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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 somewhat larger tubes (up to approximately 35 mW) are structurally similar (except for size) to these but are not quite 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). Even more powerful ones have been built as research projects. The largest HeNe lasers in current production are rated between 35 to 50 mW. A few smaller HeNe tubes may exist, perhaps 4" long and 0.5" in diameter but these were probably not for continous use.
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. Common HeNe lasers operate CW (Continuous Wave) producing a steady beam at a fixed output power unless switched on and off or modulated. (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!
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 (611.9 nm), and even IR (1,1152 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 but this may actually be a 'defect' resulting from a combination of high gain and insufficiently narrow band optics - these tubes tend to be unstable.
Manufacturers include Melles-Griot, Spectra-Physics, Uniphase, and several others. (You may also find Aerotech and Siemens HeNe lasers though these companies have gotten out of the HeNe laser business.) 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, laserdisc players.
- Advertising/entertainment: Holography, small laser shows.
- Measurement: Optical surveying, interferometric metrology and velocimetry, ring laser gyro, other non-contact measurement and monitoring.
- Construction: Laser level, tunnel boring, alignment of saw mill wood cutting.
- Industrial: Automotive and other alignment, parts detection, counting, and positioning.
- Biotechnology: Blood cell analysis (cytometry), laser induced fluorescence of everything from whole cells to single DNA bases, laser tweezers, confocal microscopy.
- 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.)
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 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. 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.
- 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.
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