Tag: diode

A Smorgasbord of Acronyms

There are ANSI, OSHA, FDA (CDRH), NRPB, and military standards. The CDRH (Center for Devices and Radiological Health is part of the Food and Drug Administration and is the most relevant regulatory organization in the USA for commercial and scientific lasers. The complete CDRH document may be found at: Performance Standard for Light Emitting Products.

As of Summer 2007, there is an updated “ANSI Z136.1 (2007) Safe Use of Lasers” which among other things substitutes Class 3R for Class 3A, add Classes 1M and 2M, changes some of the control measures and terminology, and more. Nothing earth shattering though. I have not yet seen a full copy. However, there is a summary article in the June 2007 Photonics Spectra magazine. And a narrated slide show of the changes can be found in the Laser Institute of America ANSI Z.136.1 Presentation. However, without details or access to the full document, it’s an excellent cure for insomnia at best. 🙂 Also see: Wikipedia: Laser Safety, which includes many references and some links.

The best discussion of the various classifications, plus general treatment of the topic, is a book by Sliney and Wolbarsht, “Safety with Lasers and Other Optical Sources”, Plenum Press, New York. While they will agree with each other in most respects, some differences will result in a particular laser changing classes depending on which standard is used. The major criteria are summarized below.

Note: I may use Class 1 and Class I, Class 2 and Class II, Class 3 and Class III, and Class 4 and Class IV interchangeably. They are equivalent.

The following is based on material from the University of Waterloo – Laser Safety Manual.

All lasers are classified by the manufacturer and labelled with the appropriate warning labels. Any modification of an existing laser or an unclassified laser must be classified by the Laser Safety Officer prior to use. The following criteria are used to classify lasers:

1. Wavelength. If the laser is designed to emit multiple wavelengths the classification is based on the most hazardous wavelength.
2. For continuous wave (CW) or repetitively pulsed lasers the average power output (Watts) and limiting exposure time inherent in the design are considered.
3. For pulsed lasers the total energy per pulse (joule), pulse duration, pulse repetition frequency and emergent beam radiant exposure are considered.

Lasers are generally classified and controlled according to the following criteria:

• Class I lasers – Lasers that are not hazardous for continuous viewing or are designed in such a way that prevent human access to laser radiation. These consist of low power lasers or higher power embedded lasers (e.g., laser printer or DVD burner).
• Class II visible lasers (400 to 700 nm) – Lasers emitting visible light which because of normal human aversion responses, do not normally present a hazard, but would if viewed directly for extended periods of time. This is like many conventional high intensity light sources.
• Class IIa visible lasers (400 to 700 nm) – Lasers emitting visible light not intended for viewing, and under normal operating conditions would not produce a injury to the eye if viewed directly for less than 1,000 seconds (e.g., bar code scanners).
• Class IIIa lasers – Lasers that normally would not cause injury to the eye if viewed momentarily but would present a hazard if viewed using collecting optics such as a magnifier or telescope).
• Class IIIb lasers – Lasers that present an eye and skin hazard if viewed directly. This includes both intrabeam viewing and specular reflections. Class IIIb lasers do not produce a hazardous diffuse reflection except when viewed at close proximity.
• Class IV lasers – Lasers that present an eye hazard from direct, specular and diffuse reflections. In addition such lasers may be fire hazards and produce skin burns.

Here is another description, paraphrased from the CORD course: “Intro to Lasers”. (Cord Communications. Lasers.) It relates the laser classifications to common laser types and power levels:

• Class I – EXEMPT LASERS, considered ‘safe’ for intrabeam viewing. Visible beam.Maximum power less than 0.4 uW for long term exposure (greater than 10,000 seconds). Looking at a Class I laser will not cause eye damage even where the entire beam enters the eye and it is being stared at continuously.A laser may also be labeled as Class I if it is entirely enclosed and not accessible without disassembly using tools. Thus, a DVD burner with a 150 mW laser diode (normally a Class IIIB laser) would still be considered Class I.
• Class II – LOW-POWERED VISIBLE (CW) OR HIGH PRF LASERS, won’t damage your eye if viewed momentarily. Visible beam.Maximum power less than 1 mW for HeNe laser.
• Class IIIa – MEDIUM POWER LASERS, focused beam can injure the eye.HeNe laser power 1.0 to 5.0 mW.
• Class IIIb – MEDIUM POWER LASERS, diffuse reflection is not hazardous, doesn’t present a fire hazard.Visible Argon laser power 5.0 mW to 500 mW.
• Class IV – HIGH POWER LASERS, diffuse reflection is hazardous and/or a fire hazard.

The classifications depend on the wavelength of the light as well and as noted, there may be additional considerations for each class depending on which agency is making the rules. For example, the NRPB (British) adds a requirement for Class IIIa that the power density for a visible laser not exceed 25 W/m² which would thus bump some laser pointers with tightly focused beams from Class IIIa to Class IIIb. For more information on laser pointer safety and the NRPB classifications, see the NRPB Laser Pointer Article.

In the US, start with the Center for Devices and Radiological Health (CDRH), part of the Food and Drug Administration (FDA). See the section: Regulations for Manufacturers of Lasers and Laser Based Equipment for more info on how to find the relevant guidance documents.

For additional information on laser safety and laser safety classifications, see the section: Laser Safety Sites (May Also Include Other Laser Information).

Here is a table of the CDRH classification and labeling requirements for commercial laser products:

Here are some excerpts from the Center for Devices and Radiological Health (CDRH) regulation 21 CFR 1040.10 and 21 CFR 1040.11, the standard classification for lasers are as follows with some additional comments by Wes Ellison (erl@sunflower.com):

• Class I laser productsNo known biological hazard. The light is shielded from any possible viewing by a person and the laser system is interlocked to prevent the laser from being on when exposed. (large laser printers such as the DEC LPS-40 had a 10 mW HeNe laser driving it which is a Class IIIb laser, but the printer is interlocked so as to prevent any contact with the exposed laser beam, hence the device produces no known biological hazard, even though the actual laser is Class IIIb. This would also apply to CD/DVD/Blu-ray players and recorders (which might have Class IIIb laser diodes of 100 mW or more) and small laser, as they are Class I devices).
• Class II laser productsPower up to 1 milliwatt. These lasers are not considered an optically dangerous device as the eye reflex will prevent any occular damage. (I.e., when the eye is hit with a bright light, the eye lid will automatically blink or the person will turn their head so as to remove the bright light. This is called the reflex action or time. Class II lasers won’t cause eye damage in this time period. Still, one wouldn’t want to look at it for an extended period of time.) Caution labels (yellow) should be placed on the laser equipment. No known skin exposure hazard exist and no fire hazard exist.
• Class IIIa laser productsPower output between 1 milliwatt and 5 milliwatt. These lasers can produce spot blindness under the right conditions and other possible eye injuries. Products that have a Class IIIa laser should have a laser emission indicator to tell when the laser is in operation. They should also have a Danger label and output aperture label attached to the laser and/or equipment. A key operated power switch SHOULD be used to prevent unauthorized use. No known skin hazard of fire hazard exist.
• Class IIIb laser productsPower output from 5 milliwatts to 500 milliwatts. These lasers are considered a definite eye hazard, particularly at the higher power levels, which WILL cause eye damage. These lasers MUST have a key switch to prevent unauthorized use, a laser emission indicator, a 3 to 5 second time delay after power is applied to allow the operator to move away from the beam path, and a mechanical shutter to turn the beam off during use. Skin may be burned at the higher levels of power output as well as the flash point of some materials which could catch fire. (I have seen 250 mW argons set a piece of red paper on fire in less than 2 seconds exposure time!) A red DANGER label and aperture label MUST be affixed to the laser.
• Class IV laser productsPower output >500 milliwatts. These CAN and WILL cause eye damage. The Class IV range CAN and WILL cause materials to burn on contact as well as skin and clothing to burn. These laser systems MUST have:A key lockout switch to prevent unauthorized use Inter-locks to prevent the system from being used with the protective covers off, emission indicators to show that the laser is in use, mechanical shutters to block the beam, and red DANGER labels and aperture labels affixed to the laser.

  The reflected beam should be considered as dangerous as the primary beam. (Again, I have seen a 1,000 watt CO2 laser blast a hole through a piece of steel, so imagine what it would do to your eye !)

• Registration of laser systemsAny laser system that has a power output of greater than 5 milliwatts MUST be registered with the FDA and Center for Devices and Radiological Health if it has an exposed beam, such as for entertainment (I.E. Laser light shows) or for medical use (such as surgery) where someone other than the operator may come in contact with it. (This is called a ‘variance’ and I have filled them out and submitted them and they ARE a royal pain in the backside!)

Sometimes, you will come across a laser subassembly that has a sticker reading something like: “Does not Comply with 21 CFR”. All this means is that since the laser was mounted inside another piece of equipment and would not normally be exposed except during servicing, it does not meet all the safety requirements for a laser of its CDRH classification such as electrical interlocks, turn-on delay, or beam shutter. This label doesn’t mean it is any more dangerous than another laser with similar specifications as long as proper precautions are taken – such as adding the missing features if using the laser for some other purpose!

(From: Johannes Swartling (Johannes.Swartling@fysik.lth.se).)

It is not the laser in itself that is given a class number, but the whole system. A system which is built around a very powerful laser can still be specified as Class I, if there is no risk of injury when operating the system under normal conditions. For example, CD players are of class I, but the (IR) laser diode may in itself be powerful enough to harm the eye. CD players are designed so that the laser light won’t escape the casing.

When it comes to laser safety and exposure levels the regulations are fairly complicated and I will not go into details. Basically, there are tables with ‘safe’ levels of exposures. The exposure has to be calculated in a certain way which is unique to each case, depending on among other things: laser power, divergence, distance, wavelength, pulse duration, peak power, and exposure time.. Although it is true that near infrared lasers are potentially more dangerous than visible because you can’t see the radiation, it is incorrect to say that it must be, say, Class III. The level of exposure may be so low that it can be a Class I (note that Class II lasers are always visible though, so infrared lasers are either of Class I or Class III or higher).

(From: John Hansknecht (vplss@lasersafetysystems.com).)

OSHA STD-01-05-001 – PUB 8-1.7 – Guidelines for Laser Safety and Hazard Assessment is an “open source” release of the ANSI Z136.1-1986 standard. It is not as up to date as the present ANSI standard (ANSI Z136.1-2015), but it’s close. The ANSI standard is considered to be the authoritative guide for safe work practices and would be a better source than a University safety manual. The key point to understand is if a laser accident ever occurs and a lawsuit ensues, the lawyers will be checking to see if the facility was following the “recognized best work practices”.

Laser Safety Systems LLC, a supplier of laser safety products, has a good intro page for anyone serious about becoming ANSI compliant at A Practical Guide to Laser Class 4 Entryway Control Requirements.

Hobbyist Projects and Laser Safety Classifications

While many of the partial circuits and complete schematics in this document can and have been used in commercial laser products, important safety equipment has generally been omitted to simplify their presentation. These range from simple warning labels for low power lasers (Class I, II, IIIa) to keyswitch and case interlocks, beam-on indicators, and other electrical and mechanical safety devices for higher power lasers. Laser safety is taken very seriously by the regulatory agencies. Each classification has its own set of requirements.

The following brief summary is just meant to be a guide for personal projects and experimentation (non-commercial use) – the specifics for each laser class may be even more stringent:

• For diode lasers and HeNe lasers outputting 5 mW or less (Classes 1, II, IIIa), packaging to minimize the chances of accidental exposure to the beam and standard laser warning labels should be provided.
• Where the case can be opened without the use of tools, interlocks which disable the beam are essential to prevent accidental exposure to laser radiation (Class IIIa and above). Their activation should also remove power and bleed off any dangerous voltages (ALL HeNe and argon/krypton lasers).
• A beam-on indication is highly desirable especially for lasers emitting invisible IR (or UV).

Aside from their essential safety function, laser warning or danger stickers DO add something in the professional and high-tech appearance department. Companies selling laser accessories will likely offer genuine CDRH approved stickers. If you are selling any laser based equipment, you’ll need them (and a lot more). For hobbyist, some semi-standard unofficial samples can be found in the next section.

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There are many uses for even a 1 mW helium-neon laser. Most of these same sorts of things can also be done with a collimated diode laser (though some laser diodes may not have the needed coherence properties for applications like interferometry and hologram generation).

Below are just a few possibilities.

(Portions from: Chris Chagaris (pyro@grolen.com).)

• Basic optical experimentation such demonstrating the principals of refraction, reflection, diffraction and polarization.
• Interferometer construction: With a small laser and a few simple optics, this device will allow you to perform many interesting experiments.
• Free-space optical communications: using some basic electronics for modulation.
• Fiber Optic Experimentation.
• Viewing of holograms.
• Hologram generation: However, I’d suggest a slightly higher powered laser for this.
• Construction of a basic laser light show. A higher power laser could be substituted when budget constraints allow. 🙂
• Laser surveillance.
• Laser tachometer.
• Laser burglar alarm.
• Laser gyroscope.
• Studying fluid dynamics.
• Applications in construction for calculating distance, leveling, aid in pipe laying, etc.

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A helium-neon (henceforth abbreviated HeNe) laser is basically a fancy neon sign with mirrors at both ends. Well, not quite, but really not much more than this at first glance (though the design and manufacturing issues which must be dealt with to achieve the desired beam characteristics, power output, stability, and life span, are non-trivial). The gas fill is a mixture of helium and neon gas at low pressure. A pair of mirrors – one totally reflective (called the High Reflector or HR), the other partially reflective (called the Output Coupler or OC) at the wavelength of the laser’s output – complete the resonator assembly. This is called a Fabry-Perot cavity (if you want to impress your friends). The mirrors may be internal (common on small and inexpensive tubes) or external (on precision high priced lab quality lasers). Electrodes sealed into the tube allow for the passage of high voltage DC current to excite the discharge.

Note that a true laser jock will further abbreviate “HeNe laser” to simply “HeNe”, pronounced: Hee-nee. Their laser jock colleagues and friends then know this really refers to a laser! 🙂 While other types of lasers are sometimes abbreviated in an analogous manner, it is never to the same extent as the HeNe.

I still consider the HeNe laser to be the quintessential laser: An electrically excited gas between a pair of mirrors. It is also the ideal first laser for the experimenter and hobbyist. OK, well, maybe after you get over the excitement of your first laser pointer! 🙂 HeNe’s are simple in principle though complex to manufacture, the beam quality is excellent – better than anything else available at a similar price. When properly powered and reasonable precautions are taken, they are relatively safe if the power output is under 5 mW. And such a laser can be easily used for many applications. With a bare HeNe laser tube, you can even look inside while it is in operation and see what is going on. Well, OK, with just a wee bit of imagination! 🙂 This really isn’t possible with diode or solid state lasers.

I remember doing the glasswork for a 3 foot long HeNe laser (probably based on the design from: “The Amateur Scientist – Helium-Neon Laser”, Scientific American, September 1964, and reprinted in the collection: “Light and Its Uses” [5]). This included joining side tubes for the electrodes and exhaust port, fusing the electrodes themselves to the glass, preparing the main bore (capillary), and cutting the angled Brewster windows (so that external mirrors could be used) on a diamond saw. I do not know if the person building the laser ever got it to work but suspect that he gave up or went on to other projects (which probably were also never finished). And, HeNe lasers are one of the simplest type of lasers to fabricate which produce a visible continuous beam.

Some die-hards still construct their own HeNe lasers from scratch. Once all the glasswork is complete, the tube must be evacuated, baked to drive off surface impurities, backfilled with a specific mixture of helium to neon (typically around 7:1 to 10:1) at a pressure of between 2 and 5 Torr (normal atmospheric pressure is about 760 Torr – 760 mm of mercury), and sealed. The mirrors must then be painstakingly positioned and aligned. Finally, the great moment arrives and the power is applied. You also constructed your high voltage power supply from scratch, correct? With luck, the laser produces a beam and only final adjustments to the mirrors are then required to optimize beam power and stability. Or, more, likely, you are doing all of this while your vacuum pumps are chugging along and you can still play with the gas fill pressure and composition. What can go wrong? All sorts of things can go wrong! With external mirrors, the losses may be too great resulting in insufficient optical gain in the resonant cavity. The gas mixture may be incorrect or become contaminated. Seals might leak. Your power supply may not start the tube, or it may catch fire or blow up. It just may not be your day! And, the lifetime of the laser is likely to end up being only a few hours in any case unless you have access to an ultra-high vacuum pumping and bakeout facility. While getting such a contraption to work would be an extremely rewarding experience, its utility for any sort of real applications would likely be quite limited and require constant fiddling with the adjustments. Nonetheless, if you really want to be able to say you built a laser from the ground up, this is one approach to take! (However, the CO2 and N2 lasers are likely to be much easier for the first-time laser builder.)

However, for most of us, ‘building’ a HeNe laser is like ‘building’ a PC: An inexpensive HeNe tube and power supply are obtained, mounted, and wired together. Optics are added as needed. Power supplies may be home-built as an interesting project but few have the desire, facilities, patience, and determination to construct the actual HeNe tube itself.

The most common internal mirror HeNe laser tubes are between 4.5″ and 14″ (125 mm to 350 mm) in overall length and 3/4″ to 1-1/2″ (19 mm to 37.5 mm) in diameter generating optical power from 0.5 mW to 5 mW. They require no maintenance and no adjustments of any kind during their long lifetime (20,000 hours typical). Both new and surplus tubes of this type – either bare or as part of complete laser heads – are readily available. Slightly smaller tubes (less than 0.5 mW) and much larger tubes (up to approximately 35 mW) are structurally similar (except for size) to these but are not as common.

Much larger HeNe tubes with internal or external mirrors or one of each (more than a *meter* in length!) and capable of generating up to 250 mW of optical power have been available and may turn up on the surplus market as well (but most of these are quite dead by now). The most famous of these (as lasers go) is probably the Spectra-Physics model 125A whose laser head is over 6 feet in length. It was only rated 50 mW (633 nm), but new samples under optimal conditions may have produced more than 200 mW. Even more powerful ones have been built as research projects. I’ve seen photos of a Hughes HeNe laser with a head around 8 feet in length that required a 6 foot rack-mount enclosure for the exciter.

Its output power is unknown, but probably less than that of the SP-125A. The largest single transverse mode (SM, with a TEM00 beam profile) HeNe lasers in current production by a well known manufacturer like Melles Griot are rated at about 35 mW minimum over an expected lifetime of 20,000 hours or more, though new samples may exceed 50 mW. However, HeNe lasers rated up to at least 70 mW SM and 100 mW MM are available. Manufacturers include: CDHC-Optics (China), Spectral Laser (Italy), and PLASMA, JSC (Russia). However, the lifetime over which these specifications apply is not known and may be much shorter.

Highly specialized configurations, such as a triple XYZ axis triangular cavity HeNe laser in a solid glass block for an optical ring laser gyro, also exist but are much much less common. Most HeNe lasers operate CW (Continuous Wave) producing a steady beam at a fixed output power unless the power is switched on and off or modulated (or someone sticks their finger in the beam and blocks it!). (At least they are supposed to when in good operating condition!) However, there are some mode-locked HeNe lasers that output a series of short pulses at a high repetition rate. And, in principle, it is possible to force a HeNe laser with at least one external mirror to “cavity dump” a high power pulse (perhaps 100 times the CW power) a couple of nanoseconds long by diverting the internal beam path with an ultra high speed acousto-optic deflector. But, for the most part, such systems aren’t generally useful for very much outside some esoteric research areas and in any case, you probably won’t find any of these at a local flea market or swap meet, though eBay can’t be ruled out! 🙂

Nearly all HeNe lasers output a single wavelength and it is most often red at 632.8 nm. (This color beam actually appears somewhat orange-red especially compared to many laser pointers using diode lasers at wavelengths between 650 and 670 nm). However, green (543.5 nm), yellow (594.1 nm), orange (604.6 and 611.9 nm), and even IR (1,152, 1.523, and 3,921 nm) HeNe lasers are available. There are a few high performance HeNe lasers that are tunable and very expensive. And, occasionally one comes across laser tubes that output two or more wavelengths simultaneously. Although some tubes are designed this way, it is more likely to be a ‘defect’ resulting from a combination of high gain and insufficiently narrow band optics. Such tubes tend to be unstable with the relative power varying among the multiple wavelengths more or less at random.

Note that the single wavelength described above usually consists of more than one longitudinal mode or lasing line (more on this later). However, some HeNe lasers are designed to produce a highly stable single optical frequency or two closely spaced optical frequencies. These are used in scientific research and metrology (measurement) applications, described in more detail below.

Current major HeNe laser manufacturers include Melles-Griot, JDS Uniphase, and LASOS. This is far fewer than there were only a few years ago. So, you may also find lasers from companies like Aerotech, Hughes, Siemens, and Spectra-Physics that have since gotten out of the HeNe laser business or have been bought out, merged, or changed names. For example, the HeNe laser divisions of Aerotech and Hughes were acquired by Melles Griot; Sieman’s HeNe laser product line is now part of LASOS; and Spectra-Physics which was probably the largest producer of HeNe lasers from the very beginning gradually eliminated all HeNe lasers from its product line over the last few years. HeNe tubes, laser heads, and complete lasers from any of these manufacturers are generally of very high quality and reliability.

HeNe lasers have been found in all kinds of equipment including:

• Consumer: Supermarket checkout UPC and other barcode scanners. early laser printers, early LaserDisc players.
• Advertising/entertainment: Holography, small laser shows.
• Measurement: Optical surveying, interferometric metrology and velocimetry, other non-contact measurement and monitoring, ring laser gyro.
• Construction: Laser level, tunnel boring, alignment of saw mill wood cutting, general surveying.
• Industrial: Automotive and other alignment; parts detection, counting, and positioning; particle counting.
• Biotechnology: Blood cell analysis (cytometry), laser induced fluorescence of everything from whole cells to single DNA bases, laser tweezers, confocal microscopy, Raman spectroscopy, anesthesia and other gas analysis.
• Medical/surgical: Patient positioning systems for diagnostic and treatment machines, alignment of high power CO2 and YAG treatment lasers and pointing beams.

Nowadays, many of these applications are likely to use the much more compact lower (drive) power solid state diode laser. (You can tell if you local ACME supermarket uses a HeNe laser in its checkout scanners by the color of the light – the 632.8 nm wavelength beam from a HeNe laser is noticeably more orange than the 660 or 670 nm deep red from a typical diode laser type.)

Melles Griot (now part of IDEX Optics and Photonics Marketplace. Catalogs used to include several pages describing HeNe laser applications. I know this was present in the 1998 catalog but has since disappeared and I don’t think it is on their Web site.

Also see the section: Some Applications of a 1 mW Helium-Neon Laser for the sorts of things you can do with even a small HeNe laser.

Since a 5 mW laser pointer complete with batteries can conveniently fit on a keychain and generate the same beam power as an AC line operated HeNe laser almost half a meter long, why bother with a HeNe laser at all? There are several reasons:

• For many applications including holography and interferometry, the high quality stable beam of a HeNe laser is unmatched (at least at reasonable cost, perhaps at all) by laser diodes (though this is apparently changing at least for some diode lasers. See the section: Holography Using Cheap Diode Lasers. In particular, the coherence length and monochromicity of even a cheap HeNe laser are excellent and the beam profile is circular and nearly ideal Gaussian TEM00 so that simple spherical optics can be used for beam manipulation. Bare edge emitting laser diodes (the only visible type currently available) on the other hand always produce a wedge shaped beam and have some amount of astigmatism. Correcting this to the equivalent quality of a HeNe laser is difficult and expensive.
• As noted in the chapter: Diode Lasers, it is all too easy to ruin them in the blink of an eye (actually, the time it takes light to travel a few feet). It would not take very long to get frustrated burning out $50 diodes. So, the HeNe laser tube may be a better way to get started. They are harder to damage through carelessness or design errors. Just don’t get the polarity reversed or exceed the tube’s rated current for too long – or drop them on the floor! And, take care around the high voltage!
• Laser diode modules at a wavelength of 635 nm (close to the 632.8 nm wavelength of red HeNe lasers) may still be somewhat more expensive than surplus HeNe tubes with power supplies. However, with the increasing popularity of DVD players and DVDROM drives, this situation probably won’t last long.

However, the market for new HeNe lasers is still in the 100,000 or more units per year. What can you say? If you need a stable, round, astigmatism-free, long lived, visible 1 to 10 mW beam for under $500 (new, remember!), the HeNe laser is still the only choice.

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