Please, save yourself likely trouble and get a laser pointer or laser diode module from someone who has already taken the time and trouble to get them to do what you probably want.
Red laser pointers are cheap - some around $5 and a few for even $1 - and are now the way to go if you just want a laser to play with.
Laser diodes are often fussy. The junction or main light-emitting working part of a typical laser diode is about the size of a bacterium and can overheat within a microsecond if its limits are exceeded. Furthermore, the minimum current to achieve laser operation ("laser threshold") can sometimes be near or over 80 percent of the "fatal dose" of current.
The light-emitting area of a laser diode typically emits more light than a similar area of the surface of the sun. Since more power must go into this area than into a similar size piece of the sun just to get things working at all, it should be understandable that things can easily go wrong.
Laser diodes can be damaged by exceeding the maximum safe optical output, which may be hardly at all over the rated optical output. Damage can occur in this way even if no other limits are exceeded or reached. Excessive amounts of light can fry particularly vulnerable subregions of the bacterium-sized working region, by means of "catastrophic optical damage". This seems to be from high optical frequency electric field. This is probably how most instant damage occurs. Please note that laser diodes make laser light more easily and more efficiently at lower temperatures, and it is sometimes possible for a laser diode to be ruined by a given current at low temperatures that it survives at warmer temperatures. Many diode laser modules have feedback systems based on the photodiode included in the laser diode to stabilize beam output. The feedback system must be free of overshoots, even during power-up and power-down.
Laser diodes are easily ruined by static electricity. Static electricity can briefly exceed maximum safe forward currents even if barely noticed. If static voltage should be applied in reverse polarity, things are even worse.
Yes, some laser diodes have survived worse than looking at them the wrong way. Some are better-built and/or more conservatively rated than others. You may get lucky and be able to just throw something together and have it work. Maybe, maybe not...
If you already have a laser diode that you can't return, and you want to push your luck, then go to Sam Goldwasser's laser FAQ (see links below or jump in here). Good luck, try only at your own risk to get a laser diode to work.
This can be largely fixed by placing a convex lens in front of the diode
laser, with a distance nearly equal to the lens's focal length. This reforms
the beam, giving it a new, wider initial width with a correspondingly lower
divergence. You will have to adjust the focus (or lens distance) yourself
for best results.
Once you do this, you might wonder what happens with the beam, since the beam from a typical laser diode is not round, but oblong. This occurs because the light emitting part of the laser diode is oblong. At best, the wider dimension of the beam will diverge less than the narrower dimension. The best to be expected from compact lenses around a centimeter in diameter is a beam with initial dimensions (upon leaving the lens) of nearly 1 centimeter in the wider dimension, by a couple or a few millimeters in the narrower dimension. The wider dimension will expand by a millimeter every ten meters or so, while the narrower dimension of the beam will expand by a millimeter every couple or few meters. The divergence may even be greater than this if the lens is of poor quality or not exactly at the optimum distance from the laser diode, or if the beam exits the lens with smaller dimensions than just mentioned above.
The divergence may not be apparent within a few meters of the lens, if the "waist" of the beam occurs at that point. The beam "waist" is a region that sometimes occurs if the lens is trying to make the beam converge at the same rate that the wave nature of light is trying to make the beam diverge. At long distances, the beam *will* diverge, at best, at a rate in milliradians roughly inverse to its initial width in millimeters.
To get less divergence, you need more complex optics or a He-Ne laser, which has a very close-to-ideal round beam. Although a He-Ne laser's beam is fairly narrow and would diverge roughly by a millimeter every meter, this can easily be "fixed". Simply fire the He-Ne through a telescope, into the eyepiece and out the other end. If the telescope is optimally focused, the beam will exit the front of the telescope with a diameter magnified by the telescope's magnification. (Try not to magnify the beam beyond the diameter of any lens it has to go through.) With luck, you could get the beam to have an extremely low divergence of around a millimeter every several 10's or even roughly every 100 meters.
Diode lasers generally don't have ideal beam characteristics, but they are fairly easy to focus to the degree that the beam does not widen by more than a millimeter or two per meter. Most diode laser "pointers" and collimated diode laser modules should achieve this.
As the wavelength gets closer to 555 nM, the visibility becomes greater,
while the color becomes closer to a yellowish green.
Wavelengths less than 555 nanometers are more blue and less visible than 555 nM.
Wavelengths less than 400 nanometers are ultraviolet, and wavelengths longer than 700 nm are infrared. Laser beams of wavelengths near or just longer than 400 nm, or near or just shorter than 700 nm are very dim compared to ones of the same power and more visible wavelengths.
"Brightness per milliwatt" as a function of wavelength would be roughly the Photopic Function.
Sometimes scattering by dust is more significant, and that is essentially independent of wavelength. However, many people want lasers whose beams are visible from the side in clean air.
Since a laser beam shows a visible path mainly in dark rooms or outdoors at night, perceived brightness of the beam's trace through air depends mainly on scotopic vision. This is usually true even when the color of the beam is visible - in which case "mesopic vision" is operating. Mesopic vision is a combination of photopic and scotopic vision, with photopic vision providing a sensation of color and scotopic vision providing most of the sensation of illumination.
So, a reasonable predictor of visibility of the beam's path tracing through clean air, is output power times the scotopic function, divided by wavelength to the 4th power.
Following is a table of scotopic function divided by wavelength to the 4th power, normalized to have maximum value of 1, for some common laser wavelengths (6.324E10 times scotopic function divided by wavelength^4), along with an alternative.
The alternative is normalized (.64 * normalized scotopic_function/wavelength^4, +.16 * scotopic function, + .16 * normalized photopic_function/wavelength^4, +.04* photopic function). I have done only very limited testing of this for correlating well with beam path visibility in fairly clean air in viewing conditions that are dim but bright enough to see the color of the laser light.
Wavelength nm, color, type Norm. V'/WL^4 ALT. ------------------------------------------------------------------ 405 violet diode laser .0435 .0378 441.6 He-Cd violetish blue .580 .492 445 violetish blue diode laser .634 .548 458 deep blue DPSS .781 .688 473 turquoise-blue DPSS .897 .814 488 main green-blue argon wavelength .986 .934 515 emerald green diode laser .877 .975 532 green DPSS .617 .804 543.5 green He-Ne .428 .638 593.5 orange-yellow DPSS .0264 .153 594.1 orange-yellow He-Ne .0253 .15 612 red-orange He-Ne .0062 .0838 632.8 red common He-Ne .00105 .03574 635 less-common red diode laser .00087 .03233 647.1 red krypton laser .000307 .01654 650 common low cost red diode laser .00024 .0148 660 many high pwr. red diode lasers .000104 .00805 694.3 ruby laser .0000072 .0007Since photopic vision and scattering by dust will remain to a small extent, visibility of beam path will be greater for longer wavelengths and less for shorter wavelengths than the above chart indicates.
Visibility of beam path through air for red lasers can easily be a a few times what the above chart indicates, but is still low.
As of late September 2010, diode lasers are only available in wavelengths in the infrared, red and orange-red range, and in the blue, violet and near-UV range.
Commercially available laser pointers whose output is a laser diode wavelength as of this time have only been red or violet, with the exception of a recent slightly violetish blue one. Lower cost red diode laser pointers have had wavelength mainly close to 645-650 nm.
Laser pointers of other colors using DPSS technology have been available for several years, though they cost more than red diode laser pointers.
The most common and least expensive non-red laser pointers are 532 nm green DPSS ones. (Exception: As of summer 2010, violet laser pointers are becoming less expensive than green ones, but violet ones are still less widely available than green ones.) As of September 2010, green laser pointers generally cost near or over $20, mostly well above $20.
UPDATE 3/26/2009: Radio Shack now sells a green laser pointer, with catalog number 63-132. Craig Johnson reviews it here.
More expensive still, generally costing at least a few hundred dollars, are "yellow" (very orangish shade of yellow) 593.5 nm DPSS laser pointers and 473 nm blue ones (color is a "turquoise" cyanish shade of blue).
Other DPSS wavelengths/colors such as 458 nm deep blue are known, but are less easily achieved than 473 nm blue, 532 nm green, and 593.5 nm orangish yellow.
He-Ne laser pointers have been known to be marketed, as far as I know only in 632.8 nm red, and only before red diode laser pointers became easily available at low prices.
UPDATE 6/23/2007: Violet diode laser pointers are starting to become available at moderate prices. One online supplier is dinodirect.com, who refers to these as blue lasers because they use laser diodes intended for "Blu-Ray" players.
Craig Johnson reviews several violet laser pointers in the laser section of his "LED Museum" website.
WARNING - Many violet laser pointers have output power well above 5 milliwatts, sometimes around 50 milliwatts. Some of these are said to be and labelled as 5 mW laser pointers. These can cause permanent eye damage faster than a human can blink. DO NOT SHINE LASERS NEAR OR OVER 5 MILLIWATTS INTO YOUR EYES OR ANYONE ELSE'S EYES FOR EVEN A FRACTION OF A SECOND! Lasers well over 5 milliwatts can also have hazards of the beam being reflected by shiny objects. The beam can be very dangerous to eyes without appearing very bright because human vision has low sensitivity to the typically 405 nm wavelength of violet laser pointers. Please read my warnings for Class IIIb and Class IV lasers.
Update 9/14/2010: 445 nm cordless handheld lasers are now becoming available. See below here.
UPDATE 12/27/2004 - I got a 4 mW 532 nm "greenie"! (green laser using frequency-doubled neodymium laser wavelength produced by a rod/crystal "pumped" by an infrared laser diode) It is one that I got from Roithner Laser with the catalog number GPL-105.
According to Roithner, the output power of this item can be anywhere from 2 to 5 mW, and according to my tests mine produces slightly over 4 mW.
It is noticeably brighter than the cheap 645 nm red ones. 4 mW of 532 nm theoretically looks about 5 times as bright as 5 mW of 645 nm and about 3 times as bright as 5 mW of 632.8 nm! Even if you get one that produces only 2 mW, it should outshine any 5 mW red laser of any common type.
As for the color, it usually looks not quite a deep pure green but a very slightly yellowish green, sort of "between pure green and lime green". It is slightly less yellowish than the green line of mercury. If you look at a CD reflecting the light of a compact fluorescent lamp and see a bright green band among the colored bands, a 532 nm laser has a shade of green slightly less yellowish than that but still yellowish in comparison to "emerald green".
The beam of my "greenie" is slightly visible when viewed from the side in a dark room. I did not see this effect with a 5 mW 645 nm red laser.
The beam quality of my 532 nm green laser pointer was much better than that of any cheap red laser pointer that I ever tested, but was slightly worse than that of a He-Ne laser.
Look for more on green and other laser pointers including dangerous high power ones in the Craig Johnson links towards the bottom of the Links section below.
UPDATE - Craig Johnson has a page on one that produced 190 milliwatts! This is an extremely dangerous one that can easily cause permanent eye damage in as little as a few milliseconds. If the beam reflects from shiny glass, plastic or other shiny smooth objects, it can be powerful enough to possibly cause permanent eye damage faster than one can blink! It even has a slight burning capability!
However, I believe that orange, yellow and green lasers should be no more dangerous and at least a little safer than red ones of the same brightness as opposed to the same power. This is because a red laser needs more power to match the brightness of an orange, yellow or green laser.
If you want to try blocking any infrared output yourself, use a "dielectric interference" bandpass filter for the wavelength that you want to use. Too many other filter types pass the infrared wavelengths in question, including many dichroic filters, nearly enough all stage lighting filter gels, and most to nearly all colored transparent acrylic sheets such as colored "Plexiglas" or colored "Lucite".
DO NOT STARE INTO A DPSS LASER POINTER WITH VISIBLE OUTPUT ATTENUATED BY A FILTER UNLESS INFRARED CONTENT IS KNOWN TO NOT EXIST AT AN EYE-HAZARDOUS LEVEL!
1064 nm and longer wavelengths of infrared are widely considered completely invisible at any eye-safe level. Do not expect to see it through a filter that attenuates the visible output, although safely seeing a dim red glow for other reasons (pumping diode output or weak fluorescence caused by the visible beam) is possible.
You might have noticed that supposedly infrared wavelengths slightly over 700 nanometers are not completely invisible. In fact, wavelengths around 800, even 900 nanometers are very slightly visible. However, one should be cautious with visible quantities of light in the 800-900 nanometer range, since large quantities of light that could be hazardous to the eye might be involved in order for such light to be easily visible.
445 nanometers appears to most people as a very deep pure blue or as a violet-blue, depending on intensity and viewing conditions.
Output power of these is usually very high - typically a few hundred miliwatts to over 1 watt, and over 2 watts has been achieved. When a narrow laser beam has .5 watt or more of laser radiation, the laser is a Class IV one, the highest classification.
MAJOR UPDATE 9/28-2010 - Craig Johnson reviews one of these here.
That particular one comes with safety goggles. Use them.
If you get a high power 445 nm laser without goggles, get appropriate goggles. Yellow UV-blocking safety glasses such as ones available at hardware stores or included with many fluorescent leak detection kits are better than none at all, but proper laser goggles are better. One should use goggles that block stray reflected beams that could hit one's eyes from any angle. Rubber-sided welding goggles with the dark green glass replaced with yellow or orange "plexiglas" or similar acrylic are likely to be OK - if tested by an expert to actually work here.
Avoid more than the briefest exposure of a spot on safety goggles to a high power laser beam. The beam may burn, melt or crack the filter material, possibly in less than a second.
There is extreme danger of causing permanent eye damage in as little as a few to several milliseconds if such a beam enters the eye. Stray reflections have significant risk of causing permanent eye damage faster than one can blink. .5 watt or more in a narrow beam can easily cause burns and can start fires. In fact, .19 watt or more in a narrow beam has some ability to start fires and can somewhat easily cause burns.
A laser beam of power around 1 watt, directly entering the eye, may cause a blood vessel in the retina to burst faster than one can react, resulting in bleeding inside the eye. This is obviously a very serious eye injury. Furthermore, a retinal nerve may get cooked within a fraction of a second, which would cause permanent serious vision loss.
Please read some legal stuff on Class IIIb and IV lasers.
Furthermore, even looking at the spot that is formed when a Class IV laser beam hits a white surface or other diffusely reflecting surface can be hazardous. If a narrow 1 watt laser beam hits a perfectly white surface, viewing the spot straight-on from 4.5 inches away results in eye exposure of approx. 2.5 mW per square centimeter, which is borderline between Class II and Class IIIa for laser radiation coming from a point but not in a narrow beam. This could result in permanent eye damage in about a second if a focused image of the spot is formed on the retina and the pupil is fully dilated. Even at greater distances and with less dilated pupils, the spot is still not safe to stare at. Such a spot would expose one to 1 microwatt per square centimeter from a distance of 5.6 meters, approx. 18.5 feet. This is the borderline between Class I and Class II for laser radiation coming from a point on a diffuse surface.
One thing to keep in mind is that this wavelength is often considered more dangerous to eyes than most other visible wavelengths. The danger of permanent eye damage may be even greater than I mentioned above.
Without collimating optics, the beam from these laser diodes has been mentioned as approximately 7 by 16 degrees. If the beam is a uniform ellipse, then 1 watt in such a beam would have an intensity of 37 watts per steradian. However, this 7 by 16 degree beam is not uniform and the center of the beam likely has an intensity roughly twice that, or roughly 74 watts per steradian. This works out to 12.5 mW per square centimeter at 77 centimeters away (give or take). This is the threshold of Class IIIb intensity for laser radiation coming from a point but not in a narrow beam. This is approximately (give or take) the threshold of being able to cause permanent eye damage faster than one can react if one's pupils are fully dilated. Obviously, even without collimating optics, these laser diodes are extremely dangerous to look into when they are producing laser radiation.
One eBay seller of these laser diodes has mentioned that one that he was selling produces 7 milliwatts of radiation even when operated at a little less than laser threshold. One thing about laser diodes is that when operated below laser threshold, their output takes a path similar to that taken by their laser radiation when they are lasing. A pointer producing a narrow beam from one of these operated below laser threshold can produce a non-laser beam of 5 milliwatts or more and having the same eye hazards as a laser beam of the same power - which would be a Class IIIb one.
These are mentioned in Sam Goldwasser's Laser FAQ here.
Don Klipstein's mirror copy at donklipstein.com. Some GIFs and some other files "cleaned up" for slightly faster download. Updated on 9/1/2009 to a "later V. 9.10" having been updated on 8/15/2009.
Official copy at the University of Pennsylvania. V. 13.00 as of 2/1/2013 through 11/20/2013.
Official copy at repairfaq.org. V. 13.00 as of 2/1/2013 through 11/20/2013.
Official copy at Drexel University. V. 13.00 as of 2/1/2013 through 11/20/2013.
The whole thing including .GIF, .PDF, .JPG files, etc. in a zip file -
Laser FAQ zip file at the University of Pennsylvania. This is a "later V. 9.10" having an update date of 8/15/2009 despite an early notation of 3/1/2009, as of 9/1/2009, likely updated since. Size approx. 45 meg at that time, official version.
The laser section of the table of contents of Craig Johnson's main page.
Also check into:
Craig Johnson's "What's New" Page. A new laser item noted was on 11/18/2013.
UPDATE 6/23/2007! Craig Johnson's page on an EXTREMELY DANGEROUS Class IV 2-watt ~445 nm deep blue portable laser! New page 11/8/2013, updated 11/13/2013.
Craig Johnson's page on a VERY DANGEROUS 190 mW 532 nm green portable laser!
He reviews many other green ones.
UPDATE 10/13/2004: - Craig Johnson's page on orange-yellow 593.5 nm DPSS laser pointers!
UPDATE 6/2/2006 ("born" in mid-2005): - Craig Johnson's page on a blue 473 nm DPSS laser pointer!
Craig Johnson's Main Page (mostly LEDs and flashlights rather than lasers). (Known to be updated 11/18/2013 with a newly added laser and likely to be updated significantly once or twice a week.
DANGER - the 15 mW and especially the higher power
lasers can cause permanent eye damage faster than a human can react!
Glass objects, shiny plastic objects and many other shiny objects, even ones
that are not metallized, can produce reflected beams of the 190 mW unit that
are strong enough to cause permanent eye damage faster than one can react!
Use of lasers 5 mW or more is regulated in the USA and forbidden by law in
public places in some areas (check state and local laws)! Please read my
section on legal stuff about Class IIIb lasers.
Use of lasers 5 mW or more is regulated in the USA and forbidden by law in public places in some areas (check state and local laws)! Please read my section on legal stuff about Class IIIb lasers.
M. Csele's Homebuilt Lasers Page. Updated 3/29/2007.
TEA (Transverse Electrode Atmospheric) Laser at Sparkbangbuzz!
This is a variation of a nitrogen laser. The lasing medium is air at
atmospheric pressure! No mirrors are needed!
The beam is not a true coherent laser beam, but it is reasonably narrow and the radiation is actually laser radiation.
CAUTION - High voltage is required, and the laser beam is invisible ultraviolet. Also, these lasers involve spark gaps that produce nitrogen oxides and ozone, possibly in quantities unhealthful to breathe for more than a few minutes - you may need good ventillation if you operate such a laser for more than a few minutes! The sparking can also be very loud!
WARNING - Transverse electrode nitrogen lasers tend to be Class IIIb lasers even if the output power is less than 1 milliwatt. The main reason is invisibility of the beam. Please read my legal warnings about Class IIIb lasers.
These are a subset of ultraviolet nitrogen lasers.
Although these are reality now, there has been fiction proposing ability to build these around 1800 or maybe even in the 1700's.
Please read this fiction about such a laser being so easy to build that it is described in fiction of the Victorian times in an obvious link from here.
UPDATE 12/27/2000 - I tried hacking some cheap bullet style laser pointers as suggested by Craig Johnson. I have been getting some amazing solar cell readings on one of these hack jobs indicating at least 6.9 milliwatts absolutely worst case of consistently reproducible readings, and more likely 7.3 mW! Your mileage may and probably will vary but 6.9-plus mW is known achieved without doing anything really extreme! All you need to do is get some spares of the right bullet style laser pointer, and then not get in trouble! :)
Actual solar cell readings were 3.6 mA easily consistently reproduced and occaisionally 3.8 mA under favorable circumstances. The laser was a good foot or two away from the solar cell to rule out most of any radiation not in the laser beam. The wavelength seemed to be 648 nm and was nominally 650.
UPDATE - try removing the bullet tip "plain lens" for more output since this seems to catch and block a small portion of the beam. All milliwatt figures in this section are for with this tip removed from the pointer.
CAUTION - One of my "better" samples of this pointer with the usual three batteries and with the bullet tip removed hit 5.4 mW with fresh, non-tired batteries. This power level sagged to 4.4 mW in about a minute. But let it be known that just by removing the bullet tip makes some of these pointers sometimes slightly exceed the 5 mW limit of Class IIIA. Use with caution. Lasers over 5 mW are in a legal class of definitely not a toy - serious federal regulations apply and serious state/local laws may apply.
UPDATE 1/2/2001 - My pointer with 4 batteries managed 9.2 mW for a few seconds with well-rested batteries, sagging to 7.2 mW within a minute as the batteries got tired.
NOW FOR RESULTS OF MORE ADVENTUROUS HACKING! (1/2/2001)
I tried attaching clip leads to the spring and the case of one of these pointers. This went to a variable voltage DC power supply. I cranked it up to 4.9 volts, which was the highest I dared to go.
Results - power output was 10.6 mW, but this sagged to 10.2 mW within a minute as the laser diode heated up.
WARNING - 10-plus mW has a significant risk of causing permanent eye damage faster than one can blink or react if the beam enters someone's eye. A 10 mW laser is definitely not a toy!
WARNING - Such abuse may ruin the laser pointer. The laser diode may degrade within minutes, possibly within a minute from overheating. In addition, thermal stress may break things in the laser diode assembly. There is also a threshold of instant damage ("catastrophic optical damage" occurring in less than a microsecond) which may be somewhere around 10 milliwatts of laser output power.
CAUTION - non-regulated power supplies may have the voltage change significantly between an unloaded condition and a loaded condition. If the voltage with a light load is safe, the no-load voltage may charge up a capacitor in the supply past the threshold of a rapid damage mode! I recommend attaching a light bulb that draws 75-300 mA to the output of any unregulated supply to guard against the voltage rising past some critical level when you release the button on the laser pointer.
CAUTION - if you remove the diode assembly from the pointer, beware that any metal part of the diode assembly that the laser diode chip is attached to may need to be pressed against a larger piece of heat-conductive metal to keep the laser diode from overheating.
This modification is sometimes known as the "pot mod". The pushbutton switch cap can be popped off with a sharp knife or a utility knife blade. This exposes a very small potentiometer or "pot" that can be turned with a very small screwdriver. Turning it counterclockwise (to the left) increases output power.
Do not turn the pot all the way up - it is recommended to at least back it down a little, otherwise the wiper of the pot may fail to make contact.
It may be a good idea to back the pot down enough to get somewhat noticeably less than maximum possible output, so that current flowing through the pump diode is limited by the circuitry rather than by resistance of the batteries. One should also verify that current draw of this laser is less than 500 milliamps, preferably less than 400 milliamps - and beware of ammeters having enough resistance to have combination of ammeter and battery resistance being the limiting factor.
Adjusting the pot with the laser on can short the pot and a resistor in series with one of the pot's leads, resulting in extreme output and possibly damaging the pump diode.
This modification can achieve 15-plus milliwatts of output. Adjusting the pot for maximum possible output can achieve output of 20-plus milliwatts, sometimes over 30 milliwatts. I would do this modification conservatively to achieve longer life of the pump diode.
More extreme modifications are known, such as replacing resistors and placing a short from the wiper of the pot to the positive battery connection. Output as high as 100 milliwatts has been achieved with such extreme modifications. However, life of the pump diode is likely to be significantly shortened, especially if current draw exceeds 500 milliamps.
DANGERS AND WARNINGS!
1. Boosting output past 5 milliwatts makes this laser a Class IIIb one, subject to serious laws and regulations, as well as serious policies at many workplaces and most schools, colleges and universities. USA regulations for Class IIIb lasers include appropriate labelling, and an emission indicator, a key switch, a turn-on delay, and generally to not use such lasers in public with some exceptions.
Please read my stuff on Class IIIb and IV lasers.
2. If a green laser beam much over 5 milliwatts enters one's eye, there is a significant chance that permanent eye damage will occur faster than one can react or blink. If the beam power involved is around or over 15 milliwatts, then this will *probably* cause permanent eye damage.
After what I say in the above disclaimers, as best as I know the main USA Federal regulations affecting lasers are 21 CFR 1040.10 and 21 CFR 1040.11.
It appears to me likely that the USA's FAA imposes some regulation upon at least some lasers being fired into the sky. And OSHA probably has some regulations on use of lasers in workplaces.
There is also Federal legislation affecting lasers. One example is an early 2,000's Federal law against firing lasers at flying aircraft. As far as I know, that law has no exemptions for lasers that can be rightfully fired into the sky, or for ones of output power low enough to not be dangerous. The penalties are very severe.
There are also state and local laws. For example, it is illegal for minors to posess laser pointers in New York City and some localities in Illinois. It is illegal for students or persons of student age to posess laser pointers in kindergarten, elementary, junior high and high schools in California or New York City. Many locations have laws specifically against harassing people with lasers or laser pointers.
Laws are different in countries other than the USA. In the USA, laser pointers are legal to retail-market if they are visible and have total output power under 5 mW. But in the United Kingdom and Australia, these have to be under 1 mW.
It is illegal to posess laser pointers in New South Wales, Australia.
Class IIIb lasers include visible lasers producing 5 to 499 milliwatts in a narrow beam. Class IV lasers include ones producing half a watt or more in a narrow beam. The main reason these are regulated is because 5 milliwatts or more being focused into a single spot of the retina of the eye can cause permanent visual damage faster than a human can react.
Furthermore, for most UV and IR wavelengths, there is no such thing in 21 CFR 1040.10 as Class II or Class IIIa. Lasers producing beams of invisible wavelengths are Class IIIb as soon as they achieve power exceeding the upper limit of Class I. This includes narrow-beam-producing nitrogen lasers even if the beam is not coherent.
It is generally not legally permissible in USA to operate Class IIIb or IV lasers where anyone other than the laser operator is at risk of being exposed to the beam, especially in terms of eye exposure. It is generally considered to be illegal in USA to operate a Class IIIb or Class IV laser in public outside the frameworks permitted by 21 CFR 1040.10 and 21 CFR 1040.11.
These regulations do apply to workplaces. Schools are workplaces. Schools and workplaces often have their own rules and/or guidelines concerning operation of lasers, and they are often strict. Various schools and workplaces can have various plans (or lack thereof) for complying with 21 CFR 1040.10 and .11 and whatever OSHA requires for operation of lasers in workplaces. They may have stricter or made-up-on-the-spot rules to satisfy their liability concerns, whether valid or not.
If you want to bring a laser into your school, especially if it is a Class IIIb or IV one, please negotiate with appropriate persons in your school in advance, such as a science teacher or physics teacher. Please give your science or physics teacher sufficient time in advance to get that worked out with the school's management.
You may be restricted to bringing in your homebrew transverse electrode atmospheric nitrogen laser without its power supply. There is a slight chance you can get it to fire a pulse of a beam from static electricity and within Class I limits, though your school may not allow even that degree of actually operating it.
And if your school allows you to bring in and operate laser pointers, please avoid ones of IIIb or IV class.