-site weak signal operation using low baud rate long term integration and advanced DSP techniques with coherent signals!

    The Laser Reflector Web site provides archives of past discussions indexed by date (year and month) and a large set of links to other laser and laser communications sites.

    Offers of inexpensive lasers, laser components, and other related items also appear from time-to-time via this email discussion group.

    Anyone with an interest in laser communications is welcome to join. You don't need to be a ham radio operator. Just send email to with 'subscribe laser' (without quotes) in the message body.

    See the section: Laser (Email) Listservers for more information about these private email discussion groups.

    See the section: Amateur Laser Communications Sites for additional Web sites related to this endeavor.

    Early Laser Communications Experiment

    Not surprising, the potential of optical communications was recognized by researchers even long before the laser was invented. The following is just an example of how easy it is to turn a laser that can be modulated and solar cell into a line-of-site comm link. This was just an ad-hoc experiment but

    Bell Labs may have actually developed and produced some number of portable demonstrators to promote the idea of optical communications. The typical unit appears to have consisted of a HeNe laser tube, power supply, and modulator, along with a separate receiver based on a solar cell, all packed in a handy traveling salesman's type sample case. :) I say "may have" and "appears" because I can't quite tell from the limited information and photos I have if it actually had a working laser or just a cool-looking neon sign-type tube for show - and actually did the communications with a separate conventional modulated lamp (an arc lamp is mentioned in the description I have and its presence doesn't make much sense otherwise). In any case, laser or not, this unit was used in community relations and school programs to show how telephone signals could travel over an optical beam. Some photos of one of these units rescued from the dumpster can be found in the Laser Equipment Gallery (Version 1.76 or higher) under "Assorted Helium-Neon Lasers" (giving it the benefit of the doubt in actually containing a laser!).

    (From: George Werner (

    Back in the middle 60's our group at Oak Ridge National Laboratory had built a HeNe laser for the purpose of demonstrating to interested groups. One time when I had brought it home in preparation to taking it "on the road" I decided to test its long distance transmission. For distant transmission we used a beam expander which was half of an 8x binocular with a 30 mm objective. We also had built into our power supply a jack into which we could plug in an audio modulation. I set up the laser on the kitchen table near a window with a little pocket radio supplying a signal to the modulator from the local radio station. With a mirror I directed the beam out the window and across the valley to the parking lot I could see where the city maintenance department has a number of vehicles parked.

    It was about a mile away. Looking with another telescope I could see that my beam was getting there when it retro-reflected from a car's tail light.

    Then, taking with me a Fresnel lens and an audio amplifier attached to a solar cell, I drove over there to see what it looked like up close. This was at about 5:30 in the afternoon, still bright daylight, so the red spot was not obvious, but I soon found it. About that time the night watchman, as he should, came to see what it was about. I explained that I was checking on this light that I was beaming down from halfway up the hill across the Turnpike. He looked in that direction but didn't see anything. Where he was standing, the beam was landing between his belt and his shoulders. "You'll have to scootch down a little bit to see it," I said. He found this hard to believe but he tried it and there was no mistaking there was a light. I would compare it to the brightness of a locomotive headlight about a half mile down the track at night (except that it was red).

    Then I put my 18 inch f/1 Fresnel lens in the beam and put the solar cell at the focus (now bright enough to see the reflected light) and the radio station came through loud and clear. With a Polaroid camera I photographed the light coming from my house. Shot from that distance, all the houses are very tiny, but magnification shows a white blob where my house should be.

    P.S. I did not get arrested for trespassing. :)

    (From: Sam.)

    Although George was definitely not an amateur in the laser field of the day, this could very well have been the earliest (or at least one of the earliest) examples of amateur laser communications since it I bet it wasn't part of his job description!

  • Back to Laser Instruments and Applications Sub-Table of Contents.


    Use of Laser to Identify Stars in the Sky to a Group

    Of course you can't reach the stars but there may be enough scatter in the air to show the direction. :)

    (From: Louis Boyd (

    In my experience a 5 mW red laser does not do the job unless there's a lot of dust or water droplets in the air. The problem is the dark adapted human eye is very insensitive to red. Also backscatter from small particles is reduced as wavelength increases. I can't give a specific power level because it's so dependent on the particles suspended in the air. Under the right conditions a 3mw green pointer would be easily visible for a few people standing together but probably won't be adequate in very clean air. Blinking the laser can make it easier to detect and reduce power consumption. You also didn't state the size of the group. The distance of the observer from the emitter makes a difference.

    The "vanishing point" for off axis viewer isn't at infinity and is dependent on the power level and the hight of suspended particles. The effect is that what you are pointing at may not be exactly where other's perceive the end of the "beam" to be. You may actually be better off with a larger beam diameter using a modified flashlight with a halogen bulb.

    One of the more powerful "MagLight" or "Surefire" flashlights with a an extension of a couple of feet of ABS plastic with internal baffle rings to prevent side scatter does a good job. This can put out around a watt of light and it's a lot cheaper than an adequately powerful laser. If this is for a large group get one of the "million candlepower" lamps and make the baffle out of a "honeycomb" of tubes with black flocking blown into them. Those have over 10 watts of light output. If you need to do this for a large crowd like a stadium use a xenon short arc lamp spotlight with hundreds of watts of output.

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