Viewing Spectral Lines in Discharge, Other Colours in Output

For accurate measurements, you’ll need an optical instrument such as a monochromator or spectrophotometer or optical spectrum analyzer. But to simply see the complexity of the discharge spectrum inside the bore of a He-Ne laser tube, it’s much easier and cheaper.

(Spectra for various 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.)

Instant Spectroscope for Viewing Lines in He-Ne Discharge

It is easy to look at the major visible lines. All it takes is a diffraction grating or prism. I made my instant spectroscope from the diffraction grating out of some sort of special effects glasses – found in a box of cereal, no less! – and a monocular (actually 1/2 of a pair of binoculars).

  • If you missed the Kellogg’s option, diffraction gratings can be purchased from places like Edmund Scientific. You don’t need anything fancy – any of the inexpensive ‘transmission replica gratings’ on a flat rigid substrate or mounted between a pair of plane glass plates will be fine. In a pinch, a CD disc or other optical media will work but only as a reflection grating so mounting may be a problem. A spectroscope can also be made with a prism of course but a diffraction grating is likely to be less expensive and better for this application since it is much lighter and easier to mount.
  • The plasma tube of a bare He-Ne laser is an ideal light source since it provides its own slit as the glow discharge is confined to the long narrow capillary bore. However, this approach can also be used with other lasers as long as the beam can be focused to a spot on a wall or screen. This will produce a ‘bright spot spectra’ instead of politically correct lines but you can’t have everything. 🙂
  • The diffraction grating can be used by itself but the additional optics will provide magnification and other benefits for people with less than perfect eyeballs.
  • Glue the diffraction grating to a cardboard sleeve that can be slipped over the (or one) objective of a monocular, binocular, or small telescope – or the telephoto lens of your camera. Orient it so that the dispersion will be vertical (since your slit will be horizontal).
  • Operate the HeNe tube on a piece of black velvet or paper. This will result in optimum contrast. This is best done in a darkened room where the only source of light is the laser tube itself. Just don’t trip and zap yourself on the high voltage!
  • A diffraction grating produces several images. The zero’th order will be the original image seen straight ahead. The important ones are the first order spectra. Tip the instrument up or down to see these. The dispersion direction – order of the colours – will depend on which way it is tipped.
  • Any distance beyond the closest focus of your instrument will work but being further away will reduce the effective width of the ‘slit’ resulting in the ability to distinguish more closely spaced lines.

The shear number of individual spectral lines present in the discharge is quite amazing. You will see the major red, orange, yellow, and green lines as well as some far into the blue and violet portions of the spectrum and toward the IR as well.

Bright Line Spectra of Helium and Neon
Bright Line Spectra of Helium and Neon

All of those shown will be present as well as many others not produced by the individual gas discharges. There are numerous IR lines as well but, of course, these will not be visible.

Place a white card in the exit beam and note where the single red output line of the He-Ne tube falls relative to the position and intensity of the numerous red lines present in the gas discharge.

As an aside, you may also note a weak blue/green haze surrounding the intense main red beam (not even with the spectroscope). This is due to the blue/green (incoherent) spectral lines in the discharge being able to pass through the output mirror which has been optimized to reflect well (>99 percent) at 632.8 nm and is relatively transparent at wavelengths some distance away from these (shorter and longer but you would need an IR sensor to see the longer ones). Since it is not part of the lasing process, this light diverges rapidly and is therefore only visible close to the tube’s output mirror.

Dynamic Measurement of Discharge Spectra

The following is trivial to do if you have a recording spectrometer and external mirror He-Ne laser. For an internal mirror He-Ne laser tube, it should be possible to rock one of the mirrors far enough to kill lasing without permanently changing alignment. If you don’t have proper measuring instruments, don’t worry, this is probably in the “Gee wiz, that’s neat but of marginal practical use” department. 🙂

(From: George Werner (glwerner@sprynet.com).)

Here is an effect I found many years ago and I don’t know if anyone has pursued it further.

We had a recording spectrometer in our lab which we used to examine the incoherent light coming from the laser discharge. This spectrum when lasing was slightly different from the spectrum when not lasing, which one can expect since energy levels are redistributed. As with most detectors, ours used a chopper in the spectrometer light beam and a lock-in amplifier.

Instead of putting the chopper in the path of light going to the spectrometer, I put it in the path of the internal laser beam, so that instead of an open/closed signal going to the amplifier it was a lasing/not-lasing signal. What was recorded then was three kinds of spectrum lines: some deflected positive in the normal way, others deflected negative, and the third group were those that were unaffected by chopping, in which case when we passed over the line we only saw an increase in the noise level. Setting up such a test is easy. The hard part is interpreting the data in a meaningful way.

Other Colour Lines in Red He-Ne Laser Output

When viewing spectral lines in the actual beam of a red He-Ne laser, you may notice some very faint ones far removed from the dominant 632.8 nm line we all know and love. (This, of course, also applies to other colour He-Ne lasers.)

For He-Ne lasers, the primary line (usually 632.8 nm) is extremely narrow and effectively a singularity given any instrumentation you are likely to have at your disposal. Any other lines you detect in the output are almost certainly from two possible sources but neither is actual laser emission:

  • Plasma discharge – there are many strong emission lines in the actual discharge – and none of them are actually at the 632.8nm lasing wavelength! These extend from the mid-IR through the violet.Close to the output mirror, you may see some of this light seeping through especially at wavelengths in the green, blue, and violet, for which the dielectric mirrors are nearly perfectly transparent. However, such light will be quite divergent and diffuse and won’t be visible at all more than a couple of inches from the mirror.
  • Superradiance – As we know, He-Ne lasers can be made to operate at a variety of wavelengths other than the common 632.8nm red. The physics for these is still applicable in a red He-Ne tube but the mirrors do not have the needed reflectivity at these other wavelengths and therefore the resonator gain is too low to support true laser action. However, stimulated emission can still take place in superradiance mode – one pass down the tube and out, exiting easily for the green wavelength in particular since the dielectric mirrors are quite transparent in that region of the spectrum.The result will be a weak green beam that can sometimes be observed with a spectroscope in a very dark room room. It isn’t really quite as coherent or monochromatic as the beam from a true green He-Ne laser and probably has much wider divergence but nonetheless may be present. It may be easier to see this by using your spectroscope to view the bright spot from the laser on a white card rather than by deflecting the beam and trying to locate the green dot off to one side.Note: I have not been able to detect this effect on the short He-Ne tubes I have checked.

Since the brightness of the discharge and superradiance output should be about the same from either mirror, using the non-output end (high reflector) should prove easier (assuming it isn’t painted over or otherwise covered) since the red beam exiting from this mirror will be much less intense and won’t obscure the weak green beam.

Note that argon and krypton ion lasers are often designed for multiline output where all colours are coherent and within an order of magnitude of being equal to each other in intensity or with a knob to select an individual wavelength. Anything like this is only rarely done with He-Ne lasers because it is very difficult (and expensive) due to the low gain of the non-red lines.

He-Ne Lasers – Introduction

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.

Monster Vintage Hughes HeNe Laser System
Monster Vintage Hughes HeNe Laser System

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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Portable Power Supply

Battery
Battery

This is detailing my portable multi-purpose power pack of my own design. Here is an overview, mainly showing the 4Ah 12v Ni-Cd battery pack.

Front Panel Right
Front Panel Right

Panel Features – Bottom: Car cigar lighter socket, main power keyswitch. Top: LED toggle switch, provision for upcoming laser project, Red main Power LED, 7A circuit breaker.

Front Panel Left
Front Panel Left

Top: Toggle switch serving post terminals, USB Port.
Post terminals supply unregulated 12v for external gadgets. USB port is standard 5v regulated for charging phones, PDAs etc.
Bottom: Pair of XLR connectors for external LED lights. Switches on their right control power & the knob controls brightness.

Additions are being made to this all the time, the latest being a 2W laser diode driver. Update to come soon!