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    Argon/Krypton Ion Lasers

    Sub-Table of Contents



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  • Back to Argon/Krypton Ion Lasers Sub-Table of Contents.

    Introduction, Acknowledgement, Ar/Kr Ion Laser Safety

    Introduction to Ion Lasers

    Argon and krypton (rare gas) ion lasers find applications in many diverse fields including (1) very high performance printing, copying, scanning, typesetting, photoplotting, and image generation; (2) forensic medicine, general and ophthalmic surgery; (3) entertainment; (4) holography; (5) electrooptics research; and (6) as an optical 'pumping' source for other lasers. From the hobbyist's point of view, items (3) and (4) are generally the most important (aside from the pure project value of such higher power lasers). However, common sources for these lasers when they show up on the surplus market are mostly from (1).

    Note: For the purposes of this discussion, argon ion and krypton ion lasers are very similar - they are both rare gas ion lasers, their basic principles of operation are similar, and the same basic hardware configuration and power supplies can usually be used. Differences are primarily in gas fill of the plasma tube and the mirrors/prisms for selecting the output wavelength. Keep this in mind since where we describe something for an argon ion laser, most likely it applies to a krypton ion (or mixed gas 'white light') laser as well. However, this doesn't mean you can just replace one type with another or convert an argon ion laser to krypton by cracking open the seal on its tube and refilling it! For more information, see the section: Comparison of Argon and Krypton Ion Tube Characteristics and the chapter: Ar/Kr Ion Laser Power Supplies.

    These are the types of lasers that had generally been used for large scale light shows before the advent of the Diode Pumped Solid State (DPSS) laser. Some people still prefer the variety of colors (wavelengths) possible with argon and krypton ion lasers, not easily provided (at least for an affordable cost) with DPSS technology as yet. Ion lasers were also used in some types of high performance phototypesetters or other digital imagers as well as for holography and other optics research. Unlike diode and HeNe types, a serious interest in these also represents a very serious investment of time, money, and caution.

    If you are totally unfamiliar with ion laser technology - or you are just curious - it might be worth checking out the sections: On-line Introductions to Argon Ion Lasers and More Information on Argon/Krypton Ion Lasers before.

    Note that laser jocks will generally drop the "ion" when discussing these - saying simply "argon laser" or "krypton laser". But they are the same thing.

    Equipment Using Argon Ion Lasers

    As noted, argon and krypton ion lasers are used for many applications. Large water-cooled units (like the various Lexel models) with up to several WATTs of output power were often medical or surgical lasers. While locating one of these in working or even repairable condition at a good price might be your fantasy (it is mine), consider that such a beast will require at least 220 VAC (some use 440 VAC!) and possibly three-phase power, a tap water connection, and likely a fair amount of tinkering (and possibly cost as well) to get it going and keep it going. While your local electric company will probably be willing and eager to sell you all those kWHs, you may need a large phase converter to convert your residential single-phase power to three-phase or an upgrade your power feed (which may or may not be costly). Here are a couple of links to possible designs:

    At least the added plumbing shouldn't be much of a problem unless portability is an important consideration! This is not to say it cannot be done, just that you will have to be pretty determined to get that large laser going in an one-bedroom apartment! In any case, you can't just go and plug one of these beasts into the nearest AC outlet. :)

    A small air-cooled ion laser is probably a more reasonable toy especially if you have to share the single 3-prong outlet in your place with the family microwave! :-) And, some of these lasers still have outputs that can approach 500 mW (though most are much lower).

    The types of small argon ion (krypton ion types would be rare) lasers that are turning up on the surplus market are often from various high performance scanners, recorders, duplicators (not your ordinary office copier), printers, and phototypesetters.

    The Xerox 9700 series and older 8700 series (and possibly the 8400 as well) utilized an American Laser Corporation (ALC) 60X argon ion laser. This laser was made to the Xerox "X" standard for a high speed duplicator/printer, hence the X in the part number. The NEC-3030 is also a printer laser and OEM Spectra-Physics (SP) 161 lasers were used in a Times Graphics, Inc. printer. The IBM model 3900, 3835, and 3825 printers (circa 1996) have argon ion lasers and AOMs. Other companies that manufacture or have manufactured equipment containing ion lasers include Dainippon Screens, Hell, and Ricoh.

    Many of these older but expensive graphic arts systems are still being maintained and are now being retrofitted with newer technology such as high power IR diode lasers or Diode Pumped Solid State (DPSS) lasers. Therefore, more small air-cooled argon (mostly) ion laser heads and power supplies are showing up on the surplus market at attractive prices. (However, if you would be content with only 532 nm green, there are high quality DPSS lasers showing up surplus from these sources as well. The most common is the Coherent Compass 315M-100 which produces a TEM00 beam with an output power of up to 100 mW. However, red and blue DPSS lasers are still way too expensive for most hobbyists. For more information, see the chapter: Solid State Lasers.)

    Some DNA sequencers apparently also contain argon ion and other medium power visible lasers.

    For reference, here are the typical wavelengths and expected power output from argon ion laser heads pulled from graphics arts equipment:

    Other argon ion lasers that may turn up as pulls from graphic arts equipment include the Uniphase 2202-5BLT, 2202-30BLT, and Spectra-Physics 163, as well as several others.

    Note that some lasers that at first appear to have excellent specs may be designed for pulsed (low duty cycle) operation. One example is the HGM Spectrum Compac A Argon Laser. This uses a American Laser 68B tube which would be good for 2.5 W with a proper power supply and adequate cooling but in this case is only designed for relatively low duty cycle pulsed operation. Pulse, cool, pulse, cool, etc. If the price is low enough, it may be worth buying just for the tube (assuming it is still good) but non-trivial modifications will likely be needed for it to run CW.

    Additional more detailed information on many of these and other models from ALC, NEC, and SP can be found later in this and subsequent chapters on ion lasers. Also see the section: How to Get a Laser Without Really Trying - Part 2.

    However, many older laser printers and related equipment were based on HeNe lasers so don't assume there is an argon ion laser in that dusty thing at the salvage yard (even if quite large) just because it has a laser warning label! (Newer consumer/office type laser printers use relatively low power IR diode lasers.)

    Mike Harrison (mike@whitewing.co.uk) has a Web page in the early stages of development which lists graphic arts, industrial, medical, scientific, and other equipment which include internal lasers of all kinds. The page can be updated with your contributions as well. Take the link near the bottom of Mike's Electric Stuff Page (which also has a lot of other interesting topics).

    Here are some guidelines for determining if dragging home something bigger than your living room will be worth the trouble:

    (From: Lynn Strickland (stricks760@earthlink.net).)

    Some of the higher-end stuff from Xerox, ECRM, still have HeNe's in them. The Xerox printers are the kind of machines you'd find at places like Kinko's. (big, expensive ones). Xerox still services some argon ion based units too.

    Xerox just yanks the laser heads out after a certain number of operating hours and, last I knew, sells them off on the surplus market. The HeNe's that come out still have quite a few operating hours left in them at that point. For awhile MWK had first dibbs on them, but I don't know what's happening in the last year or two.

    (From: Dean Glassburn (Dean@niteliteproducts.com).)

    Basically here is the story on Amercian Laser's Lasers. ALC has sold 60X, 68B, 909, and 920 systems in the past. The 60X was used in Xerox graphic arts machines, the 68B, 909, and 920 in medical systems. All these manufactures of the systems that used the tubes have since either gone out of business or found other technology to replace the ion laser systems (e.g., high power diode and DPSS lasers).

    About two years ago, ALC's main source of BeO went out of business. Not a major disaster to ALC, because their replacement of those tube types were about 10 a year combined for the 909 and 68B. Ceramic for the 920 was all gone as well as the need for that tube. The current situation is that to get a new tube manufactured there is a 6 to 10 week wait and the cost for the ceramic is twice what it once was, so the tube price new is much higher.

    OK that is the current situation. We have sold and rebuilt many ALC systems for use, but the new tube issue always eventually comes up.

    The benefits to using these are easy to state: Usually you can find these cheaper than other systems. When working properly, the systems put out good power as designed.

    The design limitations are as follows. The 60X was originally designed for 7 to 9 A tube running 488 nm TEM00. This equates to a maximum of 20 mW for 8,000 hours. However, all the 60X systems currently out there are usually old tubes running at a MUCH higher current. To get a higher output of 50 to 100 mw, tube life is very limited. The 909 systems deliver about 5 to 6 watts multimode/multiline. Not the best divergence but not bad either. No fill system on the tube, so life is about 1,000 to 2,500 hours. We have regassed plenty of these and kept them running. As a krypton or mixed gas, life around 600 hours is normal. We usually leave a valve on the tube for regassing. The 68B tubes are usually cracked and cannot be repaired, or run at a low pressure and carbon tracked the bypass. The 920 made a lot of power when running, but it was designed to pump into a Dye laser or a fiber. It is a big bore tube to operate on three-phase 208 VAC at 45 amps, good for about 1,000 hours. You would have better divergence with a flashlight. Changing the optics to reduce the divergence makes a 6 watt, 45 amp laser instead of a 14 watt high divergence laser. Better off with a 909, same output, less power consumed.

    Mix and Match Ion Laser Components

    When you obtain a laser that spent its former life as part of a fancy graphics arts machine (or from other sources as well for that matter), it may turn out to be composed of pieces from several manufacturers as parts were replaced or upgraded. Don't panic! For better or for worse, this implies a high level of interchangeability among similar size lasers and the availability of replacements and substitute components. In fact, such a hybrid or may be better than the original because at least some of its parts will likely be newer than the original as they were replaced to provide an improvement in performance or life expectancy. The downside is finding documentation on a half dozen different pieces from several manufacturers!

    However, a word of caution: Just because the connectors look the same or the specs look like the power supply and laser head should be compatible doesn't make it so. Just plugging something together may result in smoke or shortened lifetime. It is a safe bet that if the components actually came from a working system, they will play happily together. On the other hand, if someone just connected a power supply to a laser head that it wasn't designed to drive, tested the combination for a couple of minutes, and sold it as a working system, there could be problems down the road.

    Regardless of whether your laser is built like Frankenstein's monster, it WILL likely be missing the cooling fan and in some cases, even the head cover. The typical Patriot style fans are available surplus typically for between $15 and $30. Other type fans or blowers with similar ratings (220 cfm and up) will also work if the airflow direction is correct (i.e., for the ALC-60X, it must be sucking out of the head). In cases where the fan diameter is much larger than the opening in the head above the tube as with the Patriot and ALC-60X, a 1 inch collar will also be needed between them to act as an adapter plenum. For laser heads like the SP-161 where the cover may be missing, replacements (including the fan) may be available from the original manufacturer or companies like National Laser. though the cost may be a good fraction of what you paid for the entire laser! (In the case of the SP-161, the cover is really only needed for safety - the fan doesn't use the cover for mounting.) But, if you are just a bit handy, they can usually be fabricated relatively easily. Any interlocks that are missing will also need to be replaced. I've even heard of people using vacuum cleaners for cooling ion lasers! (Think of the marketing possibilities!)

    Also, don't be upset if the running time meter says something like 64,500 hours! This is typical of a graphic arts pull and doesn't reflect on how much time is on the tube itself - which is the only thing that really matters. You can be sure the tube has been replaced more than once but there is probably no way to actually determine how many hours are on the one that is installed.

    Where the umbilical cable has been cut (this happens as well since whoever removed the unit may not have realized that the cable could be extracated non-destructively), a proper connector will need to be reattached. If they are the same type at both ends, the wiring is likely 1:1 so an ohmmeter can be used to determine the connections. However, if they are not the same type (e.g., a Jones type at one end and an AMP type at the other), you will need to find the wiring for each one. Ditto if either end is hard-wired. However, in the worst case, a lot of the wiring at the head-end at least can be determined by tracing connections inside the head. WARNING: A cut umbiliacal could also mean there could be compatibility problems as mentioned above if the head and power supply were not from the same piece of equipment and were never tested together. Even if they use the same AMP connector, there could still be problems. For example, an ALC or Omni power supply may melt down attempting to drive an NEC head or vice-versa without some rewiring and other changes (if it is even possible) even though the connectors mate.

    Also see the section: Spectra-Physics-161 Laser - More or Less.

    On-line Introductions to Argon Ion Lasers

    There are a number of Web sites with laser information and tutorials.

    Acknowledgement for Ar/Kr Ion Laser Information

    Much of the material on argon and krypton ion lasers has been provided in one form or another by Steve Roberts via direct contribution of text and photos and via email discussions. Many thanks to Steve for making this material available.

    Argon/Krypton Ion Laser Safety

    It is not possible to over-emphasize the hazards involved in working with argon ion lasers.

    Having said that, argon ion lasers represent the Holy Grail for laser enthusiasts who will likely turn up their collective noses at HeNe lasers once they have become hooked.

    Additional Comments on Argon/Krypton Ion Laser Safety

    Safety is a major concern, as these are HV high current devices. The major hazard is instant eye damage, and although you can slightly burn certain materials such as black thermoplastic, these are not burning and cutting lasers even when focused, and walls don't burst into flame when the beam hits them. The tubes are usually made of beryllium oxide, kovar and copper (though there are a few quartz ones out there as well). There is a hazard if the BeO ceramic is ground or powdered and inhaled, and most people who buy one don't know about that. (But then, if you are grinding the tube, you don't have a working laser anymore. However, keep this in mind should you come across a certifiably dead argon/krypton tube and become overly curious.)

    I'm sure you've seen the posts on the sci.optics or alt.lasers newsgroups that go something like: "I just got a big laser. What type is it? What can I do with it? Etc." One of these guys is going to look down the bore and get blinded or worse. So I'd also like to see a site up for that reason. Well it turns out there is such a site. There is a excellent laser safety site at: Rockwell Laser Industries.



  • Back to Argon/Krypton Ion Lasers Sub-Table of Contents.

    Basic Characteristics, Other Information Sources

    Basic Characteristics of 'Small' Argon Ion Lasers

    WARNING: These are in a whole different league (compared to common diode and helium-neon lasers) with respect to safety hazards as the optical power levels involved are generally much higher (20 mW to many WATTS - Class IIIb and Class IV) and the power supplies are more dangerous as well. There is no margin for error in dealing with either the operating laser or its power supply.

    Note: Since comparisons are made throughout this discussion between argon (and krypton) ion lasers and helium-neon (HeNe) lasers, it is worthwhile to first read the Chapter: Helium-Neon Lasers if you are not familiar with those devices.

    The basic design of the argon/krypton laser is conceptually similar to that of the HeNe (or other gas) laser - plasma tube containing the active medium (argon and/or krypton gas) mirrors forming a Fabry-Perot resonator. However, unlike HeNe lasers, the energy level transitions that contribute to laser action come from ions of argon or krypton - atoms that have had 1 or 2 electrons stripped from their outer shells. Spectral lines at wavelengths less than 400 nm come from atoms that have had 2 electrons removed. Longer wavelengths come from singly ionized atoms. There are many possible transitions in the UV, visible, and IR portions of the spectrum. With suitable optics coherent light from a single spectral line or many lines may be produced simultaneously. An adjustable intra-cavity prism can even be included to permit the desired wavelength to be selected via a thumb-screw adjustment.

    Beam characteristics in terms of diameter and divergence are similar to those of HeNe lasers. However, the coherence length (without additional optics) tends to be smaller than that of a HeNe laser of similar cavity length. This is because the gain curve for the ion laser transitions is wider than the one for the HeNe laser - around 2.5 GHz compared to 1.5 GHz. So, a larger number of longitudinal modes will be present and the coherence length will therefore be reduced. Coherence lengths quoted by various sources range from 2.5 to 10 cm for typical air-cooled ion lasers.

    To excite the ionic transitions and achieve a population inversion, much more current is needed than for a HeNe laser. A 'small' argon laser may use 10 AMPs of current (rather than the 3 to 8 mA typical of a HeNe laser tube). Even at a tube voltage of 100 VDC, this represents about 1000 W of power dissipation. (Think of a typical space heater inside a small box!) High flow rate forced air cooling is absolutely essential - the tube would melt down in short order without it. Larger ion laser tubes may pass more than 100 AMPs of current at up to 400 VDC or more - and require three-phase power and water cooling - figure on utility substation just for your laser!

    Thus, while Ar/Kr ion lasers and HeNe lasers are conceptually similar, the approximately 3 orders of magnitude greater tube current and two orders of magnitude greater power dissipation compared to a HeNe laser mean that the construction details are vastly different. You won't find one of these in a laser pointer!

    See Typical Cyonics Air-Cooled Argon Ion Laser Tube for the construction of one popular internal mirror type (which bears the most similarity to a HeNe laser tube)! Those from other manufacturers are similar.

    Ar/Kr Ion Laser Tube Electrical Characteristics

    Both HeNe and Ar/Kr ion tubes use a gas discharge in a narrow capillary bore to excite the laser medium to upper energy states, they both need a starting pulse to overcome the initially non-conducting gas, and a current controlled power supply to regulate the discharge. However, the similarities pretty much end there.

    The following assumes a small air-cooled Ar/Kr ion tube like that used in the American Laser Corporation 60X/Omnichrome 532 or the Cyonics tube described in the section: Cyonics Argon Ion Tube.

    Large frame Ar/Kr lasers may require 35 A at 400 V running on three-phase 240 VAC, 30 kV or more to start, and gallons-per-minute of tap water cooling!

    Why Does an Ion Laser Tube use a Heated Cathode?

    Modern HeNe lasers usually use cold cathodes (some older ones DID use the heated variety). What about ion lasers? You could save a lot of weight if there was no filament transformer, right?

    Think of how much larger the cathode of a HeNe tube is compared to the anode. That much surface area is needed to keep the heating and sputtering at the cathode within acceptable limits. (An HeNe tube WILL lase if hooked up backwards but its life will be significantly shortened.) And that is for a tube using only a few watts compared to a KILOWATT or more for an ion laser.

    The typical ion tube has a thick helix of tungsten for its cathode (calling it a 'filament' is really minizing the massive nature of this structure!). The hot cathode results in thermionic emission (the boiling off of electrons from its surface) reducing its work function - the potential (voltage) drop associated with pulling an electron out and away from it to free space. With less voltage drop, less power is dissipated at the cathode itself, there is less damage and less sputtering, as well as a reduced voltage requirement for the power supply. Also see the section: Effects of Improper Filament Temperature.

    Also note that the cathode isn't just made out of metallic tungsten - it is specially processed and coated to promote the formation of an sufficiently dense electron cloud so that AMPS of current can be passed through the tube with minimal additional heating at the cathode itself. The AC of the filament supply keeps arc moving in the vicinity of the cathode Even so, if conditions aren't perfect, you can end up with localized heating leading to cathode sag and eventual failure. Repeated attempts at starting, improper gas pressure, contamination, and plasma oscillations can result in erosion of the cathode. An ion tube with a cathode that is not hot enough won't start. One that loses its filament supply during operation will go out - and there may be damage to the cathode in the process.

    All in all, what you would gain in not needing a filament transformer, you would lose in having to have a truly HUGE cathode structure to handle the heat dissipation, a much HIGHER voltage power supply, even better then perfect cooling, etc. :-)

    Ion Laser Bore Temperature, Materials, and BeO Warning

    The plasma temperature is hotter then the surface of the sun, way up there. For large tubes like the Lexel-88, there is a magnetic field to push it away from the bore walls. You loose the magnet, you rapidly damage the tube. Most lasers actually use tungsten disks to form the bore and then have the tungsten mounted in copper blocks that spread the heat out over a large area so the water can cool the tube. The plasma is about 1,500 to 2,000 °C, plus it is extremely energetic. When a gas is torn apart like that and given that kind of energy, it acts like a very strong acid and will attack the tube lining.

    This arc temp is well above what would melt glass. There are only 5 or 6 materials that can go into a argon plasma tube and survive the arc: BeO, tungsten, aluminum nitride, pyrolytic graphite and molybdenum. Even the NEC-3030 which has a glass outer bottle, has a BeO tube fused on to handle the plasma and conduct away the heat. If you put enough current through the bore, you get additional wavelengths as the sodium, oxygen. barium, and who knows what else ions start to lase with the argon. Tube materials lasing is not something you want to see despite how pretty it might look. :-) Tubes (and possibly power supplies) don't last long when this happens!

    Keep in mind that even the old quartz plasma tubes have beryllium oxide or tungsten bores, and the glass is NOT in contact with the plasma. Beryllium oxide conducts heat 5 times faster then most metals.

    You will NOT be able to purchase BeO as it can be an extremely nasty thing if mishandled and you breathe the powder. A small but unknown percentage of people will have their lungs damaged leading to an early death. If somebody gives you a old plasma tube made of BeO or containing BeO components, you should NEVER grind on it, open it, clean it with acid, or breathe the dust it makes when it breaks. When they break, they have to be FLOODED with water, and all the pieces sealed in a plastic bag and sent to a special place for disposal (there should be precisely this warning on the tube somewhere). (And, then you may have to have a Hazmat team come in to clean up your house.) Don't mess with it!

    How Tube Dimensions (and Other Factors) Affect Output Power

    In addition to discharge length and bore diameter, the power output of an ion tube is a function of the pressure and current density required for a stable arc of a given length. That's generally proprietary and hard won information, so here goes what's published and what I can measure.

    The following were measured with calipers from end-bell to end-bell and then roughly compensated with eyeball for actual bore length:

    Given the overall length of these tube, the relatively short bores may seem surprising. The remaining length is taken up with the filament/cathode and support structure, and gas filled spaces of the cathode and anode end-bells. Typical Cyonics Air-Cooled Argon Ion Laser Tube where (although the diagram is not totally to scale) the cathode end-bell is actually longer than the actual bore.

    Bore diameters usually range from .55 to .75 mm for small lasers and up to 2 mm for large-frames. Longer tubes require larger bores.

    Most of the information below is from the operation manual for a Spectra-Physics large-frame ion laser.

    The formula for argon output available from a given ion tube operated below saturation all lines multimode output is:

                                  P = K * (J2) * V
    
    Where:

    Notes:

    1. Visible output saturates at about 800 A/cm2.
    2. The gain of most argon visible lines rises with magnetic field to a limit of about 1200 G then it rolls over."

    Most lasers in the less then 1 meter class like about 600 Gauss to start. Too much magnetic field in an ALC-60X type tube, can actually kill power, as we have found (--- Steve).

    Magnetic fields that envelop the plasma discharge enhances the population inversion, it tends to force free electrons toward the center of the plasma tube bore, increasing the probability of a pumping collision, unfortunately the magnetic field also causes Zeeman splitting of the laser lines, which elliptically polarizes the output, causing partial loss at the polarization sensitive plasma tube windows

    The following equation applies to any laser - not just an ion type. Output power can be calculated from:

                                              q * L
                           Po = T * A * I * (------ - 1)
                                              T + B
    
    Where:

    The following are some specific numbers for various lasers (from "Laser Fundamentals" by William Silfast, ISBN 0-521-55617-1):

    Photos and Diagrams of Some Air-Cooled Argon Ion Tubes

    The most common model likely to be found surplus is the ALC-60X:

    Here are some others you might run across:

    (Photos provided by: Marco Lauschmann (mla@sbk-ks.de). Diagram (don't you just love the fabulous colors? :) from: Sam.)

    Ion laser manufacturers and resellers sometimes have photos, diagrams, and specifications on their Web sites or in their catalogs. For example, there are some for current model air-cooled ion tubes (and laser heads) on the National Laser Web site under Argon Laser Products. See the chapter: Laser and Parts Sources for additional possibilities.

    Higher Power Ion Laser

    The unclassified world record holder is a ~2 meter long 30 mm bore tube running at ~240 A with on the order of 500 watts optical output. The limit is determined by how well the coatings on the mirrors can hold up. These tubes are constructed of oxidized aluminum disks stacked together with O-rings and are water cooled. They are actively pumped and leak like a sieve (no sealed tube), hard to start, and use a cold cathode.

    The basic design is by Dr. Sergei Babin at Novosibersk. They are available commercially as one offs at up to 75 watts. Get yours while they are hot! :)

    Compact Low Power Ion Laser?

    We all are impressed by HUGE argon ion lasers putting out 20 W or more that are 6 feet long and require 480 VAC three-phase 50 A service.

    However, what about a really compact air-cooled argon ion laser only capable of a few mW but made as small as possible?

    The problem no matter how you slice it is power dissipation and the bore length required to achieve adequate gain. The smallest commercial argon ion tubes have bore lengths of a little over 75 mm with a diameter of about .5 mm. These may have a lasing threshold as low as 2 A at perhaps 85 V across the tube. Assuming that such a tube could produce 2 mW at a current of 3 A and that amount of power is most that will be needed, the power dissipation of the discharge is reduced to just over 250 W max. For such a tube:

    This could result in a laser head about the size of a soup can (a cylinder 60 mm in diameter by 125 mm in length) with a fan at one end. The power supply would be significantly reduced in size and complexity as well. I wonder if such a laser exists?

    Now, it turns out that there is a patent on a truly tiny ion laser, with a discharge length of 30 mm and a gain region of 10 mm. The electrical input is only 300 mA at 150 V, about 45 W. Check out U.S. Patent# 4,646,313: Inert Gas Ion Laser.

    About Small Air-Cooled Ion Lasers with Huge Power Ratings

    Sometimes, ion lasers turn up that are about the size of a typical small air-cooled unit but have CDRH power ratings of multiple WATTs. In most cases, these are lasers designed for low duty cycle applications. Both the power supply and cooling will be way underrated for CW operation.

    (From: Steve Roberts.)

    For CW work, 4 units of 350 cfm Patriot fans for a model 68B, the HGM5 is an ALC-68B with shortened Brewster stems, a bigger gas ballast, and a slightly wider bore, it can be 3 watt CW laser, but is usually ran duty cycled. I have seen ALC-68s do 7 watts on the bench when freshly made. The HGM does run about 500 mW CW and can be pulsed up to 3 watts max for up to say 15 seconds with the existing HGM fan, which is a big squirrel cage type.

    The warning label is 5 watts on the HGM5, the medical circuitry clips the power at 3 W.

    If you could cool it enough, an ALC-60X size tube can do 2 to 3 watts easily, in fact there is a medical unit that uses a small internal mirror tube at 3 watts using a closed loop water-to-air-cooler in a power-on-demand application at a 5% or so duty cycle. What limits you on an ALC-60X is the glowing red undercooled anode that will open up when you try it - spoken from experience, not conjecture. :-)

    Other Types of Ion Lasers

    While argon and krypton ion lasers are the most well known, there are other types. Two of these are xenon ion and oxygen ion (pulsed only) and neon ion. They are not super-radient and require cavity optics like Ar/Kr types.

    Just about every gaseous element has been shown to lase in the IR and some cases visible or UV, but few will lase CW. Xenon is used for resistor trimmers because in its pulsed mode, its green lines are able to be focused tightly, and its per pass gain is much higher then any other gas laser except copper vapor resulting in a compact high power green laser before frequency doubled solid state lasers were available.

    More Information on Argon/Krypton Ion Lasers

    While most of what you need may be contained in this and the chapters that follow, here are some additional sources for photos, diagrams, theory, and more related to argon and krypton ion lasers:

    Ion Laser System Patents

    General information on argon ion laser tube construction and power supplies can be found in U.S. (and other) patents. A search based a major laser manufacturer like Coherent or Spectra-Physics, or keywords like 'ion' AND 'laser' should get you more than you ever wanted to known. (See the section: Patents for information on on-line patent databases.)

    The following patents are particularly relevant with respect to small ion lasers:

    Each of these patents references others that may be useful as well. You can easily spend centuries surfing the various patent sites!

    While patents do not provide all the details needed to construct your own system, they are valuable nonetheless as a starting point for understanding basic principles of operation and system design. Some of the electronics are described in substantial detail.

    However, some of these appear to match actual hardware very closely. Of particular interest are the two ALC patents. These outline the principles of operation and provide fairly complete schematics of the power supply for the ALC 60X/Omnichrome 532 laser.

    Some information may also be available from the major manufacturers of ion lasers. See the chapter: Laser and Parts Sources for addresses and links.



  • Back to Argon/Krypton Ion Lasers Sub-Table of Contents.

    Wavelengths, Spectra, White Light Lasers

    Argon Ion and Krypton Ion Laser Wavelengths

    While most HeNe lasers output a single wavelength, argon and krypton ion lasers are often designed and/or set up to output many wavelengths at the same time. Not all lines will lase simultaneously in a given laser, some of these are only available in larger lasers with more current density. Others will compete with each other for gain. Therefore special mirror coatings or an intracavity prism (etalon) may be required to obtain output on a few of these lines. Consult factory for details about which optics set is needed for your application. Large output powers at UV and IR will require special tube processing and/or crystalline quartz Brewster windows to avoid losses, solarization, and color center formation in the optics.

    Which lines actually lase are sensitive to both tube current and gas pressure and thus the color balance (relative intensity of the various wavelengths) will shift as the tube heats up and with age.

    To get an idea of the actual perceived color at each wavelength, see the section: Color Versus Wavelength.

    Effects of Optics on Laser Performance

    Different types of optics may be used on a laser to optimize or select a particular performance characteristic. Where one or both mirrors are external, optics sets can be swapped to change behavior. See the section: Substituting Optics Between Lasers.

    Single-line or multiline: This refers to the output spectral lines in the beam. For ion lasers, several wavelengths can be generated simultaneously. The reflectivity curve of the Output Coupler (OC) mirror and tube current determine which subset of the possible lasing lines are active.

    Single-mode or multimode: This refers to the axial mode structure of the output beam.

    The table below shows the distribution of output wavelengths as a function of the type of optics and tube current for an ALC-60X/Omni-532 compatible argon ion tube (specific model unknown).

       Plasma   -------- Laser Output Power (mW) --------
        Tube     Multi- ------ Gaussian TEM00 Mode ------
       Current    mode           ------- Pure Line ------
       (Amps)   -- All Lines --   457 nm  488 nm  514 nm   Lifetime (MTBF) Hours
      ---------------------------------------------------------------------------
          4        20      10       1.0      7.0     0.0      15,000 - 25,000
          6        50      30       2.0     17.6     7.5       8,000 - 15,000
          8       110      70       5.0     27.0    23.0       4,000 -  6,000
         10       220     130      10.0     44.0    42.0       1,500 -  2,000
         12       325     200      15.0     60.0    68.0       1,000 -  1,500
         14       430     280      22.0     81.0    98.0         500 -  1,000
    
    Notes:
    1. The total power output for all lines using multiline optics is greater than the sum of the single-line outputs at the same tube current. This is probably due to the contribution of the additional wavelengths (the other 5 of the 8 common argon ion lines) not shown in the table.

    2. With multiline optics, the relative output power of each wavelength will be roughly the same as those indicated for single-line optics.

    3. With multimode optics, 50 to 100 percent higher power output can be obtained for the same tube current (and tube life).

    Single-Line and Multiline Output

    Since the argon and krypton lasing mediums have substantial gain at several spectral lines (see the section: Argon Ion and Krypton Ion Laser Wavelengths), a given laser can be set up to output on a single line or more than one at the same time depending on how the optics are designed and adjusted. The tube current also affects this to some extent as increasing the current will bring in progressively more lower gain lines.

    A laser set up for multiline operation will usually result in highest total output power but there are many applications where a monochromatic beam is required.

    multiline operation requires a set of mirrors with reflectivities designed to achieve laser operation for all the desired spectral lines. Any intracavity prisms are removed.

    Single-line operation can be implemented in a couple of different ways:

    Etalons for Ion Lasers

    (From: Steve Roberts.)

    The typical ion laser etalon is 15 mm diameter, 12 mm thick, fused silica or quartz, coated to reflect about 10% from both sides, usually with as few coating layers as possible. It's heated 10 to 20 °C above room temperature, stable within 0.001 °C. Without temperature control, it won't be very stable. The laser will flash at 2 to 3 Hz as the etalon temperature stabilizes. You peak it on a bright flash.

    Even a thick plate of window glass will get you started, the trick is to position it correctly and keep it warm. Trust me, try making a simple holder using a (Newport) MM1 or MM2 and stick a piece of high quality glass or quartz intracavity.

    Etalons need to first be adjusted to what is referred to as "Flash". In other words, extremely parallel to the cavity where minute changes in the angle take the cavity in and out of resonance very quickly. They are then walked to a offset angle that sets the choice of single frequency once they heat up.

    I have one of these rugs I maintain running 24/7 locked to an iodine cell with a etalon,

    More Comments on Argon/Krypton Spectral Lines

    An air-cooled argon ion tube will only lase at: 457.9 nm, 465.8 nm, 472.7 nm, 476.5 nm, 488 nm, 496.5 nm, 501.7 nm, and 514.5 nm. The 457.9 nm, 488 nm, and 514 nm lines are common as single lines for argon ion air-cooled tubes due to their strength. The majority of power will be in those lines but the relative strengths will also depend on tube current. See the section: Argon/Krypton Ion Laser Tube Life for a chart which includes argon ion spectral line power versus tube current for single-line, multiline, and multimode optics for a typical air-cooled tube.

    For krypton, the lines are 647 nm and 530 nm. Krypton lines are sensitive to pressure and magnetic field strength. All water-cooled ion lasers have axial electromagnets around the bore to concentrate the arc. A krypton laser will have a high/low field switch as well.

    The tables below list the relative strengths of all the important lines for a typical 30 watt argon/7 watt krypton laser with:

    Normally, optics are selected to support the mission of the laser - i.e., surgery wants only the blue lines; ophthalmology needs green, red, and yellow; Raman Spectroscopy needs 647 and 676 nm; laser shows use argon for blue, green, and violet, and krypton for red and yellow. Mixed gas lasers use optics selected for 55% red, 20% green, and 25% blue and violet. To kill a line, one of the optics is made more then 15% transmissive at that line.

    The 488 and 514.5 nm lines are lower then normal on this list - other manufacturers claim more power for these 2 lines. Note: The total power for all wavelengths adds up to more than 30/7 W because these lines are selected with a prism and are not lasing simultaneously which would result in wavelength competition.

    Argon lines:

         Wavelength    Relative Power    Absolute Power
        ------------------------------------------------
          454.6 nm         0.03              0.8  W
          457.9 nm         0.06              1.5  W
          465.8 nm         0.03              0.8  W
          472.7 nm         0.05              1.3  W
          476.5 nm         0.12              3.0  W
          488.0 nm         0.32              8.0  W
          496.5 nm         0.12              3.0  W
          501.7 nm         0.07              1.8  W
          514.5 nm         0.40             10.0  W
          528.7 nm         0.07              1.8  W
    

    Krypton lines (magnetic field optimal for majority of lines, but not all).

         Wavelength    Relative Power    Absolute Power
        ------------------------------------------------
          406.7 nm         0.036             0.9  W
          413.1 nm         0.07              1.8  W
          415.4 nm         0.02              0.28 W
          468.0 nm         0.02              0.5  W
          476.2 nm         0.016             0.4  W
          482.5 nm         0.016             0.4  W
          520.8 nm         0.028             0.7  W
          530.9 nm         0.06              1.5  W
          568.2 nm         0.044             1.1  W
          647.1 nm         0.14              3.5  W
          676.4 nm         0.048             1.2  W
    

    (From: Skywise.)

    Here's what I have. I gathered it from commercial manufacturer brochures and Jeff Hecht's "The Laser Guidebook".

    Argon Ion:

       Wavelength (nm)  Rel Pwr
    ----------------------------
        1090.0
         528.700         0.16
         514.533         1.0
         501.717         0.2
         496.508         0.35
         487.986         0.78
         488.1
         476.488         0.29
         472.689         0.10
         465.795         0.07
         457.936         0.18
         454.504         0.06
         437.073
         363.8
         351.4
         351.1
         334.0
         305.5
         302.4
         300.3
         275.4
    

    Krypton Ion:

     Wavelength (nm)  Rel Pwr
    --------------------------
         799.300
         752.5
         687.096
         676.457       0.22
         657.000
         647.100       1.0 
         631.2         0.03
         593.3         0.03
         575.3         0.03
         568.192       0.31
         530.868       0.33
         520.832       0.16
         484.666
         482.518       0.11
         476.571
         476.244       0.12
         468.045       0.14
         461.917            
         457.720
         415.4         0.08
         413.1         0.53
         406.7         0.30
         356.4
         350.7         0.32
         337.4
    

    Depending on gas fill, current, optics, and luck, there may be other weak lines present including: 437 nm (argon), and 457.7 nm, 461.9 nm, 657.0 nm, 687.0 nm, and 799.3 nm (krypton).

    As a side note, the color saturation with an ion laser is unbelievable, it's possible to get 16.8 million distinct shades with off the shelf hardware. I know the eye can't resolve that but the results you can see are beautiful.

    (From: Tom Yu (tlyu@mit.edu).)

    I found the following interesting comments on relative power of the various argon ion lines in my Spectra-Physics 164/166/167/168 manual. (These are the medium-frame 1 meter water-cooled Argon or Krypton lasers that want 3-phase 208 VAC at 40 A per line or so.)

    "A more interesting effect in the case of argon, specifically, is that of relative intensity and gain ratios in the case of the two strong lines, 488.0 nm and 514.5 nm. Most of the visible laser transitions in the CW argon-ion laser have approximately the same gain-to-power ratio as 488.0 nm, although they are weaker than that line and generally have less gain. The 514.5 nm line, however, has only about 1/4 the gain of the 488.0 nm line, but has approximately 25% more power output when the gain is sufficient to overcome internal losses. This effect is largely due to the difference in the atomic constants that determine the power-to-gain ratio, owing to the fact that the 514.5 nm upper state comes from a different family of levels than do most of the other transitions."

    It seems that this explains nicely why the 514.5 nm line is quite weak at lower powers but quickly becomes as strong as the 488.0 nm line at higher tube currents.

    (From: Steve Roberts.)

    Here are some additional lasing lines from Alan B. Peterson, In "New Developments and Applications in Gas Lasers" Lee R. Carlson, chair/editor SPIE Volume 737, based on the proceedings of a 1987 conference on gas lasers, pp. 106-111.

    Nobel gas ion lines not previuosly reported (nm):

     Argon: 307.816, 276.223, 437.594
    
     Krypton: 379.270, 330.473, 322.062, 317.22, 304.692, 302.230 
    
     Neon:  372.710, 372.684
    
     Xenon: 377.629, 376.897, 376.226, 373.022, 367.662, 366.675, 365.461, 364.831,
            312.569, 310.863, 304.425, 297.051, 295.478, 282.251, 281.968, 276.778
    

    The book has lasing conditions and power but no details about transmission on the optics. A magnetic field up to 1,600 gauss was used, as most of these are III transitions. The experiments used a type SP-171 and a type SP-2020 tube. The author is/was an employee of Spectra-Physics.

    White Light Lasers

    The term 'white light laser' typically refers to one that is capable of producing a set of wavelengths in the red, green, and blue portions of the spectrum which if mixed in the proper proportion can 'simulate' the effect of a white light source in full color displays and laser shows, and for some spectroscopy or other applications. There may also be colors like yellow produced in addition or in place of others. However, white light lasers generally don't produce a broad spectrum like an incandescent light bulb (though there are apparently some exceptions, at least in the research lab). The individual wavelengths are coherent but not with each-other. The beam characteristics (i.e., diameter, divergence, polarization, etc.) of each wavelength are similar to what would be expected if that were the only one wavelength present. There are no optical interactions between colors (light waves that are not the same wavelength/frequency do not interact with each other in a linear medium) but there may be affects in terms of one lasing transition stealing power from another. For many purposes, the white light laser is equivalent to several individual single color lasers combined into one beam, but in a smaller package at lower cost.

    The most common white light lasers are large frame ion types with a mixture of argon and krypton for the gas fill.

    White light lasers are now even available in air cooled format. All use a mix of argon and krypton. Many are made for a roughly 60:20:20 ratio of red, green, and blue lines for proper white balance. Their reliability is increasing with cost staying a little above normal Ion laser prices. Spectra-Physics, Coherent and Lexel all manufacture tubes for this. And LaserPhysics, Inc. sells the air-cooled version that runs off single phase 220 VAC and does 400+ milliwatts. Most of these lasers are modified for reduced operator skills with sealed mirrors and simplified power supplies. So, yes, they are out there, and laser company reps tell me the demand is going up as people start to use them for lab and industrial applications as well as display.

    There are other ion lasers that aren't optimized for best laser show or TV color rendition but for other applications. For example, some biological mixed gas and biological krypton will kill green, lase red, yellow, and blue, With RYB optics, there will never be more than say 4 lines and no green, not 514 nm, not 520 nm, nor 530 nm. The RYB optic will have a 15% or greater transmission from 500 to around 550 nm. The lasing transmissions are about 1.5% for blue, 0.8% for yellow, and 1.2% for red. If it has RYGB optics, there will be about 7 lines. Note that for the laser enthusiast, these have a high novelty value but are less than ideal for for display due to their wavelengths as noted above. With external mirror lasers, the optics sets can be replaced but this may not be ideal if the gas fill ratio and pressure isn't optimal.

    CREOL in Florida and quite a few other labs have demonstrated RGB as well in diode pumped frequency doubled YAG lasers so smaller and more practical is just around the corner as soon as ways are found around the materials and quality control problems with solid state laser components. Right now they have to test 4 or 5 crystals for every good one they get.

    Note that other technologies can be used for white light lasers. For example:

    (From: Colin Evans (c.j.evans@goose.ac.uk).)

    A white light laser was developed in this department several years ago. It was based on a helium-cadmium mixture which could lase simultaneously at red, green and blue wavelengths. There was no automatic balance between the three colours and had to be carefully adjusted using the pressure and temperature. Also, I don't know whether the three colours could be regarded as "coherent" in any sense. Advantages are very strict polarization, and narrow parallel beams, neither of which are much use in a projector.

    (From: Marco Lauschmann (mq-laser@gmx.de).

    The only real white laser I know of used a Bucky-ball (carbon) compound which was optical pumped by the 488 nm line of a argon ion Laser. The emission was a real white light continuum - not like the 488 nm, 514, nm and 647 nm lines of an Ar/Kr ion laser system which looks like white light to the human eye. Researchers at the University of Manchester Institute of Science and Technology have demonstrated that confined buckyballs emit strong white light when excited by blue light from an argon-ion laser. Although work is at an early stage, the group has already identified some possible applications for this new material. They suggest that it may form the basis of a new laser material or new types of optical displays.

    Another source for a white light continuum is a Ti:Sapphire regenerative amplifier with a frequency doubler. So, a white light continuum could be produced with 800 nm output of 150 Fs, 500 uJ pulses at 1 kHz from a Ti:Sapphire regenerative amplifier, which extends from 400 to 1500 nm. A 5 cm long piece of fused silica is the non-linear element and was used to generate the continuum. This is only an example - there are systems which deliver a white light spectrum with average power of more than 1 W with repetition rates of 250 kHz or more.

    Effect of Tube Pressure on White Light Ar/Kr Ion Laser Spectral Balance

    Here are four photos courtesy of John S. Rubacha that illustrate the laser lines from the LP 300WC Ar/Kr Laser as a function of tube pressure.

    1. White Light Ar/Kr Ion Laser Spectral Lines shows the laser output as seen with a reflection diffraction grating. Note the zero-order line has a degree of pink shade in its color due to lots of Kr red power.

    2. White Light Ar/Kr Ion Laser Diffracted Spots at High and Low Tube Pressure shows the distribution of spots and approximate intensities as seen with the grating. At lower pressure, note the appearance of the deeper blue Ar lines, the brighter 520 nm Kr green line, the much brighter 568 nm Kr yellow line, and the weak appearance of a Kr 687 nm red line.

    3. White Light Ar/Kr Ion Laser Spectral Balance at High and Low Tube Pressure shows RGB and white at the same tube pressures as in the previous photo. The overall white color balance is definitely shifted towards pink/red at high tube pressure, it is near normal at low tube pressure.

    A word of caution. The digital camera does not record with the same range of brightness the eye can perceive visually. Thus the intensities of the laser lines are somewhat "compressed".

    Please note that the 530 nm Kr green line is suppressed on YBR Krypton and White Light lasers as it suppresses gain on the 568 nm yellow line. That's why the normally very strong green line at 530 nm is also missing in the spectrum and pictures. Killing the 530 nm line also kills the 528 nm line in argon because the coating is not that selective. This is in addition to the red/yellow pressure branch problem. This laser was shipped initially very high in pressure - these pictures are the result of what happens when the excess gas is "burned off" after many hundreds of hours of operation. What is missing is the picture of the initial state of this laser, which had just the 488 and 482 nm lines lasing with the red lines, which results in a sort of strange magenta color.

    Ion Laser Dielectric Mirrors

    (Also see the sections: Mirrors used for Lasers and Laser Applications and Estimating Dielectric Mirror Spectral Reflectivity.

    (From: Steve Roberts.)

    Argon ion lasers are generally shipped with broadband optics installed, they are usually a 100 to 200 layer dielectric stack. The high reflectors are coated for 99.999% reflection at all wavelengths (that the laser may be set up produce - they will still be transparent at others). The OC is what is changed. Wavelength selective output couplers are coated for a minimum 15 to 20% transmission on lines that are *not* supposed to lase, and the transmission for desired lines varies from .5 to to 12 %, depending on the length of the laser. Higher power tubes have more gain and thus use a higher transmission. An air-cooled laser will have transmission in the .5 to 1.5% range, short-frame water-cooled lasers will be in the 2 to 5% range and 25 watt large-frame types will be in the 8 to 12% range. It is possible to adjust the coatings for a given color balance if you have a large number of identical tubes. Ion optics are sold as matched pairs, and loosing half a pair can ensure that you will play musical mirrors and maybe not even lase. However some manufacturers have different OCs you can use with a given HR.

    In the case of lasers with a intracavity prism, unless the client pays extra for special optics for a weak line that doesn't have that much gain, a broadband set is still used.

    White Light lasers using Ar/Kr mixes are using a mirror coated for 400 to 700 nm broadband high reflectance and an OC that usually kills the 531 nm line of krypton, as these compete with the yellow line for gain, thus killing the yellow if they are allowed to lase. The 647 nm red and 568 nm yellow lines share the same upper state and thus the optics must be tailored for a given red/yellow balance on a given laser tube design at a given gas pressure, thus making these optics even more expensive.

    A typical 1/2" diameter large-frame optic is $400 to 500 from the factory PER optic. White Light optics are about $2,000 a set minimum.

    Comments on Brewster Window Seals

    Although it may appear that these are glued in place, the Epoxy/TorrSeal is only a backup. Modern tubes usually have their Brewster windows optically contacted whereby the two pieces are superpolished flat and are so clean that they stick together vacuum tight on contact. This weak covalent bonding is then put into a hot oven in an electrostatic field to increase ion mobility and make a more permanent bond, but it is still subject to shear (sideways stress) failure - i.e., you flick it with a finger or tool and it might dislodge, often taking some part of the stem with it. The epoxy prevents the shear stress problem and does provide some small leak prevention.

    Still it's a selected silica filled epoxy with a high Tg. low cost 5 minute epoxy from the drug store or department store wont do it.

    If you dare, take a very sharp razor blade and very slowly press its sharp edge in against a unglued older brewster where it joins to the stem and POP! Off comes the now ruined window and a flat undamaged polished face is usually left on the stem. If you stick that window between polarizers in a stress analyzer, it is now very deformed.

    Comparison of Power with Single-Line Versus Multiline Optics

    The basic question is: Will replacing a multiline OC with one for a particular wavelength result in more power on that particular line because the others are suppressed or will it be pretty much the same? (HRs are often broadband regardless of the particular OC fitted to the laser).

    (From: Steve Roberts.)

    This depends on the line and size of the tube. In a long bore laser, there may be a 10 to 15% gain on some lines while on other lines there will be little or no net gain. Some lines share a given upper state and tube conditions such as pressure and magnetic field determine where they fall. For example, the red and yellow lines of krypton will fight each other. If I recall my Spectra-Physics manual correctly, there are two weak argon lines that can also fight, but I can't remember which ones and it's an insignificant difference in normal operation anyhow.

    For a short tube argon, there may be few percent increase in power with single line optics. They are used for spectral purity in a short tube ion laser. For example, each line comes to focus at a slightly different point on the film or drum in a printer or copier which means you would have to filter out one of the lines or use expensive achromatic optics. Or, a particular line is used for spectroscopy and the other one would increase the noise level if not removed.

    Narrow band dielectric mirrors are easier to manufacture anyhow so they should be cheaper as well - a win-win situation for many applications.

    Substituting Optics Between Lasers

    For lasers with external mirrors, it may be possible to swap optics to change the lasing wavelengths or switch from single-line to multiline, for example, for an ion laser. However, just because a mirror can be made to fit, doesn't mean it will work well or at all. For example, the optics of the ILT-5490 and ALC-60X are the same physical size but will result in less than optimal performance if swapped (for one thing, the radii of curvature are different - 45 versus 60 cm, respectively).

    There are several parameters which must be closely matched to achieve enough resonator gain and make alignment something that doesn't share too many characteristics with Chinese Water Torture. :)

    The best option if you really want to do this (realizing that a partial mirror alignment will almost certainly be needed in any case), is to acquire a replacement OC or complete mirror sets designed for your particular model laser. Some companies sell what they claim are 'high output optics' at similarly high prices for this purpose. Unfortunately, I don't know of any reliable way of determining whether a given product will do anything for you or your laser other than waste an afternoon or more in installation and alignment. Specifications are rarely detailed enough to make a decision on technical merit. So, if you are willing to spend the time, at least get a binding money back warranty.

    However, where you have something sitting on the shelf or a potentially good deal arises, here are some considerations:

    These issues are discussed in more specific detail for a related application in the section: Comments on Acquiring Optics for Home-Built Lasers.

    Which Optics to Use on a Large-Frame Ion Laser

    The following addresses questions along the lines of the following:

    "I have a XYZ corporation small or medium-frame ion laser. It's old, but it seems to have gas. I didn't get optics with it. I don't even know if it's argon or krypton. Heck, it may be mixed gas. What optics should I buy?"

    (From: Steve Roberts.)

    Recent experience suggests that just about any 1 meter class ion laser will lase with the following mirror specs, at moderate power with no major sensitivity to alignment. You may or may not get peak possible power, but IT WILL lase, over a wide range of pressures, gas mixes, and magnetic fields. Mode quality is not guaranteed, and we observed everything from TEM00 to doughnut mode to high order multimode. However none of the odd modes such as 1,2 or 2,2 or 3 to 5 were seen and in each case, a round beam was obtained. When doughnut mode was lasing, the band to hole ratio was at least 20 to 1, i.e., a very small hole.

    These combinations has been tested on a Lexel 88, Lexel 95, American Laser 68, HGM 5, HGM 20, Spectra-Physics 164/168, and Omni 543. In krypton, argon, and mixed gas. In every case, it lased well but not always on every line.

    The HR had a 2 meter radius with 100% reflectivity at desired wavelengths.

    The OC had a 2 meter radius with transmission as follows:

    There is a second yellow line in Kr. It is never spec'd in manufacturers' data because it is hard to obtain without a prism and special optics. There is also no major demand for it, and its gain is much lower then the 568 nm yellow. But we had it lasing day before yesterday. One tube rebuilder specs his White Light optics for this line and kills 568 nm to obtain a stable yellow for planetarium displays.

    530 nm green was killed at the HR during some tests by use of a standard argon or KR OC. In one case, a standard argon HR produced lasing on 2 blues and two yellows in pure krypton. In no case was the 528 nm argon line observed as far as we know, it seems to require a low transmission. (But see the section: How to Get 528 nm in an Argon Ion Laser.) Lasers tested were a mix of Kr only, Ar only and MG lasers. The optic used for the OC was 7.62 mm in diameter. This optic was good for some lasers, but resulted in extra high intracavity powers for others. In some cases performance was better then factory optics. A few larger lasers were tested with a 1 meter mirror at each end and this worked well, but 1 meter mirrors were not recommended for the ALC-68/LExel-88/Omni-543 sized lasers as power was weak due to lack of mode volume.

    Argon optics pairs produced poor low power lasing on pure krypton as their transmission was too high, i.e., the Lexel-95 standard argon optics were very poor and resulted in only a faint blue from the same sized krypton.

    Thanks to Dale Harder, and Bruce Rodgers, and Dr. S. for access to their lasers. Thanks to Karl at Promethius Photonics for providing the high grade chemicals used in this study.

    How to Get 528 nm in an Argon Ion Laser

    (From: Steve Roberts.)

    I wanted 528 nm from an argon ion laser. I'd only seen it once in a huge Laser Ionics tube at very high pressure. Strangely, that tube had a more or less orange glow at the cathode sheath, much more orange then normal. Now I know why. After a year of research and wondering if my prism was walking out of alignment when cranked to the 528 nm position, I found the solution in a old gas laser text the library was throwing out. It turns out you need a trace of neon in the tube to get the right upper state. No wonder 528 is always labeled "special testing required" from the manufacturer.



  • Back to Argon/Krypton Ion Lasers Sub-Table of Contents.

    Ion Laser Power Output and Efficiency

    Expected Output Power from Surplus or Previously Owned Ion Lasers

    So you saw an advertisement for a 100 mW Cyonics-2201 or 300 mW ALC-60X. Should these be believed? Probably not, at least not if they are pulls from graphic arts equipment or some other similar source. There may be versions of these lasers that will do higher output power (possibly with identical model numbers or ones that are close) but they usually don't show up at bargain prices - e.g., the ALC-60A, a current model, will do over 100 mW on all lines, but this isn't the same laser as the ALC-60X!). Lasers from reputable dealers will have their *minimum* guaranteed output power clearly listed and may actually produce much more when new. That Cyonics *may* do only 10 mW and that ALC *may* do only 20 mW - or even less.

    About that 300 mW claim, all I can say is: Ha Ha Ha Ha Ha Ha Ha....

    150 mW yes, 175 yes, 225 to 250, yes on a factory select tube. 300, hum... Rarely and not for long unless it was designed that way. Note where the PSU current limit is set when they claim this. Note that newer high-tech tubes can do this running on 115 VAC. One manufacturer does make a 300 mW sealed mirror retrofit for the 60X. Laser Physics' Reliant series certainly does.

    What happened is when large quantities of these units were in use, a few companies made money rebuilding them in quantity. They bought large quantities of pulls for rebuilding. They didn't care which tube they installed in a unit, as long as it met spec and lasted out the warranty. So therefore once in a while you can hit the jackpot on a used laser and get a hot tube. Once in a while you can also pick up a head that was designed for high power.

    It's with special multimode optics and a high divergence doughnut mode or worse beam shape on a selected tube. Notice how vendors have power graded pricing, this lines up with the factory catalog of tubes. Note that lasers almost always are shipped doing well above the factory rating when new. Yes this gets you 275 mW or so on a fresh new high power tube at 10 A with brand new optics and a sweet new cathode. To sustain it for any length of time it takes 11 A. But I'd want a iron clad warranty I could enforce.

    The idea is buy a hot laser and run it lower then its rated power, and thus enjoy longer life. Thats why you could retune that tube to 110 mW or so, it was derated for longer service and the optics were tuned to run at a fairly constant power over its life. Run it at 60 to 70 mW and enjoy it for a long time.

    And, as with any laser, the CDRH safety sticker or catalog listing may not be an accurate indication of useful or possible output power. Actual performance may be a small fraction of what you expected! This is a significant issue with ion lasers since they have many variables affecting output power (compared to internal mirror HeNe lasers, for example, where the output power is pretty much fixed - it isn't affected in a significant way by tube current or often not even much by age and use). The output power of an ion laser is a strong function of tube current and life expectancy is inversely proportional to tube current! So, the rating on the CDRH safety sticker is likely to be much much higher than what could be used with expectations of a reasonable tube life. And, unscrupulous or unknowledgeable people can list the power based on a ridiculously high tube current where life might only be a few hours! Ion tubes that are physically the same size and interchangeable in a laser chassis also can vary by a large factor in power ratings even if they are new depending on manufacturer and model. For tubes with external mirrors, the type of resonator (single-line fixed, single-line with line selecting prism, multiline) as well as alignment and cleanliness, strongly influence output power. At least you can remedy problems with some of these with some basic maintenance or parts replacement. However, age, total operating hours, and possible prior abuse, are also significant factors affecting ion laser performance and there is little you can do to revive a weak tube.

    Also see the sections: Locating Laser Specifications and Buyer Beware for Laser Purchases.

    (From: Dean Glassburn (Dean@niteliteproducts.com).)

    Most of these lasers came from xerox machines which were set up for single line 488 nm TEM00 running at about 6 to 7 amps when installed. New they would do about 15 to 20 mW in that configuration. There were also slightly different tubes (bore diameter) which would preclude higher current densities as the cross sectional area of the active region was smaller. You can and many do install broadband mirrors which would more than double the output. And, you can increase the current as much as 100% (double) as installed, which would give you the higher power limits advertized (and, of course, much shorter tube life). Additionally if the optics were not carefully aligned it was real easy to catch the rubber between the photocell and the front tilt plate when beam walking the laser which would smoke the rubber onto the front optic and beam splitter.

    A used laser is just that - and priced accordingly unless the tube meets specifications as originally installed. And some units due to the smaller bore diameter will never attain the higher power levels.

    (From: Mike Kenney (MKenny1989@aol.com).)

    Nearly all of the 60Xs made for Xerox came with 200 cm, 488 to 505 nm TEM00 optics - very flat and wern't coated for all of the argon wavelengths. That's why if you had a new tube from American Laser with Xerox optics you might get 85 mW at 10 amps. However, with 60 cm, 450 to 530 nm multimode optics, you can get 250 mW at 10 amp. I just sold one with a new tube (not regassed or refurbished) that was putting out 200 mW at 9 amps. The true capability of these lasers is only really understood by American, Omnichrome, and National laser service and that's about it.

    (From: Steve Roberts.)

    I'd tend to agree, although the hottest 60X I've ever seen on a laser power meter was 225 mW. However when you figure out the uncertainty in the power meter calibration, that tends to jive. Xerox needed a small focal point and the AO used on the sled would tend to diffract out the 514 and 477 nm lines a lot, so the narrow band was needed to get a tight spot. Xerox Sled AOs are carefully AR coated and have a wedge on the backside of the crystal that probably corrects for the diffraction.

    My source for used optics was a rebuilder who had Xerox contracts, he bought heads where ever he could get them, and stashed all the old optics in 55 gallon drums. He gave me a couple of hand fulls of optics from each drum and put them in optics shipping boxes from Coherent Auburn division. I'd get a laser off a used xerox sled and reoptic it using something from the barrel batch, or some bought from a laser engineer living in SLC., usually just the OC, and WHAM! 110 to 150 mW and all lines on many of the units. It would take some matching of the optics to get the best power. Tubes with better heatsinks also did better, so I suspect heat transfer plays a part.

    I also agree that the factory can dial in the lifetime and power to anywhere they want it. There also were other tubes that I knew were brand new but I could never coax more then 20 mW out of them as well.

    The middle of the road optics that I prefer all had 120 cm radius, its a nice tradeoff between beam diameter and divergence compared to the 60-60s, and some customers would gladly sacrifice some power for a tight beam. The hottest 60X I ever saw was a dual side fan 60C that had a TEM01 structure and all lines and was from a HELL Typesetter, it would easy burn holes in the wall. I didn't have a PM with me, but that laser would give my Lexel 88 a run for the money.

    The really good high power "X" OCs have a deep cherry red color when viewed in transmission, as opposed to the straw or yellow color of the tailored copier optics. But you can't just go by the color, for example I've seen I've seen optics from Auburn that have a radial gradient, or "bullseye" pattern that performed just as well as the cherries and were almost transparent. Usually just the OC is doctored, its kind of expensive to change anything but the radius on both optics, and that coating is really dialed in. We have a excellent UV/VIS spectrometer here at work and I can barely make out the changes in transmission from a known high power all lines optic except they bleed out much more 514/528 nm green and have reflectivity almost to the UV. The difference is a fraction of a percent in transmission. I can also easily see the 10% or more changes in transmission on a line that is killed.

    However the thing I always wanted to read was the document that spec'd the OC transmissions versus wavelengths for all the different factory shipping powers, it would be very interesting. I did once find the curves for the RYB krypton lasers developed for confocal microscopy, and the changes needed for a given power were very minute.

    Determining the Power Output of an Ion Laser

    So, you obtained a surplus argon ion laser from one of those printer things and would like to have an idea of whether it is performing anywhere close to what was claimed by the seller. Or, you are just curious.

    However, it should be possible to estimate the power output of a small ion laser using a simple laser power meter such as the one described in the sections: Sam's Super Cheap and Dirty Laser Power Meter or Simple Laser Power Meter Using Photocell. In fact, about all you really need is almost any type of photodiode (those from old computer mice floppy drives are fine), a working 9 V battery (even if it is tired and puts out only 7 V), a multimeter that measures DC mA, and a 1K ohm resistor (to protect your multimeter should the photodiode be connected backwards or decide to turn into a blob of solder). :)

    The approach below makes use of the relative brightness sensitivity of the human eye to provide a reference in comparison to a HeNe laser at 632.8 nm with a known power output. The simple laser power meter can then be used as a relative indicator of ion laser's output at various tube currents. To perform the comparison, the ion laser must have adjustable output (from just above threshold) and the actual power output of the HeNe laser must be known - not just the typically much higher value printed on the CDRH safety sticker.

    This same basic approach (with minor modifications) can be used for other types of lasers with variable power output or through the use of a variable laser attenuator or set of neutral density filters.

    1. Set up the ion laser and HeNe laser side-by-side with their beams an inch or so apart shining on a white diffuse surface. This probably works better if the beam diameter is large enough so that the brightness of the HeNe laser isn't overpowering - maybe an inch or so.

    2. Adjust the output of the ion laser so that it appears to be the same brightness as the HeNe laser. Take your time - this will ultimately determine the accuracy of the result.

    3. Use the chart in the section: Relative Visibility of Light at Various Wavelengths to compute the approximate actual power output of the ion laser.

      For a single-line ion laser, use that wavelength. For argon or krypton ion lasers with multiline (all lines) optics, use 488 nm and 647.1 nm respectively, since those wavelengths will be dominant at low power. Multiply the (actual) HeNe power, P(HeNe), by the ratio of the eye's relative sensitivities at 632.8 nm and 488 nm.

      So, for a single-line 488 nm or multiline argon ion laser, the output power, P(488)0, will be:

                                            .171
                   P0 = P(488)0 = P(HeNe) * ----- = P(HeNe) * .895
                                            .191
      

    4. Without changing the setting on the ion laser, put your laser power meter kludge (LPMK) in the beam path. It may be desirable to remove any beam spreading optics for greater sensitivity if the spot size is larger than the sensor area but this is not critical if you already have enough photons for a strong reading.

    5. Note the reading on LPMK. This is your reference value, I0.

    6. Increase the ion laser output to whatever maximum you desire and/or set it at several outputs and note the reading(s) on your LPMK as well as the tube current or current/light control setting.

    7. The ratio of the LPMK reading to I0 is the relative output compared to the reference level. Multiply by P0 to get the actual power at each setting. A photodiode or solar cell based power meter is quite linear with intensity. Though there will be a slight shift in the wavelength distribution of output spectral lines as the ion tube current is increased - and sensor's response depends on wavelength - this shouldn't affect the accuracy very much.

    It may be possible to do this without any instruments by comparing the power determined by comparison with the HeNe laser to values in the chart shown in the section: Argon/Krypton Ion Laser Tube Life. However, depending on your actual current threshold, it is possible for the relative power versus current relationship to be quite different for your tube and optics.

    Efficiency of Argon Ion Lasers

    (Portions from: Steve Roberts.)

    The output couplers on argon lasers are 5 to 7% transmissive, (much greater than the helium-neon output side mirror). (Note: These values are on the high side for short tubes at least compared to the table out of the service manual for a commercial argon ion laser in the section: Substituting Optics Between Lasers. --- Sam) So an argon has more gain and it scales as a semi-log function of current density. The upper limit is the tube material melting - about 100 watts output at present in experimental (very large) tubes. (A HeNe tube peaks in output power and then declines as current is increased.)

    However, for a typical small air-cooled argon ion laser, 100 mW beam power out for 1,000 W electrical power in is only about .01 percent efficient which is not quite as 'efficient' as a HeNe laser (e.g., 6 mA at 2,450 V for a typical 10 mW HeNe laser - about .07 percent).

    For a mid-size water-cooled argon ion laser - say 4 W out 7,000 W in, the efficiency is somewhat better - about .057 percent. :)

    Some specific numbers for maximum output using multiline optics from a few common argon ion lasers:

                               -------- Tube Input -------   Output
               Laser Model     Current   Voltage   Wattage   Wattage  Efficiency
          -----------------------------------------------------------------------
           Cyonics-2301-20ML     7 A      100 V     .7 kW      .02 W   .0029 %
           Omni-532             10 A      105 V    1.05 kW     .13 W   .0124 %
           Lexel-88             20 A      165 V    3.3 kW     1.5 W    .0454 %
           Coherent-CR18SG      50 A      550 V   27.5 kW    18.0 W    .0654 %
    
    Power outputs (and efficiency) for krypton ion lasers is must lower - perhaps 1/10th to 1/5th of the numbers listed above at the same power input.

    So, from this very comprehensive listing, the larger the laser, the more efficient it is likely to be when operated at full power. Note that this doesn't take into consideration the losses in the power supply - figure another 10 to 30 percent reduction in efficiency for that!

    On the Abysmal Efficiency of Ion Lasers

    Now, in the miscellaneous trivia department, here is something to ponder at night if you are unable to sleep (this is a guaranteed cure for insomnia): So, for a tube producing 100 mW of laser light (2.5*1017 photons per second), each argon atom must be getting ionized and participating in stimulated emission only about 25 times a second (though, due to gas circulation, they aren't likely to be the *same* atoms!). And, on average, it takes 240 electrons to produce one photon of useful light! I might be off by a factor of 10 here or there. Now, think of how much power we could get out of one of these things if it were possible to increase the number of lucky argon atoms involved in the lasing process and they were kept busy continuously! :)



  • Back to Argon/Krypton Ion Lasers Sub-Table of Contents.

    A Typical Small Air-Cooled Argon Ion Laser - the ALC-60X/Omni-532

    Description of a Typical Air-Cooled Argon Ion Laser

    Unlike HeNe lasers, there are not that many models of argon ion lasers out there and even fewer available to the hobbyist-scrounger type. Therefore, the description below, while generally applicable to most low to medium power argon ion lasers, is written from the point of view of the American Laser Corporation Model 60X which was second sourced by Omnichrome, Inc. as the Model 532.

    The Omni-532 is for all practical purposes a exact drop-in replacement for a ALC-60X. Or, that could be reworded that an ALC-60X is a exact drop-in replacement for an Omni-532. Both lasers were made to the Xerox "X" open standard. While there are minor differences in the electronics, there are major differences in the construction. For example, Omni-532 heads have a cast aluminum alloy L shaped resonator while ALC-60Xs have the traditional rod and end-plate resonators floating on a baseplate.) There are also proprietary differences in the ion tube construction, but their I-V curves are very interchangeable.

    Other air-cooled argon/krypton ion lasers are similar but not identical. Keep this in mind where specific component values or designs are described - variations are likely where a different laser is concerned.

    An air-cooled tube is a neat little thing about four times the diameter of an average glass HeNe tube. Most have external mirrors and Brewster windows, but many are of the sealed mirror variety. What they all have in common is a heated cathode (like a vacuum tube such as a magnetron) requiring 3.2 volts at 10 to 25 amps. They operate from a range of 4 to 10 AMPs through the arc (Yes, that is AMPs) at around 100 VDC. The tube current is fed to the cathode via a center tap on the filament winding of the transformer to balance the arc on the center of the cathode to avoid plasma etching of the cathode supports. Hence the need for a beefy transformer with #14 or #10 wire on the secondary. Rewound microwave oven transformers work well for this purpose.

    The tube is designed for a 100 to 105 V voltage drop, and is ran directly off the rectified and filtered AC line. This makes regulating the tube current a very interesting problem in design because we also have a series injection igniter (similar in function to a HeNe starter) which is a 3" toroid with 80 turns on the secondary and one turn on the primary. A 10 uF cap is charged to 110 or 400 V depending one the model of laser and is dumped directly into the 1 turn primary through an SCR which has a reverse connected fast switching (10 ns) diode across it. You end up with a 500 Khz 30 kV ringing wave pulse applied to the tube, which can blow the arc out as well as ignite it. The winding on the igniter transformer is also #14 wire as it also carries the entire tube current. There is no ballast resistor (as would be found in a HeNe laser power supply) as it would have to dissipate up to 1,000 watts at times. There is a .2 ohm resistor in the anode lead inside the head to sense the current for feedback to the supply, and a beam-splitter sampler that drives a solar cell for the fine loop, which keeps the light level constant to .05% and is used to cancel out noise and oscillations in the beam.

    An air cooled tube's current may be regulated in a variety of ways including a series pass-bank of 4 high power NPN power transistors in linear mode, with two 700 V, 20 A PNP transistors ahead of them in switch mode; two 400 V, 25 amp FETs are used in a buck mode converter at 80 Khz; or just a linear regulator. Larger water cooled lasers which run off three-phase and need 20 to 35 amps of tube current use about 100 large NPNs in series/parallel strings for fine adjust and SCR's on the incoming phases for course adjustment.

    The fun part starts when you buy the laser, the power supplies are scarce and run about $900 to $1,250 used. When the laser tubes are pulled for a rebuild every 5,000 hours the PSU stays in the photocopier/printer/medical instrument/typesetter or whatever until the whole unit is discarded. So the laser heads show up, but supplies keep their initial value.

    A tube is good for 2 to 3 rebuilds, and after 5,000 hours they usually have 1,000 to 2,000 or more hours left for they hobbyist to enjoy. Most of the lasers are built as 150 milliwatt units and ran at 20 milliwatts to enhance lifetime, so even an old laser still has a lot of potential.

    There is no book on how to maintain these things either and since it is the Holy Grail of laser hobbyists to own one, maybe it's time they learned how to maintain them, clean the optics, align the mirrors, peak the performance and find out how to avoid paying $3,800 for a used one when you can get one for less then $1.000. I (Steve Roberts) paid $125 for my head, and built my own power supply.

    Also see the section: Maintenance, Alignment, and Modifications of the ALC-60X Laser Head for much more detailed information on the ALC-60X/Omni-532 laser.

    Photos of the Major Components of the ALC Model 60X Laser Head

    This laser was also second sourced as the Omnichrome Model 532.

    (Some of the following photos are from Steve Roberts and Jeff Keyzer (jkeyzer@ucsd.edu).)

    Additional photos of the ALC-60X useful in conjunction with maintenance and alignment procedures can be found starting in the section: Maintenance, Alignment, and Modifications of \ the ALC-60X Laser Head.

    Also see the Laser Equipment Gallery for for many more detailed views of ALC laser heads and power supplies.

    Physical Description of ALC-60X Laser Head

    Refer to photos in the section: Photos of the Major Components of the ALC Model 60X Laser Head while reading this description.

    The ALC-60X laser head is a box about 13-1/8" (L) x 6-1/8" (W) x 4-1/2" (H) made of goldish colored (alodyned) aluminum with thick ends plates - actually not part of the box structure but rather the movable parts of the mirror mounts.

    The primary structure is composed of three, 3/8" InVar rods placed near 3 of the 4 edges of the box. They are bolted to the fixed portion of the mirror mounts at each end. The rods extend through these plates and another pair of thick plates - the moving part of the mirror mounts. They terminate in the large hex (you have to use a wrench) mirror adjustment screws.

    For the complete schematic of the ALC laser head, see the section: Omnichrome 150R Power Supply and 532 Laser Head (Omni-150R/532). The Omni-532 is identical electrically to the ALC-60X in all major respects.

    The interior is dominated by the 60X ion tube, its riser box with the main core heat sinks (2) and the anode and cathode heat sinks (the latter with its thermal protector). The tube is mounted on four 6-32 studs that are insulated from the chassis and provide height adjustment to center it with respect to the optics at either end.

    A pair of desiccant flasks are mounted on the wall on the panel-side of the box. These are connected by rubber tubes to the mirror mounts (or should be). They are filled with a silica gel desiccant to absorb moisture and thus keep the optics in a dry environment. They also allow the air inside the optics assemblies to expand as the temperature changes but remain isolated from the dirty outside air (like all that messy toner). If your 60X arrived with the rubber tubes disconnected or missing, the silica gel likely needs to be revived (heated in an oven to drive out the moisture) or replaced, and the optics will probably benefit from a good cleaning.

    The ignite card is mounted on the opposite wall and contains everything except the large blocking diode which is mounted separately on its own heat sink.

    The light sense preamp card is mounted at the output-end of the box.

    All the wiring in the ALC-60X is the same boring white (at least after you get all the toner off of it!) and bundled together so tracing anything is virtually impossible. You have to use the schematic and an ohmmeter!

    Steve's Favorite Questions About the ALC-60C/Omni-532

    The Intracavity Prism

    This won't be present if the laser was set up for all-line or fixed single line operation using single-line optics as is quite likely with surplus heads.

    On the opposite end of the output aperture, there may be something that looks like an angled mirror or prism covered with a metal cap. This is called an intracavity prism and is used to select which line lases. It operates by deflecting each of the lines by a slightly different amount - as any ordinary self-respecting prism would to create a spectrum. :-) Only photons of the selected wavelength make it through the prism-mirror combination in such a way that they bounce back down the bore of the cavity and contribute to the lasing process. (The intracavity prism can be replaced with a broadband mirror for all the lines to lase.) Turning the vertical adjust nut/screw/knob on the rear mirror will select the line. Set the current at around 8 to 9 A when you adjust it. Beware of touching adjustments on the prism itself.

    A common type is called a "Brewster prism". An example is shown in Wavelength Tuning Assembly using Brewster Prism. This shows such an optic mounted between the Brewster stem of a 60X type tube and the HR mirror. A precision screw (not visible in the photo) allows the angle of the prism to be adjusted to select wavelengths.

    One elegant way of implementing the prism-mirror combination is with a single element called a Littrow prism. The Littrow prism is shaped in such a way that light entering its front surface at its Brewster angle is refracted by the precise amount needed such that it reflects back along the identical path from the prism's rear surface. If this is broadband HR coated for the wavelength range of interest, there will be essentially no losses when oriented correctly within the cavity of a polarized laser (e.g., using a tube with Brewster windows). However, the angle of refraction does change very slightly with wavelength so that the Littrow prism can be used for line selection by adjusting its precise orientation with a micrometer knob.

    Also see the section: Typical Behavior of Wavelength Tuning Assembly.

    Typical Behavior of Wavelength Tuning Assembly

    This is from the ALC-60X/Omni-532 user/service manual and lists the typical optical power levels on each spectral line as a function of the thumb-screw position. It is altering the precise angle of the intra-cavity prism.
          Spectral       Plasma Tube Current         Thumb-Screw Rotation
            Line        6 A      8 A     10 A      Clockwise from 514 nm Line
       -----------------------------------------------------------------------
            514 nm     6.8 mW  24.0 mW  48.0 mW                0 Turn
            501 nm     0.0 mW   1.2 mW   5.0 mW              1/4 Turn
            496 nm      .9 mW   4.5 mW  10.8 mW              3/8 Turn
            488 nm    17.6 mW  37.0 mW  60.0 mW              1/2 Turn
            476 nm     2.4 mW   7.3 mW  14.3 mW              3/4 Turn
            472 nm     1.0 mW   3.5 mW   7.5 mW              7/8 Turn
            465 nm     1.5 mW   2.3 mW  11.5 mW            1     Turn
            457 nm     1.3 mW   4.6 mW  10.0 mW            1-1/4 Turn
            454 nm     0.1 mW   1.1 mW   2.5 mW            1-1/4 Turn
    

    ALC-60X Tube Voltage and Life Expectancy

    (From: Steve Roberts.)

    I've seen tubes down to 99 V (at around 10A) and still working, but the usual range is 101 to 115 V for a X tube, with 101 V being dying and 115 V being overfilled. This depends on many factors including tube bore (not all tubes you may find in a 60X head have the same size bore) and fill pressure. It's a rough guide used by techs working on a system to judge performance and remaining life, usually done by comparing the voltage at a reference current while watching a power meter, and then checking against the factory test sheet for that given tube. You really need a reference to go by, it is not an absolute method unless you have a pile of tubes for comparison tests or the factory sheet for a given model and submodel. That is why nothing more specific can really be given here.

    About Those Extra Electrodes on Some ALC-60X Tubes

    On some older 60X tubes, you will find a thin wire electrode on each Brewster stem which connects to the interior via a glass-metal seal (but clears the bore) and is wired into the starter card - but have nothing to do with starting.

    These are called "catenodes" and are there to aid in keeping residual dust inside off the Brewster windows. They were included before the source of carbon dust in these tubes was eliminated. The catenodes work by neutralizing the charge on any particle before it could hit the window using a soft glow discharge from the wire to the inside of the tube. The plasma greatly accelerates any dust in the tube toward the most neutral place - the Brewster windows, which are the last places you want it! They were no longer required once better processing techniques allowed by hard-sealed windows became available. A few older SP medium and large frame ion lasers also used to have them.

    The catenodes do not particularly aid in starting and the tube will run just fine without them. However. the boost voltage on the start card is the only voltage high enough in the PSU to fight the tube and cathode drop. They only need a few hundred microamps to function so the connections are via 88K resistors. All 60X start cards have the pads for the resistors, even after they stopped making tubes with the catenodes.

    Comparison of Argon and Krypton Ion Tube Characteristics

    (From: Steve Roberts.)

    A fresh 60X argon ion tube should drop about 106 to 109 Volts at 10 amps doing 95 to 107 mW, all lines, with TEM00 optics. This would typically mean a 60 or 200 cm radius output coupler (OC) and a flat high reflector (HR). It outputs about 200 mW with a 60 cm radius OC and a 60 cm radius HR. Of course the diameter and divergence suffer!

    Larger bore 60X tubes designed for argon can be converted to krypton. However pure krypton is usually not an option for most 60X tubes as the bore and gas return are much smaller then required. Although adding 30 to 40% argon will help with the problem of the igniter pulse blowing the krypton out, stability is hard to achieve. Adding a ballast tank with a large flow path to the cathode-end can cure some of the problems by surrounding the cathode with gas when the krypton is sucked down the bore by cataphoresis. Gas velocities in an ion tube can reach 30 cm/second in larger tubes and the argon and krypton have a tendency to separate to different ends of the tube making the discharge unstable. Krypton really needs larger gas returns and some form of ballast. In modern water cooled lasers, ballasting is handled by the spaces between the bore disks and the large cathode bell.

    Due to Paschen's law, in some cases the krypton gas will find a low current, low amperage discharge down a long return path to be more sustainable then a direct path down the short bore. One company compensates for this by adding a card to their mixed gas power supplies with a comparator. If the supply senses a ignited return path the current will be much lower then the normal idle and the supply will then start pulsing the igniter till the tube lights down the bore.

    When filled with krypton, the same tube with a 45 cm radius OC and a 45 cm radius HR outputs 647 nm and 676 nm red at about 35 mW while dropping ONLY 85 volts at 10 A. These were the only optics we could find, and were less then optimal.

    Krypton runs at a lower voltage, but unlike argon which is a semi-log curve in output versus current, krypton has a knee curve for gain. There is a certain threshold above which all hell breaks loose. I doubt we were at that threshold and we didn't have time to experiment with the pressure of the fill. Below the curve you get mostly 676 nm. A 60X emitting a cherry red beam is a rare sight indeed and we did it just to see if it could be done as many people told us it could not be! We even took it to a conference to ensure witnesses!

    More recently, Steve did the following:

    An air-cooled 60X filled with krypton will tend to lase on just the 482 nm blue line at lower pressure then transition over to some 568 nm with less blue, then finally the 647 and 676 nm lines will lase with yellow being long gone. This is if you can get it to ignite stably in pure Kr and add a big ballast. There actually can be a crossover region between yellow and red with NO lasing of either!

    Note that Gold box power supplies are highly stressed to nearly explosive running a 85 volt tube at 10 or 11 amps. Note also that a Kr+ tube needs to be driven much harder for a given power level, so the 11 amps at a lower tube voltage is approaching the more normal conditions of 1,600 watts in argon operation, but is still hard on the PSU and is vulnerable to acoustic plasma oscillations inside the tube. Acoustic in this case means 20 to 60 kHz and these oscillations can rip apart a PSU. I have already distroyed one PSU testing this, but switching to the destruction proof resistor PSUs decreases your chances of a long tube life during initial processing by ignoring oscillations you may not see in the beam. A hybrid of resistor and linear is required, as well as a variable energy igniter, as its easy to blow out a krypton arc with a hot ignite pulse. While a different igniter helps, having more voltage across the unlit tube boosts chances of a cathode spot forming and the discharge starting. A "simmer" power supply similar to those used in arc lamps may also be a aid in starting.

    You also need white light or red optics with a 0.5% or less transmission in the red and need the multimode 60/60cm RoC configuration to achieve any decent level of power. We're talking about maybe 30 mW here unless you have a real large bore high power tube or a factory krypton tube and the tube redone was a working argon doing 110 mW or more with a fresh cathode. It will be hard to achieve white balance, with pure Kr you generally end up with one of the following three combinations: "bright yellow, dim blue", "dim red, dim yellow, and dim blue at very low powers", or "brighter red and bright blue" at higher pressures. Adding argon gets you RGB and higher overall power up to a point at the cost of total loss of yellow. Argon raises the tube voltage, making it more acceptable to the PSU, but red starts falling as the pressure rises and the percentage of Kr decreases in the tube. It's a tricky balancing act. The 488 nm line of argon will tend to dominate if argon is added, with the 482 nm Kr blue falling off due to its higher ionization voltage.

    (From: LaserguruChris (laserguruchris@aol.com).)

    Krypton gas gets "burned" off a lot faster then argon requiring a much bigger gas ballast so they physically will not fit inside a 60X laser head. Power output is really low as well at this size, no more then about say 20 or so mW at balls to the walls current at a wavelength that is laser pointer red at 647 nm. This is not cost effective compared with say diode lasers or helium neon lasers.

    Nevertheless one such laser does exist, and while not pure krypton it does contain it. This tube is designed to lase on lines of both krypton and argon. While basically similar to a typical air-cooled argon ion laser tube, it has a large external gas ballast tank and would not fit inside a 60X head. It is used in a strange medical device called cytomeric analyzer (sort of a cross between a laser particle counter, an absorption spectrophotometer, and a high presure liquid chromatograph). These machines used to use a HeNe laser for counting and a filtered broadband light source to do the detecting. The multiple wavelengths of the white light laser make this machine work much faster as the sorting and counting can be done at the same time.

    The Larger Omni-543 Laser Head with Omni-160 Power Supply

    (From: Steve Roberts.)

    The Omni-543 comes in variants from 300 mW to 800 mW. The PSU is very similar to an Omni-150, but may have 4 MOSFETs in series-parallel instead of the normal 2 in series. The filter caps will be in series instead of parallel and the filament transformer will be a 220 VAC model.

    There is a long glass tube in the laser head which is a gas ballast. Take extreme care as that ballast tank is exceedingly fragile!!! Don't even set it down on a table hard! Maybe 1/2 G of force max! The short rubber tubes are the connectors to the desiccant flasks for the B-windows and mirrors. Otherwise the thing is basically an Omni-532 with twice the arc length and different bore diameter. As expected, it needs about twice the cooling as the Omni-532.

    The tube voltage is high enough that it probably only will run 220.

    The Omni-160 power supply is basically similar to the Omni-150 but with additions specifically for the higher tube voltage and possibly for use with White Light (Ar/Kr) ion laser heads:

    Converting a Small Air-Cooled Argon Ion Laser to Whitelight

    So you have an ALC-60X tube that's dead or produces unimpressive power and you'd like to convert it to spectacular RGB whitelight. In a few words, it's probably not worth it.

    (From: Steve Roberts.)

    The four big issues are:

    1. Cost of optics: $700 to $1,300 a set, plus tighter tolerance on low transmission optics. A tenth of a percent difference can kill lasing on a small tube, while it would be tolerated on a small frame with a longer tube. Getting a coating house to agree to that tight a tolerance means you need to pay for their mistakes. Since the minimum order is about 15 sets to get a run done......

    2. Gas return bore diameter: On most air-cooled lasers, the gas return line is not big enough for adequate krypton flow. (Note: This is probably true even of commercial systems like the Omni (now Melles Griot) 643, see the previous section).

    3. Cataphoresis: This separates the argon-krypton mix, meaning your Kr is at one end of the tube and Ar is at the other. This is a function of bore size and it kills power in small tubes.

    4. Tube voltage drop: This is lower with a mixed gas fill. Asking a supply designed for a 106 V tube to drop down to 75 to 80 V really stresses the supply.

    Minor issues:

    As for small 488 only tubes designed for tem00, the bore is WAY too small to support multimode multigas operation, the bore diameter is designed to limit the mode and prevent you from upgrading a 10 mW tube to a 300 mW tube. usually small tubes have a minimal amount of ceramic in them, and couldn't take the additional heat stress from full current operation. Cost is important for small argons, and lots of steps are taken to get that cost down, most of which really limit bore energy.

    You're better off using a nice 80 mW 658 nm diode and an air-cooled all lines argon ion laser. Since a good power supply design can drive an argon ion laser head to do 200 to 300 mW off of 115 VAC these days, diving down to ~70 mW of whitelight power is a drag. Whitelight tubes do not excite argon directly, a charge transfer effect from the much heavier Kr atoms pumps the AR, so you can loose a lot of 514 nm and a big amount of visual punch.

    Exception: Certain tubes do well in a green killed low power red yellow blue mode, using mostly krypton, at a 5-5-5 mW or 15-15-15 mW power level. It's visually a blue white in appearance, but not too useful outside of instrumentation/scientific applications.

    Comments on Omni White-Light Conversion

    Melles Griot now sells what is basically an Omni-643 under their label. There are a number of argon and argon/krypton versions available. The Ar/Kr versions aren't of course a conversion as described below, but probably do use the same tube as the argon-only 643s.

    I have tested a 643-RYB-A01 which is an Ar/Kr ion laser which produces red (647 nm), yellow (568 nm), and blue (several lines including 488 nm). Green is suppressed by the mirror coatings for its intended application, possibly because green would overwealm the expected fluorescence signatures being detected. It reaches rated power (47 mW total) at 6.4 A but will produce over 135 mW at 9.4 A (according to the test sheet that came with it).

    However, a funny thing happens at about 8.6 A (assuming the test point calibration is correct): The color balance suddenly changes to less yellow and more blue and/or red AND the current drops to about 6 or 7 A. It has been suggested that this is hitting the gas return limit whereby the argon and krypton circulate at different rates and thus the gas fill mix in the bore changes. It's necessary to drop the current back down to near idle at which point the color balance changes back, equally suddenly. So, I'm not sure how it was tested at 9.4 A or whether perhaps it needs to be run for awhile to correct the gas pressure, having not been used for at least several months.

    In fact, I measured approximately 150 mW total at just under the transition current, maybe 8.4 A. So, too bad about the gas return limit. Otherwise, it could go much higher!

    (From: Steve Roberts.)

    White Omni-643s have the same basic tube bore diameter as the higher powered argon version. Converting an argon ion 643 to white-light is possible. Max power would be about 60 to 70 mW assuming the tube has a good cathode. The mix would be almost all krypton, much more then a few percent argon, and it fails to start easily. Whenever we try to up the argon ratio for more green power, we can't start the thing, and you need a 220 VAC supply or a boost transformer. Others who have white-lighted these have made the same comment, getting a gas mix that will stay lit and deliver green power is tedious.

    However, I wouldn't white-light it, I'd yellow/red it. They scream with a RYB optic and you can get mondo yellow. that yellow is a gorgeous golden yellow, and if you have a little blue coming out you get a beam that appears bluish white. Add the red, dial out some blue and a it's a color you'd never see on a TV or monitor phosphor or any other light source, a very radiant saturated "Sunkist" orange. If you ever get, say a Lexel-95 krypton, put in a RYB OC optic from an air-cooled laser, and a broad-band HR, and you get the same orange, but much more of it. Most laser techs like the 647-488 or 633-488 mix, known in the laser show biz as "Atomic Flamingo Magenta", but I like the 647-482-568 (or 575) Orange!

    If you use the standard Omni optics you get a really intense yellow line, a nice 647 nm red line, 488 nm argon and 482 nm krypton blues, some krypton greens, and the 676 nm Kr line. A buddy of mine has one on station right now and is experimenting with different mixes. Strangely, 514 nm argon green hasn't shown up in our tests.

    However, it's a crap-shoot on rebuilding small tubes. If your tube is factory and still has a positive or neutral delta-T it is probably a candidate for repump with a 60/40 chance of some success, if it's been run with a negative Delta-T, it probably would need a new cathode installed. This assumes the Brewsters are squeaky clean on the inside. Aim a bright HeNe laser beam down the recently cleaned Brewster. If there is much scatter from the inside face, then forget it. Argon with the much higher gain doesn't get attenuated as bad from the inside scatter.

    The real determination of how well krypton will work in a given tube is very dependent on the gas returns as well as bore size. Higher power argon tubes will go krypton, as their returns have higher capacities. Tubes that were designed for 5 to 30 mW of argon with a small bore to limit power just wont go red, we can get stable plasmas, but not in the lasing region.

    Getting the Green Out of a Melles Griot 643-RYB-A01

    On many multiline ion lasers, some lasing wavelengths are deliberately suppressed either because the application doesn't need or want them, and/or because they rob power from the desired lines. For example, the Melles Griot 643-RYB-A01 Ar/Kr ion laser described in the previous section has an OC mirror that suppresses *all* green from the output. So, there are several blue lines, yellow at 568 nm, and red at 647 nm. But I really wanted to see green as well. I know that the HR mirror is broadband coated but the obvious approach of replacing the OC mirror with one coated for all lines would have required a mirror that fit physically, partial disassembly, and realignment of the laser. Even if I had such a mirror, this was not something I was looking forward to doing.

    So, I set up an adjustable mirror mount just in front of the output aperture of the laser and installed the broadband OC from a large-frame argon ion laser. Its reflectivity was about 99% from 470 to 520 nm, RoC unknown. Since there is always output from the normal pair of mirrors in the laser, it is trivial to align the additional mirror. There are multiple spots from the multiple reflections between the auxiliary mirror and original OC. When all these spots merge into one, the mirror is perfectly aligned, about 30 seconds on a bad day. Near that point, a nice strong green output appeared in addition to the original red, yellow, and blue. However, the green was excessively powerful, cutting the intensity of the yellow line significantly. The reflectivity of the new mirrors was too high at the green wavelength.

    So, I replaced the first mirror with one having a reflectivity of around 96.5% from 470 to 520 nm and an RoC of 2 meters. That restored most of the yellow with some reduction in the green. Only one green line, probably at 530 nm, was present. But there was an additional blue line not prevsiously present at idle current. A mirror coated for somewhere between 90 and 95% at 530 nm (narrow band) would probably be ideal to produce a balanced RYGB laser but I had exhausted my inventory of suitable mirrors. :)

    In both cases, the output was quite stable with none of the unpredictable behavior associated with trying similar stunts with lower gain HeNe lasers. The color balance remained about the same (visually) as the current was increased from 4 A to 8 A.

    CAUTION: With the high photon flux between the OC mirror and auxiliary mirror, the monitor photodiode is totally saturated so that light feedback always operates at the lowest (idle) power setting regardless of knob position. Therefore, only current mode could be used to increase the output power. The photodiode was undamaged but I'm not sure that can be guaranteed at higher power in current mode and/or with a higher reflectivity auxiliary mirror.

    Boosting Air-Cooled Ion Tube Power Using Magnets?

    Water-cooled ion lasers generally have a solenoid type magnet surrounding the tube to concentrate and center the discharge. This both stabilizes the arc and boosts power output. Either a set of permanent magnets or an electromagnet (probably with adjustable current) fed from the main power supply may be used.

    Some companies offer permanent magnet kits for use on specific models of air-cooled ion lasers. Do these work or are they in the same class as magnetic water treatment? What if anything can you expect from a $50 to $200 investment? Is there an alternative using materials found around the house?

    The first problem is that there really isn't any good place to put magnets in most air-cooled ion laser heads. With the massive heat sink/cooling fins surrounding the tube, the magnets can't be placed close to the bore where they will do the most good. Therefore, they must be much more powerful than the type used with water-cooled tubes. There really isn't much space to put a large solenoid in a 60X type laser head which limits you to a series of permanent magnets with the same pole facing inward toward the axis of the bore. They would have to be very powerful magnets being so much further away from outside the head sinks. Getting any sort of uniformity out of such an arrangement would be tough.

    With some tube/head designs like those used by SP and NEC, this might just be impossible with their square/rectangular heat sinks. For the ALC-60X and Cyonics tubes with cylindrical heat sinks, there may be space - barely. But, being unable to implement a continuous, uniform, axial magnetic field, don't expect anything dramatic.

    In the end, even the advertising for these add-ons only claim a 20 percent improvement at most. As noted below, actual performance may be much less than this. And, the power may even decrease. The only way to confirm any change would be with a laser power meter - your Mark-I eyeballs and mushware based CPU won't be able to reliably compare beam brightness before and after the magnet installation.

    If you have some high strength disk drive positioner rare earth magnets - the kind requiring a small crowbar to get apart, some spare time on your hands, a laser power meter, and are willing to risk your laser head should something go wrong like a magnet working loose at the wrong time, you can try some experiments. Several identical (same strength) magnets will need to be arranged equally spaced around the tube - preferably at multiple locations. They must NOT interfere with airflow and be placed such that their fields are not short circuited by ferrous parts close to the tube. They must also be securely mounted. All must have the same poles (N or S) facing toward the bore.

    In the end, increasing the tube current by .5 A will probably be a more reliable way of boosting power output by an equal amount (of course, there will be some decrease in tube life, but you can't have everything!).

    (From: Steve Roberts.)

    One, you need a high temp magnet, most wont hack the repeated thermal cycles. If they heat up above their Curie point, any magnetism is history. One company had been offering an add-on electromagnet. Radio Shack magnets need not apply for this position - they will heat up, drop off and block airflow. (They are also quite weak in comparison to what may be needed - Sam.)

    Two, you need a fresh tube, with lots of green, as the green lines appear to be the field sensitive ones. The gain using a non-focused magnet structure (which is what you're likely to get) is a pittance of that obtained with a proper electromagnet around the bore. It's also strongly current dependent, and doesn't work at low currents or on tubes at low pressure. Don't bother unless your laser is already doing at least 80 mW. That's from multiple tests on multiple tubes.

    Three, only certain 60Xs work with this, the ones with soft iron rings around the cooling fins, inside the riser box. Because thats where the magnets go. They get stuck on the rings, like poles facing inward (usually 3 magnets per ring or 6 magnets per ring) to form a soft theta pinch on the plasma. One magnet having more or less strength then any of the others can mess you up. We're talking magnets you can barely separate by hand.

    Some stats:

                    ---- Output -----  --- Net Gain ----
                     Before   After     Power   Percent   Cost
                    -------------------------------------------
                      62 mW    67 mW     5 mW     8.1 %    NA
                      80 mW    87 mW     7 mW     8.8 %   $50
                     117 mW   135 mW    18 mW    15.4 %   $70 
    
    However, on many tubes, no net gain was observed, and never on any tube below 45 mW. The largest gain was still much less than 20 percent even on the 'hottest' tube. (Where cost is given, it is for an array of magnets.)

    What you really are doing is not creating a true radial plasma pinch, but perhaps moving a distorted streak of plasma around inside the tube and just happen to stumble on a path with more gain, as these magnet arrays have poor field shapes. And, it's possible to go down as well as up.

    In conclusion, in cases where this works, it's only really at high current, it's hard to get those magnets into place, and you have to disassemble the riser to position them. Not a task for the timid.

    Power Limitation of Air-Cooled Tubes

    So, why can't you replace the killer fan with a jet blower or some sort of water cooling system and get much more power from your air-cooled tube? Aside from the practical issues of plumbing and having line voltage in the midst of water, there are more fundamental reasons why this probably wouldn't work even though some ion tubes like the Cyonics/Uniphase may use pretty much the same cathode/filament for their 10 mW tubes as their 5 W ones.

    One might also want to do this simply to eliminate the roar of the killer fan without exceeding the normal current limits. However, even this may not be easy. Here is a post mortem report from someone who actually tried water cooling an air-cooled argon ion tube:

    (From: Dr. Destroyer of Lasers.)

    I got the bright idea that it would be swell to water cool an air-cooled laser head, not so much to boost power as to not have to deal with the noise of the fan. After doing some experimentation, I constructed a shroud out of food tins and shim stock. I had decided to use the wet anode and just accept the fact that it would result in some current leakage. This of course, is very dangerous from a safety with electricity standpoint. I took appropriate precautions, grounded the chassis, never went near it with power applied, etc.

    For several hours, it appeared to be working and I was so happy having a silent laser! But then the anode seal failed. What seemed to be the factor was the apparent electrolysis of the anode. When I dissected the laser after removing all electricity and disconnecting the power supply from the laser, it appeared that it had rusted!!! Although iron/steel does rust in water, it shouldn't fail after less than 10 hours.

    The temperature didn't seem to be a problem, but for all I know this was what hurt it. Perhaps the differential of outside to the inside of the anode. I do not know what exactly what did it in but I am sure the anode corrosion played a significant part

    The bottom line is don't try to water cool an air-cooled argon laser - or at least you now know how not to do it!

    (From: Steve Roberts.)

    The only reason you can't scorch a 60X or other air-cooled tube is the anode and the anode seal (and others agree with this). If you could make a longer anode with an order of magnitude better cooling and then boost the airflow as well, it would be a 350 mW laser. Spectra-Physics' older "glass" 161s are the same tube design as the larger 164/168/171s, just a shorter bore and smaller anode - the only other changes being the lack of a fill system, water jacket, and a different gas return. Expensive experiment, however. The scaling limit is actually the length of the tube and lack of a magnet. So if the SP-161's gas return were redesigned to fit inside a magnet and a water jacket were glassed on, and the anode had a wet "anode block" mechanism for cooling like the big ones, yes it would be a high power tube. But you also need a electrical system that lets you have a "wet" anode, and that's not easy when powered single-phase.

    National does make a couple of wet versions of its tubes, and NEC also once made a 20 mW wet unit, as National has replacement tubes for it on their Web page.

    Notes on National Laser Corporation 60X Replacement Tubes

    National Laser manufactures air-cooled ion laser systems using permanently aligned internal mirror tubes, replacement ion tubes, conversion kits available to install their tubes in laser heads from UP, SP, ALC, MG, ILT, and NEC.

    (From: Stethman Grayson (sgrayson@attbi.com).)

    The NLC-60i tube has a large cathode end-bell with a 2.5" 'extension' on which the HR is mounted. That extension basically terminates 1/8th of an inch from the original 60X external HR mirror cavity. (Close fit, huh?) The tech at National Laser said that these tubes were used as replacements for the 60X and NEC 3030 and that the nominal voltage drop for a new tube is about 105 to 107 V at 9.5 A with with end-of-life at about 90 to 92 V.

    The NLC-61i is the next-generation replacement tube for the 60X heads. The 61i is the same form-factor as the 60i, just superior output quality and power.

    Comments on Conversion to Single Frequency Operation

    The following was prompted by a request for information on adding an etalon to the 60X laser head from an Albion Instruments anesthesia analyzer for use in holography applications. The tubes in these machines (shown in Albion Instruments Anesthesia Analyzer Argon Ion Laser Head) have very high quality optically contacted Brewster windows and an extended intracavity space (where the gas sample chamber original was located).

    (From: Steve Roberts.)

    The 60X/532 tubes from anesthesia analyzers are medium power tubes, they do have quite a bit of gain but probably not enough to run single frequency with a true etalon and single line prism. An etalon is usually a 15 mm diameter solid block of fused silica about 12 mm long, with special coatings on usually one if not both surfaces for 1 to 10% reflectivity, carefully picked by the design team for a given wavelength and intracavity power. For higher power ion lasers the etalon is a set of two air-spaced silica disks, again with a highly specialized coating that is optimized for a specific wavelength and gain.

    Since it's not a standard option on air-cooled lasers, you'd need to develop your own chunk of custom coated silica. You might get it with just sticking in a carefully tuned microscope slide, for short periods of time, but it would be a VERY frustrating exercise to keep that stable.

    The etalon is typically mounted in a heater assembly that is stable to about 0.001 °C or better, and thats going to be extremely hard to achieve on a air cooled laser. You then end up with the normal trouble of tuning the end mirror and prism plus having a unstable element that is very alignment sensitive and temperature sensitive, it typically takes 45 minutes for the system to settle down on a water cooled small frame, and small changes in how the intracavity beam heats the etalon by adsorption can cause major, major fluctuations in output power, not to mention huge mode jumps.

    At best on a large frame water-cooled laser you get about 0.5 to 0.6 times the single line power measured without the etalon, do to the short gain space on a 60X, I'd say you'd get maybe 10 mW max of stabilized 488 with a hot tube.

    On a naturally unstable laser like an air-cooled argon ion, additional steps would need to be taken to slightly and actively adjust the cavity length, like the addition of a piezo tweeter on the rear mirror to adjust cavity length to favor a single mode and single gain line, locked to an iodine cell absorption line.

    We do that here at my day job, we have a etalon equipped medium frame lexel with a active iodine cell that is kicked in to adjust the cavity length. You can visually watch it mode jump and fluctuate in intensity for about a hour. it will fluctuate from 0.1 watt to 1.5 watts as it warms up.

    You're better off getting a used lexel 85 with a prism and water cooling. It will run off single phase 220. Some experimenters on this group have modified DPSS lasers with heaters and TE coolers to achieve much better stability then you could ever get out of a air cooled argon.

    If you can get the cooling fan vibrations damped out enough for holography, you may find coherence length is not a big issue, holograms are made with diode lasers all the time.

    As for etalons, you won't find many of them on ebay (maybe one every two years has show up) as there are very few of them out there, most users of ion lasers don't have the patience or the training or the instruments to use them properly. May I suggest that you might consider using a 15 mW HeNe laser instead. :)

    (From: Sam.)

    Interesting that the remains of my Carson dual tube Ar/Kr ion laser which had both a line selecting prism and etalon didn't have any thermal stabilization within the etalon assembly. Unless the entire laser was temperature controlled, there was none. (See the section: The Really Strange Carson Dual Tube Ion Laser)

    (From: Steve Roberts.)

    That would be a witchy job to tweak, it probably would come into its own in about two hours of running, then very very slight movements of its mount or the prism would get you your desired mode.

    If you think a 60X mirror mount is crude, an etalon is 20X more touchy. But at least its behavior is somewhat predictable.

    There is a reason laser scientists of old started the lasers at 5 or 6 in the morning, then graded or wrote papers for two hours, then experimented for two hours after the rush hour instabilities, then ate lunch.

    The active iodine stabilization circuit for the Lexel, which is about 6 op-amps and a ton of temperature compensation devices, is well worth its weight in gold once it locks. It heats the resonator rods to lock up the loop with 20 to 40 watt phase controlled heater tapes on each resonator rod. A separate heater circuit disciplines the etalon.

    Types of Air-Cooled Argon Ion Laser Tubes

    (From: Steve Roberts.)

    There are two major subclasses of tubes,

    A. Those are small tubes with a tapered bore designed for a long life, no significant power, controlled TEM00 mode over entire life, single line or a few lines, at a few mW or maybe 20 mW max, built in such a way to be hard to rebuild or upgrade, usually 488, 457, 477, or more rarely 514 nm. They are almost never multiline and intended for graphic arts or small hospital lab applications. In older days these were glass like the NEC 2020 or Siemens. Nowadays they are built as a small compact brazed module with short or non-existent mirror stems and a small cathode. They have a very well defined lifetime, but if you crank the current up, they die quickly. They are a "minimalist" design and the manufacturers spent 20 years refining it. They have a number of cost tradeoffs designed to enhance sales. Rebuilding them is risky although there are cases where if you have hundreds or thousands of units in service its probably worth it.

    I have two 60Xs rotting in my garage. Both tubes are identical length and look identical externally. One is the tapered low power class. It has a nice fill pressure and the correct Delta-T on the cathode, so it's healthy. But, no matter what optics set I put on it, I get only 5 to 7 mW of 488 nm TEM00, and maybe a trace - a hundred microwatts - of 457 nm. No other lines lase, even single line green optics or two green HRs back to back. Just 488 nm. This is known as gain saturation. It starts to complain to the power supply if I try to run it at more then 6 or 7 amps. The gain plateaus at 8 amps no matter what. The other one is a wide bore, lets call it a SBPT. It's old and needs a gas refill and a new cathode, but it does 60 to 70 mW of multiline, multimode, and its response changes nicely as a I change optics sets and keeps climbing with additional current. I'd never run a 110 VAC ALC product at more then 10 amps anyways, so it increases in gain up to that point and I suspect would still be climbing, even slightly.

    B. The other class is what we might call "straight bore power tube". SBPT pretty much says it all. The bore is not always straight either, usually a significant flare at the ends to help with erosion. SBPTs have large shielded gas returns from cathode end bell to anode end bell. SBPTs are worth rebuilding and upgrading in some cases, especially if you have a hundred or more of them. SBPT lifetime is pretty much inverse exponential to tube current, see the lifetime chart in the section: Argon/Krypton Ion Laser Tube Life copied from the manual of a high power air-cooled laser. The old way of getting long life at 20 mW was to buy a high powered tube and run it at much lower then rated current. The old huge (room sized) Xerox high speed photoprinters that use a 70 to 100 mW tube and run it at 23.7 mW are a example of this. The newer tapered low power tubes have a long life, and lower costs, but no gain over what they need. They are also rarer and more expensive.

    If you look at the manufacturers catalog in the tables of model number versus specifications, it's pretty obvious to see where the line between the two classes of product starts and stops.

    Comments on the ALC-60 Series Lasers

    The designation "ALC-60" means an OEM sixty series laser.

    The 60X includes any of various output powers in a standard sized box designed by Xerox, hence the "X". It was originally mass produced for Xerox ultra high speed photocopiers with a printing option. These were used to print books at several tens of pages per second. The X copier laser design spec is 23.7 mW blue and violet lines only in light mode for 5,000 hours at a starting current of 4 amps and a max end of life current of 8 amps.

    However the factory made many lasers that were 60X-NNN (dash numbers after the 60X) which were OEM and scientific lasers at different wavelengths and powers. The 60X defines the box the tube is in, and thats it.

    60B is typically a medium power plate setter laser in a format for European use with metric mounting holes, two fans on the side instead of one on top, a way different hole pattern, and an emission indicator on top. It also usually has a way different pinout and a 230 VAC power supply. It is the same physical size as the 60X.

    The 60C is a top-fanned low power laser with yet another hole pattern on the base, probably matches the competing spectra physics hole pattern. The 60C also came in krypton with a way different tube.

    Omnichrome competed with ALC for the Xerox contracts, so some Omnichromes line up with the 60 physically.

    This gets confounded by the fact that many 60Xs out there were rebuilt by Evergreen and others and they mixed and matched used tubes to meet their customers needs, and bought old tubes and heads wherever they could get them. So it is not uncommon to find a 250 mW tube being used in a photocopier head with a Xerox sticker on the side.

    What actually determines what you get is the bore size, bore length, bore shape and bore diameter, and the optics installed. They all have the same overall tube length, the same cathode, and all are designed for either a 95 V or 106 V tube voltage and the same tube pressure.

    If they make the bore really tiny and with parallel sides, it can limit the power to 5 to 20 mW and blue only. ALC had a patent on that. No matter what optics or how much current you ram down that tube, it's blue only and 20 mW.

    If they make a tapered bore, you get a high powered multimode tube, etc.

    So there is NO easy chart to let you know what your getting, and Only the factory and the re-builders knew what went into the OEM lasers.

    I have some old charts for the black-cased scientific models, but the gold-ish color OEM housings could hold anything!

    And, people change optics sets if they have more then one laser to play with, so its luck of the draw if you buy one that the owner hasn't made measurements. And often they have dirty optics and some one sells it as 20 mW and single line blue when just cleaning the optics makes it a 100 mW all lines. :-(

    Some numbers of common tubes (all TEM00 vertically polarized, light controlled power regulation):



  • Back to Argon/Krypton Ion Lasers Sub-Table of Contents.

    Typical Small Water-Cooled Argon Ion Lasers - the Lexel Laser Models 75 and 88

    Description of a Typical Water-Cooled Argon Ion Laser

    Lexel argon and kryption ion lasers are probably the second most common types available on the surplus market. Depending on particular model and configuration, they are capable of between a few hundred mW and many WATTs of output power and may be fitted with single or multiline optics.

    The Lexel Model 75 (henceforth referred to as the Lexel-75) is a small water-cooled laser, a baby cousin to the Lexel Model 88. It operates from a standard 115 VAC line so you don't need to own stock in your local utility to be able to afford the electricity bills. :)

    Though the output power rating of the Lexel-75 is 100 to 300 mW, some of these babies will do 450 mW when new. Because of the better cooling, they will last a lot longer then their shorter air cooled brethren.

    Without the need for the HUGE cooling fan, the Lexel-75 laser head is a sleak, streamlined unit. Of course, there IS the need for that tap water cooling hookup!

    Photos of the Major Components of the Lexel-75 Laser Head

    Note the small (narrow) size of the plasma tube compared to air-cooled types with all their cooling fins obsuring the actual bore. Note the crud from the cooling water on the BeO core of the plasma tube. This can result in a 5 to 10% drop in laser power over time. Occasional use of a dilute solution of "Lime-away" can work wonders on plasma tube lifetime. The mirror mounts and rigid highly stable optical frame are clearly visible.

    Also see the Laser Equipment Gallery for for additional views of the Lexel-75 and other ion lasers and power supplies.

    Lexel-75 Tube Voltage Versus Current

    Rated TEM00 power per line using HR mirror part number 75-490-00 and OC mirror part number 75-490-01 (1 to 2 meter RoC, 1.6% typical transmission).
      Wavelength (nm)  Power Output (mW)
     ------------------------------------
          514.5              150
          501.7               15
          496.5               25
          488.0              130
          476.5               35
          472.7               10
          465.8                5
          457.9               15
    
    The Lexel-75 will also output 528 nm or 1,090 nm with the tuning prism in place of the HR mirror and appropriate optics.

    Additional specs:

    Changing to an HR (rear) mirror with a RoC between 1 and 2 times the cavity length will result in multimode output. Typical part numbers: HR mirror - 75-490-02, OC mirror - 75-490-03.

    The tables below show how the actual voltage across the tube vary with tube current. Note that the relationship demonstrates a positive resistance in the discharge - barely - over the range of a stable plasma. The incremental resistance (dV/dI) averages just over an ohm. I am not quite sure what the implications of the negative resistance at the point where the discharge becomes unstable but if the reading is accurate, it can't be good!

    This data was taken from a older 65 to 300 mW tube at middle of life doing 235 mW multiline multimode and its brand new replacement doing 400+ mW. Make sure your regulator volts is well in the green! Unlike the 60X PSU, the Lexel pass-bank is stressed more at lower amperages. The Lexel PSU for the model 75 has four 2N6259 transistors instead of the 12 used with the model 88.

    Older tube (lower pressure):

               Current (A)   Voltage (V)
              --------------------------- 
                   6          115.6
                   7          115.7
                   8          116.9
                   9          117.6
                  10          118.5
                  11          120.4
                  12          121.8
                  13          124.0   
                  13.7        121.5 (unstable plasma)
    
    Excellent new tube with only 20 or so hours on it:
               Current (A)   Voltage (V)
              ---------------------------
                   6          114.2
                   7          115.8 
                   8          117.7
                   9          119.4
                  10          120.9
                  11          122.5
                  12          124.2
                  13          125.7 
                  13.7        127.0  
    
    The filament tap set at 2.65 volts (higher could be better).

    I (Steve) recently serviced a pair of Lexel-75s for a friend. One was brand new and did 450 mW initially. The other did around 230 mW. (The 450 mW laser was a new spare for the machine he took the 230 mW laser out of.) When we ran the delta-T curve based on graphing the tube's voltage versus current versus changes in cathode heater voltage, we determined 13.6 A to be a good maximum number and a little less than 12 A to be a good place to run for a good long life/power tradeoff. Of course 9 to 10 A would make it last forever but you probably don't want to go any lower then 8. A good rule is that with clean optics on a Lexel, run it where all 6 major lines come on and leave it there.

    BTW, the 450 mW went down to around 325 mW after three weeks or so of evening operation. To take a dive in power and then level off 15% or so above their rated power for their rated warrranty period of operation is quite normal for a new argon ion laser. These numbers are for 477 to 514 nm multiline, multimode operation.

    Lexel-88 Tube Voltage Versus Current and Other Information

    These lasers are designed to cycle from about 20 mW idling to the treatment power. A 1% transmission filter forms the aiming beam of up to 6 mW. They have 5 line optics designed for eye surgery. So: 477, 488, and 514.5 nm are the main lines, along with one other faint green line and one deep violet line. All others are killed by the OC. The 514.5 nm line is an option, normally only the blues are present during surgery. The single green line is separated from the main beam using a dichroic filter when a specific type of coagulation is needed. The medical controller idles the unit and then pulses it to the desired power for a programmable time up to a second or so.

    Although the tube and some PSUs are rated for pulsed operation at a very low duty cycle - and half second pulses or less at 30 A for scorching 4.5 watt eye treatment bursts - in practice the machine is set for about 2 watts max. Obviously, as most of the units in circulation are pulls from opthalmic systems, it's wise to derate them from 2 watts to 1 watt for your own use.

    The manual lists typical readings for the laser operating at full power as follows:

    The measurements below were taken from a Lexel-88 tube with the filament on "B" setting and are actually 2 to 3 V low (indicating slightly low pressure) according to factory test sheet for a similar tube. Normal factory reference is 165 V +/- 1 V at 20 A. (However, sustained operation at 20 A or more is not recommended; 13 to 15 A is a nice tradeoff between lifetime and power.)

               Current (A)   Voltage (V)
               ---------------------------
                  15.5         154.8
                  16           156.9
                  16.5         157.7  
                  17           158.4  
                  18           159.4  
                  19           160.3  
                  20           161.1     
    

    This following additional information was compiled from various lexel unit test sheets for lasers intended for low duty cycle high power pulsed medical service.

    Data taken at 20 A tube current:

    Limit and trip settings

    In light mode, with properly adjusted medical PSU, voltage is applied to 15 pin remote control plug on back of unit, lightmeter output is on 15 pin plug as well.

    Notes:

    1. 20 A is an INSANE level of current for long term continuous operation. A more reasonable value is 13 to 15 A. Remember this is an industrial unit originally designed for pulsed power on demand, coming up from a low idle of 8 or 9 amps for no more then 3 seconds at high power. In medical service, the maximum an eye surgery controller would demand is 3 watts for 1 second or less. 250 to 350 mW is a more typical value used in eye surgery.

    2. A more typical value of tube voltage at 20 A is 168 V when new.

    3. The light controller can demand more power from the pass-bank then the current control card, but only for short periods of time. The pass-bank is only rated for 10% duty cycle at 30 A or 30 seconds, whichever is less.

    4. The factory can get away with straining a brand new tube at 20 to 30 A for short periods of time. The test conditions above are for VERY SHORT

    5. If no voltage is applied to the light input on the 15 pin connector and the unit is switched into light, the tube plasma will go out. Thus scientific style light control is not on a 88 psu unless it has retrofitted with a extra light control pot.

    6. The medical optics are spec'd at quote" NO MORE THEN 5% of total power can be greater then 514 nm or less then 488 nm" Hence these optics kill 528 nm light and wavelengths below 488 nm are there but very faint, 15 to 20 mW of 477 nm blue violet shows up at higher currents but no 465 nm or 457 nm violet will lase.

    7. An eye surgery system usually has a green only mode for treating certain diseases, where a blue blocking dichroic filter is inserted to avoid deep cauterization caused by the blue light, hence the need to be able to overdrive the laser for shot periods to get enhanced green. During aiming, a 95% reflective mirror is placed between the laser and the fiber coupler to reduce the aiming light to a maximum of 3 mW, which is adjustable down to zero by the eye surgeon.

    Notes on Availability of These Lasers and Alternatives

    (From: Steve Roberts.)

    Lexel-75s haven't been made in about 4 years (now winter 2000). So, any tube you get, unless its a cherry stored in some back room some place unused, is going to be dying when you get it. You need a American 909 or a HGM 5 (basically a American 68C running pulsed with reduced cooling, but the PSU will run CW). Yes, even the famous Lexel-88 is no longer made, but it is commonly rebuilt and at least two companies make after-market rebuilt tubes for it. Lexel still produces the 85 and 95 series however as well as the 3500 series. An even better goal would be a Laser Physics Reliant. Try for a medical HGM, biomed people generally don't know who to clean optics or tweak resonators, so they scrap a lot of perfectly good lasers.

    Comments on Lexel-88 Starting Procedure and Behavior

    The following was in response to concern on the part of the owner of a Lexel-88 laser:

    "The laser fired, ran for 3 to 5 seconds, then clicked off (I heard a definite click). However, the interlock and emission indicator lights remained on. I shut it off, turned the current up a tad, tried it again, and same thing happened. Third try I don't remember what the starting current was, but it was probably around 12 amps. This time the laser stayed on, though after the first couple of seconds, I did hear the same (I think) click. I don't recall whether the laser "blinked" slightly when it clicked, but it might have. But the laser ran fine for the rest of the evening (12 to 13 amps for about an hour and a half, and 15 amps for about 20 minutes)."

    (From: Steve Roberts.)

    The one blink after start is normal on lexels, and it's a consequence of the way the igniter works, not to worry, its a pretty common thing and is not a sign of a bad tube or system problem.

    My laser wants to be set at about 11 A for starts, so given that the meters are not all that accurate from unit to unit and are kind of nonlinear, your tube is fine. Remember it can be turned down lower then where it starts because once they have the arc, they have a enough hot ions to keep things going. Ions are harder to make when the tube is cold. You're fine. Starting amperage shifts upwards as pressure rises, but 1 A is not enough to worry me given the 5% accuracy of the current knob and the 5 to 10% accuracy of the meter.

    But, for the sake of your pass-bank and cathode, you should start with a much higher current then that, say 15 A. Cathode sag happens when the cathode is at a middle temperature of 600 to 700 DegC, tungsten is softened in that range and it goes through a phase change - above 700 DegC it actually gets stronger, so during warmup, it's strong, then very soft, then stronger. So, if the arcs hotter from the current being up, its out of the dangerous sag period quicker. If it stays in the sage period for long periods, it will stretch and deform. Therefore the idea is to get it out of there as soon as possible. A higher current helps with that. Cold filaments sag quickly, warm filaments can go years without sag. Sag is actually a change in the grain orientation of the cathode metal, sag occurs during a phase change when it switches state to a longer thinner grain that can stretch, instead of the normal grainy structure which crosses at right angles at random along the metal.

    As for the pass-bank, being full on means it is really almost a short circuit once the tube starts to conduct, thus protecting it from the ignite spikes, by allowing them to pass through. when that tube first glows, it nearly totally discharges the filter cap right through the pass-bank, so to protect the transistors, turn it up for starts. This is in nearly every Spectra-Physics and Coherent manual.

    Comments on Regassing Lexel White Light Ar/Kr Ion Lasers

    (From: Dan Glassburn (dan@niteliteproducts.com).)

    We do quite a large amount of regassing of units. Lately there has been more activity concerning regassing of "great whites" and lexel systems to white light. Many of these system can be regassed, but some, in particular 8100 and 5100 white light lexel systems usually don't work very well as a regassed tube. The reason is as follows. White light systems are mostly krypton and lexel in their bid to get customer loyalty gave two year warranties on these systems. Lexel did not put a fill system on these tubes initially. The tubes would use up the gas and the pressure would lower. This condition would cause more metallic vapor from the discharge components to cycle in the tube, much as is vapor deposition used today. This metal deposition would cool in the return and on the windows, making the tube hard to start, destroying the mode, and reducing the output. This effectively makes most lexels difficult to regas, unless it has low hours or light duty on the tube. Keep this in mind when looking at these to purchase, thinking that a regas would bring the tube back. We normally will evaluate the tube very carefully before we attempt a regas, and will notify the customer the condition of the tube and if a regas should be attempted. Nothing worse than having a customer bring a 5 watt argon to convert only to get a 1 watt, shaky mode white light. :(



  • Back to Argon/Krypton Ion Lasers Sub-Table of Contents.

    Maximizing Tube Life, Plasma Oscillations

    Tips for Maximizing Ion Laser Tube Life

    Unlike the typical small internal mirror HeNe laser tube which can be turned on and off with few restrictions after sitting on the shelf for years, ion lasers require a bit of tender loving care to provide thousands of hours of reliable service.

    Or, at least some of them do. From my (Sam's) admittedly limited experience, Cyonics/Uniphase tubes don't seem to care much about sitting on a shelf being ignored for many months and will be ready to start on a dime. ALC-60Xs seem to be more finicky and long periods of non-use may result in hard starting.

    Since ion laser tube current has a very significant impact on tube life (see the section: Argon/Krypton Ion Laser Tube Life), it is always going to be best to operate the laser at the lowest output power that provides acceptable performance for your application, especially when running for many hours at a time. However, this alone isn't enough to maximize tube life especially if the laser is used infrequently.

    (Note that medium and large-frame water cooled ion lasers must be run within a range of currents with the minimum being above the that needed to sustain an stable arc. They do not like to be run at very low current - tube life may suffer dramatically.)

    Here are some tips for operating a small air-cooled argon ion laser to maximize tube life and prevent it from 'dying on the shelf'. (Much should also apply to water-cooled ion lasers as well.)

    Also see the section: Hard-to-Start Ar/Kr Ion Tubes - Outgassing and Keeping Your Laser Healthy which includes more info on Oudin coils and salvaging that old tube.

    Idle or Shut Down Completely?

    When you're running an air-cooled ion laser at high power (near the maximum recommended tube current) as in a laser show application, follow the recommendations above for startup and shutdown. Between shows, if the laser can be run at idle (e.g., 6 or 7 A for an ALC-60X), that's better for the tube than a complete shutdown (with its thermal shock) even if for an hour or more.

    (From: Steve Roberts.)

    There will be some crossover point where shutting it down completely will result in longer life than running at idle (statistically, at least!) but I don't know where it is. If you look at the MTBF (see the chart in the section: Effects of Optics on Laser Performance, 6 or 7 A results in many thousands of hours. Let's call it "starts per day", each start or shutdown is stressful to the tube in terms of expansion and contraction of the ceramic to metal seals, and on ALC tubes with brazed on fins, it has to stress the whole bore. Each start also literally blows chunks off the cathode. So when starts per day exceeds a certain number N, idle is a better deal. When Total IDLE Time, T, exceeds some Z number of hours, powering down completely and restarting multiple times, (i.e., 2 to 3 starts a day) is a better deal. But if there is only half an hour between shows, idling is the preferred thing to do. Without further data. it's really a judgement call. Commercial equipment such as big printers idle down. Biological applications idle down. That photocopier is running at 7 amps to get 23 mW and its spec'd for 5,000 hours. But when you ramp it up to 10 A for kick-butt laser show, you're in a whole new ball game. I don't know when Z enters the game. But if N is large, life is greatly shortened. Not only the tube lifetime, but the power supply (pass transistor) failure odds go way up with each start.

    Plasma Oscillations and Other Instabilities in Ion Lasers

    Like other gas discharge devices, the Ar/Kr ion laser tube is susceptible to a variety of instabilities. These can occur even with a good current regulated power supply feeding the laser head. While in severe cases, there may be effects visible to the naked eye like chaotic or periodic variations in beam brightness, test equipment is needed to confirm that lower amplitude or higher frequency instabilities are NOT present as the Mark-I eyeball has its limitations. The likelihood of seeing any of what is discussed below is fairly small but it is prudent for every ion laser user to be aware of the possibilities and be on the lookout for errant behavior.

    Should instability or oscillations be present, the result of not doing anything to deal with them can include excessive high frequency noise in the laser's output, shortened tube life, and even complete destruction of the tube or power supply.

    The light control mode (particularly, the AC or noise feedback) present in most commercial ion laser power supplies will help to minimize those instabilities which have a frequency within the loop response. However, these can only go so far and the design of the laser head itself and the condition of the ion tube are just as critical. Even where the average tube current is way below its maximum specs, the unstable and constantly moving arc, particularly at the cathode, can result in continuing damage to the tube.

    There appear to be a variety of possible causes for what are collectively called 'plasma oscillations' in an ion laser and which of them (if any) may be present depends on a variety of factors:

    If you have a vacuum pump good down to a Torr (1 mm of Hg) or so, you can demonstrate some of these forms of plasma oscillation with a homemade glow discharge. Set up a long tube which is sealed except for an electrode at each end and an exhaust port for the pump. Connect it to a neon sign transformer or a current limited AC power supply that's well over the voltage needed to start the tube. For a tube 60 cm long with a 1 cm bore, 5 kV is adequate. Turn on the voltage and pump the tube down slowly. You'll find a region or two where the tube tries to stay lit but keeps fluttering in and out of conduction. Watch the plasma itself for periodic and chaotic changes in shape, structure, and appearance. That's one type of plasma oscillation. Next, do the same thing with a DC supply and ballast resistor. Finally, put small HV capacitor (even a piece of glass with a couple of aluminum foil plates) directly across the tube and see what happens. Experiment with the value of the ballast resistor and capacitor. All of the effects you will experience are possible with an ion laser tube. However, imagine what can happen where many AMPS are involved rather than the few mA you are using!

    Unlike a neon sign or HeNe laser tube that drops into a stable operating range with just a ballast resistor, an arc discharge is less forgiving. Until the current is damped and actively filtered the arc will wander. With a discharge lamp, the electrode geometry or the wall stabilizes it. However, the ion tube plasma has multiple places to go in the cathode space - possibly places that it wasn't designed to go. The arc can also be destabilized by thermal effects. A simple ballast resistor as with a brute force 'heater' supply may not provide enough damping and the tube can develop plasma regions that form relaxation oscillations especially if there is something to resonate in. In some cases, the tube can actually chirp at audio rates - and that can be destructive, especially with large-frame tubes!

    One way to test for plasma oscillations is to monitor the light sense test point or the output of a high speed photodetector intercepting the laser beam on an oscilloscope. Small amplitude (a few percent) wide band noise is to be expected but large amplitude oscillations would be cause for concern.

    Another even simpler way may be to put an AM radio near the power supply or laser head. At the onset of plasma oscillations, it will produce a continuous PSSSSTTTTTTTTTT sound due to the broadband RF noise. On some tubes, particularly high mileage ones, this will occur above a certain tube current (or output power) using an otherwise stable power supply.

    Consider the following to avoid plasma oscillations and minimize their effects:

    Depending on your specific setup, power supply and laser head combination, you may never experience plasma instability - which is just as well. However, it is wise to be aware of the possibility and know how to deal with it just in case!

    Effects of Improper Filament Temperature

    The filament/cathode of an ion tube must be within a certain range of temperatures to achieve maximum tube life - or any tube life at all!

    Note: The filament IS the cathode for ion laser tubes, microwave magnetrons, and many other power tubes. However, others like CRTs and small receiving tubes (if any still exist) use indirectly heated cathodes.

    Too hot and the filament will burn out prematurely like any incandescent lamp due to evaporation of tungsten (which may also deposit a black film on the optics). Some tubes such as the NECs, like a somewhat cooler filament for other reasons. Too hot a filament and they can outgass leading to hard starting (at least). There may be other reasons to limit filament temperature as well.

    However, a cold or not hot enough filament in an ion tube is where the most warnings apply. Electrons can be released from a cathode structure by pure thermionic emission or by secondary emission from ion bombardment or a combination of these. (The electrodes can be heated by an external current as with an ion laser tube or by ion bombardment as with a common fluorescent lamp.) The goal for long electrode life is to minimize electrode emission by secondary emission, since this requires high energy ions that will damage the cathode - possibly resulting in total failure very quickly.

    This isn't a problem for many vacuum tubes as far as I can tell. In fact, normal X-ray equipment use the filament temperature to actually control tube current. Microwave ovens may cycle the filament and high voltage every 10 or 15 seconds for the duration when cooking at less than full power. This results in HV on the tube during warmup. Other tube type electronic equipment often applies full voltage to the tube plates long before the filaments/cathodes have gotten up to temperature. But in all these cases, the current ramps up slowly as the surfaces come up to operating temperature and begin to emit electrons so that must help as well.

    I expect gas filled tubes like those used in ion lasers are a different story because a glow discharge or arc can be sustained or initiated by a power supply with enough voltage compliance range possibly even if the cathode is cold (or at least not at normal operating temperature). However, the cathode has a much higher work function (resulting in current flow due to secondary emission rather than thermionic emission) and thus there will be much greater power dissipation and other damaging effects. Starting is especially hard on these filaments/cathodes at the instant the discharge or arc is initiated, possibly even at a higher current than in the steady state. Current flow begins in a very small spot (resulting in excessive heating) until it has a chance to spread out over the cathode surface and stabilize where it is supposed to go.

    There are no doubt many other more complex phenomena going on as well. :)

    Compatibility Issues of ALC-60X and NEC Laser Heads and Power Supplies

    An NEC laser head CANNOT be run with an ALC-60X supply for several reasons including differences in voltage compliance range and connector pinouts and signals.

    The NEC uses an 80 to 95 V drop across the tube. ALC-60X supplies start to overheat and stress the transistors when the drop gets to around 99 V. The PSU then drops out of compliance and oscillates.

    I can speak from experience on the compliance issue, we tried repumping 60X tubes with krypton, resulting in a tube voltage of 85 to 90 V. We got the tube to lase, but the PSU was less then stable because it wants to see 101 to 109 V.

    NEC and 60X lasers have the same connector on the side of the head and while at first glance, most of the signals look about the same, there are some key differences in pinout and power supply characteristics. NEC tubes tend to explode when connected to ALC-60X supplies. I have witnessed that first-hand. We had to try just once. :-( The anode, cathode, and fan pins are compatible but the control and feedback pins are not and the signals produce vastly different behavior in the power supply. The tube lights up and then the power supply goes bonkers. You end up replacing $200 worth of parts in the PSU if it's a classic gold box, or about $85 worth of parts and some blown board traces if it's an Omni-155.

    Keep in mind NEC tubes were designed for the 100 volt Japanese line voltage!

    Linear bipolar based supplies are not that difficult to build and can cope with the NEC's quirks whereas a quasi-switching or switching PSU like what is used for 60X tubes can't. Each supply needs to be matched to a head, the ALC-60X and Omni-532 supplies are interchangeable, but you need to adjust the low current limit and light feedback pots for each different head. On Omni supplies, there are 4 pots that need to be trimmed for each different head or any time you replace the MOSFETs in the supply. This is why a lot of people have trouble with short life when they buy air-cooled lasers.

    See the chapter: Complete Ar/Kr Ion Laser Power Supply Schematics for several designs that could be used with either of these lasers and can be built with readily available parts.

    Powering Ion Laser Tubes that have Wet Anodes

    Water-cooled ion tubes where the high voltage portions of the tube are not isolated from the cooling water need special considerations, especially for starting.

    (From: Steve Roberts.)

    Argh!!! I just got a wet anode SP head, i.e., a SP-164/68/71. How do I hook it up to my switcher or a Lexel supply or something like that? Answer: You probably don't.

    Wet anode tubes have the tube anode at about ground potential or above it by about 1 megohm measured when the plasma is off with normal tap cooling water. The anode is cooled by the cooling water in direct contact. The starting pulse is injected through the cathode transformer center tap, meaning that both the igniter and cathode transformer must have high breakdown ratings. Injecting a starting pulse from the anode-end generally dumps the pulse to ground and you won't even see a flash. I tried both a medical switcher and a Lexel 95 supply with no luck, using a series anode end igniter in both cases. The only supplies I know that will run these are the Spectra-Physics supply designed for it and the Aurora 40 series after-market switcher. The SP supplies also double the line voltage, i.e., you have 560 volts across the tube to aid in igniting. Oh and yes, a sizable current is leaked through the tube assembly to the water, preferably from a designed-in ground wire that is installed at the cooling water input on the tube at the cathode end (thin red wire). Otherwise it comes from the anode block, which it will probably rapidly corrode.

    If you get one of these and it's not starting on a proper power supply, clean the corrosion off the plastic around the anode block. If a good meter reads less than 200K from the block to frame ground, you have a problem. (Thanks to Dale Harder for that tip.)

    Gas Pressure Considerations for Ion Lasers

    (From: Steve Roberts.)

    Gas Pressure for ion lasers is highly dependent on the specific tube in question, and can change considerably for two sequential tubes off the production line, however this is reduced with disk tubes.

    Strong variations in fill pressure occur in identical tubes that have wires or ceramic "restrictor" rods installed in the gas return bores. These are installed to control gas return flows to the cathode and anode regions. They are also installed to prevent plasma ignition down the return path. Two identical tubes with the same design rods may vary several volts either way from the design point, due to to tolerance issues in manufacturing.

    Typical "COLD" pressures from when I have adjusted pressure on Argon lasers. Ion Laser fill Pressure is traditionally measured (All the way back to Dr Bridges' days) on a "AIR" calibrated TC (ThermoCouple) gauge. "Cold" is added because I have had tubes where a built in TC gauge can run while the plasma is lit. So these values are typical on a pure argon tube when measured with a TC gauge calibrated for AIR.

    While it is HIGHLY unlikely that a person doing one tube on a vacuum station can properly process a used cathode exposed to air without training, a special "cleanup" gas mixture and a tube furnace, I will say the following:

    From experience with pressure adjusting standard ion laser tubes, DO NOT START AT VACUUM AND ADD GAS UPWARDS WHILE ATTEMPTING TO START. Start at a higher pressure then the desired fill and carefully pull downwards to lasing. Then slightly overfill. Especially on :DISK BORE: tubes, starting too low will have the tube to have a very highly conductive plasma that will most likely be out of the power supply "Compliance Range". This will severely damage the power supply. The tube will behave similarly to a triggered Thyratron at this point.

    You can pull a tube's pressure down below the operating range into the "inverse region" below the "inversion point" where tube voltage will again start to rise. Tubes typically decrease in operating voltage as they are pulled close to the inversion point then may start to climb back up rapidly.

    Laser output may climb steadily past the inversion point, resulting in a impressive laser output, but the tube will have a negative "DELTA T" characteristic and will quickly destroy itself. Tubes typically start small oscillations in output power as you approach this point. If you go far up the inverse side of the curve the plasma will become very unstable and the power supply will destroy itself as described above. Just past the inversion point, you are well into the "low pressure" range of the tube.

    Start at least two Torr higher then your desired pressure and work down. The tube will light at a pressure much higher then the lasing pressure range.



  • Back to Argon/Krypton Ion Lasers Sub-Table of Contents.

    When You Really Want an Ion Laser, Great Deals, and Other Tid-Bits

    Ross's Quest for an Argon Ion Laser

    (From: Ross McEwan (G.R.McEwan@hw.ac.uk).)

    It was a real bit of luck. I study physics at Heriot-Watt University and was walking by the physics department skip (dumpster?) and saw this dirty great metal box with Coherent Radiation Model CR-5 Ion Laser printed down the side.

    How could I resist. At this point I knew zero about argon lasers and assumed it was capable of maybe tens to low hundreds of milliwatts and would make a nice contrast to the red HeNe's and diodes that everyone thinks are so cool. Now don't get me wrong, they are cool but blue is even better. :) So I dragged it out of the skip and loaded it into the back of my car, drove it back to the flat (apartment) and opened it up.

    Oh dear. :-( The cathode end of the tube has shattered and someone has been raiding the electronics for parts. I asked the department about the head and this is the story. It worked. Perfectly. As did four other argon and krypton head they threw out later despite my attempts to get hold of them first. The tubes break when the heads drop four feet onto metal and concrete. Apparently the lasers are "too old to use" and "we use solid state now" so they throw them out . Sorry about the mild rant but it kind of annoys me to see several perfectly good lasers destroyed just because they bought new ones and they don't want curious undergrads messing with the old ones. Grrrrr...

    (It would drive me nuts to think that perfectly good lasers were trashed when I can think of so many good homes for them! Probably too many lawyers or whatever you call them over there! --- Sam)

    They're called solicitors - yet soliciting is a crime. Go figure :). I think the judge is called "My Lord" too. And they wear funny wigs. I don't pretend to understand the legal system......

    Well that is sort of the reason. For a while (10 years or so) now there has been all sorts of crud flying about worker safety and dangers at work. Now as you know a 5W argon and PSU presents a fair few ways to injure yourself and the uni is VERY strict about safe working with them in the labs. NO-ONE is allowed in a lab with an operable laser unless they have had an eye test, read the rules about laser safety and have appropriate eye wear. If you want to chat to your mate in the laser labs they have to shutter off the beam, leave the lab, close the door. If there is no-one left in the lab the laser must be turned off etc.

    So you can imagine their mild horror when I go round asking for help to get their old lasers running in my flat. They do have a valid point and I agree that they would get a good roasting if they GAVE me a laser and I fryed myself but I promised any lasers they were chucking would get anonymised (labels peeled, ID numbers ground off, etc) and I'd deny everything as it were but they still insisted on breaking them. Still, I have rescued one and should get it working so I am happy.

    In defense of the university who are (perhaps rightly) getting a mild roasting here, when Nigel (mate and laser enthusiast) and I asked the optoelectronics boss about the chucking out policy he did give us a dead 1W Nd:YAG with parts missing and said as long as we didn't take it off campus we could fix it up and tinker with it. As it is missing Q switch, cavity, PSU and most of the head electronics it is unlikely to work any time soon. We may end up converting it to pulsed operation. More likely we will let it rot in Nigel's lab. He also offered us a tour of one of the laser labs which has argon, krypton, excimer lasers, and I believe a TEA CO2 laser. I get the impression personally he'd have liked to help.

    Anyway, I phoned Coherent UK. They said "you are not a company so we can't deal with you". They wouldn't even send me schematics on a laser that is 30 years old and hasn't been supported for the past 10? 15? years. So I looked around the Web and emailed a few companies asking for schematics and/or parts. Laser Innovations said they had a tube I could have (for free! if I pay shipping so I don't mind if it isn't up to commercial rebuild standards) so I am very close to having a complete head.

    I already have a design for the PSU. It was initially going to be a line powered switcher with a small linear pass-bank but it turned out it was easier and cheaper to build a BIG pass-bank (40 - IRF740 power MOSFETs) and water cool the heat sink. It's loosely based on the original Coherent design but missing a few of the more exotic bits (due to price and parts availability). The Lexel-88 schematics are pretty much identical to those in the Coherent manual. I guess there's only so much you can do with a linear design.

    I doubt my design is as good as Lexel's or Coherent's and I doubt it meets CE standards but it will (slowly) charge the caps, fire the starter and regulate the current all without melting. :) Ferrites, chokes and other moderately exotic parts are very hard to get hold of in UK unless you are a business or university or lab.

    I will buy PSU parts soon, a major capacitor manufacturer have generously donated a big filter capacitor so I estimate 200GBP for the rest of the bits and pieces.

    Side note: I like the magnetic oscillator in the Lexel-88 starter, very clever.

    Deal of the Week?

    Well, we all know about the efficiency of ion lasers! I hope he has a power substation nearby :).

    (From: Richard and Debora Everett" (everett@oz.net).)

    I just bought two argon medical lasers from a local auction. One of them is a Coherent model 900. It has a manufacture date of 1981, and the little meter inside says 79 hours. It is water cooled and rated for 9 watts output. The funny thing is, it takes three-phase 208 V at 35 amps! This thing must really be inefficient!

    The second laser is much newer with a manufacture date of 1988. When I got it home, I was pleasantly surprised to find it has TWO tubes in it, a 10 watt argon and a 3 watt krypton. Both of these tubes are water cooled and made by Spectra-Physics. The laser itself was made by Cooper Vision and Hewlett Packard.

    Now I have little (okay, zero) experience with ion lasers, although I have worked with HeNe and CO2 lasers before. I am a little concerned about the integrity of the Cooper Vision/Hewlett Packard laser tubes, because the &#$# loading dock guy dropped the whole cabinet assembly about 4 to 6 inches from his pallet jack. I have looked at the tubes (very cool looking) and see no visible cracks, although most of the tube seems to be in a metal jacket. I am not sure how to tell if either tube survived all of this.

    Anyway, I only paid $200 for all of this, so I guess I will not be out that much if they don't work.

    A Strange Small Argon Ion Tube

    (Quoted text from: Axel Kanne (axel.k@swipnet.se).)
    "When I was visiting a local laser company (Latronix AB) to acquire my first HeNe tube, a Russian made 1.2 mW tube manufactured only a year ago and probably never used, price ~$12 :), I saw something pretty interesting on display in a locker. Stupidly, I didn't ask about it, but maybe someone here knows anything about it.

    Here's an ASCII illustration:

             +-------+
             |        \
             |         \          ____________________
          +--+          \---------+--------------------------------------+
          |                                                              |
          |                                                              |
          +--+          /------------------------------------------------+
             |         /
             |        /
             +-------+
    
    The base of the tube was wide, and the rest was maybe 1 inch or so. A thin glass tube was spiraled around the long tube (the strange looking thing on the illustration :). The total length was about 16 inches.

    3 wires stuck out from the base. The only thing readable on the label (from my viewing angle) was "made in Russia" and some obscure model number. I don't remember anything about the electrodes. I think it had mirrors on the ends, but it might have had Brewster windows. Any ideas about what this laser might be?

    Label on My Tube:

                                            PLASMA
                                        HeNe Gas Laser
                                   <Number (SN??)>    04-97
                                        Made in Russia
    
    Label on Strange Tube:
                                            PLASMA
                                            ?????
                                        ????      ????
                                        Made in Russia
    
    The only things readable from my angle was a bit of the logo and "Russia".

    Hmmm.. I will pay the company another visit in a month or so to get a larger HeNe tube and maybe a diode laser and then I'll take another look at it. I wonder if they would sell the strange tube to me. At least it would look good as a hi-tech glass display :)."

    (From Sam):

    Since there aren't that many types of low power gas lasers, it might be a HeNe but the spiral tube thing is really strange. Any chance of getting another look? Conceivably, it could be a high quality tube designed to have ring magnets on the outside to focus and stabilize the beam or something.

    Of course, it could be a lot of other things!

    "Maybe.. But the spiral tube would be in the way.. And 3 connection wires? And, it had the same type of label as my HeNe."
    (From:Chris Livingston chrisliv@aol.com).)

    I may be wrong here, but it sounds like a NEC Type Argon tube, The wide part, sounds typical of the NEC tubes, and the cathode is normally suspended inside this "Wide Part" which on my nec is about 4-5" and the "spiral" sounds to me like it may be the gas return like used on the NEC tubes, which is from the wide part back toward the anode region. The mirrors are connected on both ends.

    (From: Sam.)

    But glass? Argon ion tubes are usually made of materials like beryllium oxide and tungsten to withstand the intense heat of the discharge.

    "Today, I paid the local laser company a visit again. Result: One REALLY small Siemens HeNe tube (total length 11.6 cm, diameter 2.3 cm, power 0.7 mW, makes a nice laser pointer) and a large ~10 mW Uniphase tube which I'm going to use for a laser show.

    Most important result (for this letter's purpose at least):

    The strange unidentified beast is indeed an argon ion tube (I asked them). I thought argon tubes had large cooling fins and a lot of metal structure, but this one is mostly glass. Now, I just wonder what power it might be. Can't be too high since there seems to be no means of cooling (though the spiral tube might also be a water jacket). The tube also has a strange vent port in the wide section, In all, really strange. :-)"

    (From: Steve Roberts.)

    After talking with a Russian on laser chat from one of the few light show companies in Russia, your glass beast described as the mystery Russian tube is an LG106. It is a roughly 2 to 3 watt argon ion laser - water cooled glass jacket with ceramic core. For many years, 7250 pyrex glass tubes encasing either a BeO core or a stack of graphite or tungsten disks were the tube design of choice. They all have a water jacket lathed into the tube and all are masterworks of the glassblowers' art. Many of them are still in service today and replacement tubes are still made in production quantities of the higher wattage glass/BeO stick (Spectra-Physics) variety. Glass/BeO tubes are known for lower optical noise. The Russian tube is a short Spectra-Physics glass/BeO clone.



  • Back to Argon/Krypton Ion Lasers Sub-Table of Contents.

    Descriptions of Assorted Ar/Kr Ion Lasers (Systems Related)

    The following sections contain descriptions of ion lasers with emphasis on the overall system - resonator, optics, and so forth. Also see the sections starting with: Descriptions of Assorted Ar/Kr Ion Lasers (Power Supply Related).

    The Really Strange Carson Dual Tube Ion Laser

    Someone sent me the optics and most of their mountings for a Carson dual tube ion laser - model unknown. Apparently, the rest of it lost a fight with a MAC truck wrecking ball (either deliberately to make sure the thing would never lase again - safety regulations - or in a most unfortunate accident - also unknown). So, I don't have details ln the tubes, magnets, or anything else that was part of the laser head. For a description of the matching power supplies as well as additional speculation on the laser itself, see the section: The Really Strange Carson PSU.

    However, nearly all the optics survived and are in fairly decent condition considering the age and lack of any careful storage. Only the HR1 mirror got seriously chipped somehow (possibly valiently defending the laser against attack!) and only the central 2 mm is at all useful. Whether the soft dielectric coatings apparently used on all the mirrors are still reflective enough to be used in another laser, I don't know but most do look pristine. Note that they are *soft* coated which means we don't know what is safe for cleaning. When I put some 90 percent isopropyl on one, blisters appeared around the edges and I terminated that experiment as quickly as possible! :(

    Please refer to the Carson Dual Tube Ion Laser - Optical Layout to make sense of the following description.

    This is one of those rare lasers (you will see why) where a pair of ion tubes, possibly argon and krypton but conceivably both argon, are arranged in series rather than with some sort of optical combiner. The two tubes were side-by-side spaced about 7 inches on center with the OC and HR at one end and a periscope-like enclosure with a pair of 45 degree turning mirrors at the other. Thus the cavity is in the shape of a long U with 4 reflecting surfaces (in addition to some other stuff) inside the resonator. O-ring joints were used to seal everything within the resonator in a gas-tight environment to prevent contamination of the optics (of course, most of this doesn't exist anymore either).

    The optical arrangement is: LS, OC, Tube 1, TM1, TM2, Tube 2, MF, HR1 XOR (LSP, HR2) where:

    The components comprising the OC assembly (LS and OC) are mounted on a small three screw (equilateral - equally spaced around a 1-3/4" circle) adjuster which is in turn mounted on a large kinematic (X-Y) three screw mount (4" X to C and Y to C, C=Common point). All screws are 80 tpi. The large mounts looks like there were knobs on the X and Y screws - all the others except as noted are screwdriver adjustments. The following 4 items are part of the HR assembly: Following the mode filter, there is an optical diverter that can be flipped into position. When out of the optical path, HR1 completes the resonator; when inserted, a prism reflector diverts the path up to HR2. At first, I didn't think there were enough adjustments for actual lasing using HR2 when in this position and assumed the diverter was for some mysterious purpose like providing a way of feeding in the beam of an external alignment laser. However, I am now convinced that this is part of a wavelength tuning assembly for running the laser in single-line mode. The alignment adjustments would have to be made first for single-line lasing with HR2 (by setting the position of the overall mount - see below). Then the diverter would be flipped out of the optical path and final adjustments in all-lines mode could be made using only the screws on the mount for HR1 itself. The angle of the diverter assembly can be adjusted with a calibrated 10 turn knob affair like those on 10 turn pots. So, assuming the laser operates in all-lines mode with the fixed HR1 and in single-line mode using HR2, the 10 turn control would provide the selection, and the preset for each wavelength (up to 13 or more lines depending on the combination of Ar/Kr ion tubes used) could be restored from a table of knob settings.

    All of the components comprising the HR assembly (MF, HR1, LSP, HR2) are also mounted on a small three screw equilateral adjuster which is in turn mounted on a large kinematic mount.

    With so many precision adjustments, I don't envy the person assigned to align one of these - it must have been a full-time job. Perhaps, they shipped one (a technician or engineer, that is) with each laser! :)

    (From: Steve Roberts.)

    I'll have to concede that it looks like you have a dual tube Ar/Kr pair with the extremely rare 4 mirror cavity, which meant the required level of cleaning on the optics went up about 16 fold, and the coating quality control must have been state of the art for its day. They must of had to ram a extra amount of power down each tube to overcome the losses. Alignment is best described using the word "bitch" and relied on a surveyors' transit or autocollimator. I'd like to see the manual for THAT one!

    The only thing ever intracavity on an argon is an etalon, Brewster windowed cell, Pellein-Broca, or Littrow prism. More commonly, there is an external combiner/splitter. Any thing else, especially something with multiple coatings is a external optic because of loss and interference effects. Pellein-Brocas are unique because they have two faces at the Brewster angle in a cavity.

    However, the description above does sound like a mode filter internal to the cavity on a braodband laser. This thing is bizarre. The only reason I can think of is to stabilize it for having all those Brewster windows in the path, but an etalon is a 25 to 50% loss in power, and would have X and Y adjusts on it, anything intracavity must have X and Y - alignment is more then critical.

    Was there a heater around it or a Z-axis adjustment stressing the spacer? If not, it's really weird, as etalons are usually are adjusted by heating the spacer to get the desired mode, if the flats aren't on piezos

    Stick a HeNe laser beam into it and see if you can get fringes, may be hard to align to the HeNe.

    (From: Sam.)

    No heater or other fancy gadgetry (beyond what had been described!). Shining the beam from a HeNe laser through the etalon produces significant interference effects as one approachs normal incidence with wide fluctuations in transmitted beam power (brightness) as the multiple reflected dots (resulting from light bouncing back and forth between the two inner surfaces) approach perfect alignment.

    The Lexel-95/Dye Laser Hybrid

    Some photos of the Lexel-95 ion laser can be found in the Laser Equipment Gallery (Version 1.70 or higher) in the "Lexel Argon Ion Lasers" wing.

    (From Steve Roberts.)

    So you ask, what's that huge pile of tan painted junk in Steve's garage, with the array of heaters around it and the low temp alarm? Could it be, no, oh my, it IS an 8 to 12 watt three phase big monster argon ion laser with a 19" rack power supply and oh, gee goshes ghondi, it's a liquid cancer box, a.k.a. a dye laser. Beam me up Scottie. :)

    I Just bought a Lexel Aurora with some help from my partner, same identical PSU design as the other Lexels (e.g., Lexel-88, just add 10 more transistors, and don't even bother to beef up the driver stage or change resistor values. Do add a cap and a blocking diode, with the cap driven by a voltage doubler to kick up the start voltage across the tube. After starting, a relay drops the cap out of the circuit. The control power comes from a 30 V tap on the three phase buck/boost instead of a zener string but the control cards are the same.

    It has a lexel 295 tube with a dye head built into the same rails. It had 200 hours on the meter which is when something caused the concave mirror that focuses the argon beam onto the dye to explode. It's about 7 feet long and was mounted on a 800 pound steel table with a 300 pound optional 440 to 220 step down transformer built into the table. It has a real fancy system of 2 robot arms and two fixed arms that can punch mirrors into the argon beam and route it under the dye head to focus it exactly on the same place on the fiber feed as the dye output - no easy task - which is why the computer that controls it can fine trim the mirrors with stepper motors. This was roughly $100,000 when new, supposedly used to treat kidney disease. The argon output was up to 8 watts or so with the dye laser producing up to 1.5 W half limited by the control circuitry. The wavelength of the dye is unknown but the warning placard urges nurses to check the dye wavelength before surgery with a spectroscope. (The hospital kept the nice hand held unit used for the job.)

    The dye laser portion of this beast has a three mirror ring cavity with a single plate birefringent tuner. The dye jet is 2 parallel quartz plates with a thin gap between them mounted at the Bbewster angle to the dye beam path. The dye was fed at something like 50 psi and leaps across horizontally into the top of what looks like a coffee can about 4" across with a funnel at the bottom and then returns to the dye reservoir via a 1" hose. Total capacity is about liter of dye. The dye passes through a heat exchanger with the warm laser PSU exit water, then into a filter, into a pump into some kind of constant pressure gadget, then into the tubing to the jet. The dye simply swirls around down the can walls being directed to graze the "can" edge. The dye pump assembly is 19" rack about 14 inches tall, it fit into the 800 pound table on wheels below the PSU.

    Rhodamine 6G Perchlorate (590.1 nm) in DCM solvent, whatever the heck that is, smells weakly of cherry Coolaid, looks like orange Coolaid, but I sure as heck wont drink it!

    The 295 tube and magnet is the same diameter as the assembly in a Lexel-75 or 88 head but the arc length is about 18 inches versus maybe 13" for the 88, and 15" for the 95 series. This thing is supposed to do 12 watts multimode (all lines) when new. The length between mirrors is 28" plus another 26" of resonator rods to hold the dye head and the fiber feed and power samplers. A ND filter and a 50/50 mirror pop down to attenuate the main beam down for aiming. The power samplers are pelicle beamsplitters at the Brewster angle with silicon cells and white ceramic disks for lambertian diffusers, and layers of thin blue and red mylar for trimming the cells output.

    The brain is a stand alone 8080 based system on its own little cart with a power meter sensor that you stick the fiber into. with printer that prints out treatment times and power levels , then probably totals the bill as well.

    GPIB, RS232 and ethernet optional but not installed on the dye pump. Separate sensors for dye power, argon power and power to the fiber feed. The fiber is 400 micron glass (core size unknown). The birefringent disk just looks like a chunk of microscope slide cover Epoxied into a rotary stage with a micrometer. It's spun about its Z axis like a record on a record player. There are no visible special effects when you look through it, save for a thin rainbow at one angle if your looking at a bright point source. at 55 degrees it looks clear.

    In order to make the dye laser compact enough to fit in the rails, they sawed off parts of the dye cavity optics prior to coating to make room for the beams to pass by. So they all look like half moons or circles with flat spots.

    If it weren't for the $1,800 custom argon focus mirror being blown to shreds, I would have fired up the dye. All optics in the dye head are close together and have visible concave radiuses, looks like a funhouse mirror when you look into them. It's a twisted ring, roughly horizontal, with a flat OC. two of the mirrors in the ring are only a few CM apart, then the OC is about 12 cm away. The biref tuner is in the long path, away from the jet.

    Dye is terribly corrosive to hardware in the system, and forms a sticky residue like superglue and causes contact rust on iron and severe corrosion on aluminum.

    The Spectra-Physics-161 Laser - More or Less

    Well, actually, what you might get as a pull from a graphics arts machine. :)

    Some photos of the SP-161/162 lasers, the SP mating power supply, and the NEC clone laser head can be found in the Laser Equipment Gallery (Version 1.11 or higher) in the ""Spectra-Physics Ion Lasers" wing.

    (From Steve Roberts.)

    Ah, an OEM SP-161 out of a Times Graphics, Inc. printer. Probably just like all the others that have been dumped on the market lately: An SP head with a National Laser replacement tube resold by a American Laser and connected to an NEC power supply. Gotta love competitors making interchangeable parts to try to steal each others' sales. The good news is most of them have had a long life sealed mirror 15 mW tube installed, made by National Laser. With a typical reading of 69,000 hours on the running time meter, it's actually on tube #6, 7, or 8. National Lasers tubes go for 8,000 hours or more in life tests at rated power, but typically laser 12,000 hours according to the test data they sent to me.

    It is quite normal for these lasers to not need a side cover for directed airflow, unlike the American or omni units that need careful airflow controls. That plastic fan mount and a 300 to 450 cfm fan is all you need. The air must blow out the top of the laser. In other words, air enters the fins at the bottom of the laser and exits out where the top would be.

    I don't know how much you paid for the unit, but considering most laser PSUs cost about $800 to $900 used. In some ways that unit was/is the best PSU ever produced for air cooled lasers so even if you just end up reselling it, there is likely money to be made.

    National Laser might sell you a "shipping" cover if you need it, SP will not sell you a cover even though they have them in stock (too many lawyers). But, all you really need is some sheet metal bent on a brake by a tin shop. This is for safety, to prevent contact with the lethal voltages inside. It needs some side holes of at least 6 square inches per side for airflow.

    If you have the standard SP supply, you will not need a remote. Just make sure the switch on the front panel is in "local" control.

    SP gets $58 for a manual. The NEC-3050 manual will be nearly as good since that is pretty much an exact clone. The light meter calibration won't be worth a damn, odds are it's still calibrated for tube #1!

    Go to National Laser and see if your tube looks like a 61I series, or if you have a glass tube. I have the tube identification. National's tubes want to see 4 amps minimum and 8 amps absolute maximum current, typically running best at only 6 amps.

    (From: David Hansen (gldhansen@rcn.com).)

    I just bought an interesting hybrid argon laser system. The tube is made by National Laser, the head is a Spectra-Physics (now Newport) model 161B, and the the power supply is an NEC unit. The laser was a pull from a high-speed scanner, so it is missing the head cover and fan. I am in the process of getting these. The output power should be about 20 mW (multiline) when the darn thing is actually running. :(

    Thanks to Steve Roberts for helping me get the components needed to finish this project.

    The NEC power supply has a 15 pin "Jones" style connector for the mating cable to the SP-161 laser head and a 37 pin circular AMP connector for remote operation (there is also a local mode that ignores the signals on this connector). On the PSU front panel there is a dual-range meter (tube current and laser output power), a current/light selector switch, a main power switch, an "Emission" switch, and an interlock connector for the printer cover (this has to be bypassed for the laser to run). The back panel has the laser head connector jack, three fuses, and a power control pot. The power cord is on the front panel. The two main annoyances about the PSU are the location of the power control pot and the fact that it weighs 40 pounds!

    Spectra-Physics-2013 Laser Head

    Some photos of the SP-2013 laser head can be found in the Laser Equipment Gallery (Version 1.71 or higher) in the "Spectra-Physics Ion Lasers" wing.

    (From Steve Roberts.)

    The magnet is the long metal cylinder around the tube. It has at least 5,000 feet of magnet wire wound along the long axis of the tube. Usually SP paints their magnet assemblies black. Yes, it's a electromagnet - only a few lasers were ever made with conventional magnets. Cooling water flows between the electromagnet wall and the tube outer wall. Magnet current is quite small, on the order of 1 to 10 A. Usually the magnet is wired across the filter caps of the PSU, although many lasers have magnet regulators. SP tends to regulate their magnet current. The magnet will be between 15 and 250 ohms, depending on the laser design. You should have the following, 2 magnet leads, at least 2 leads to the hot cathode, one wire from the anode. Often times wires are doubled up or used in pairs to increase current capacity while keeping flexibility. Usually cathode leads are paired up, i.e., two reds, two blues. The cathode leads will read as almost a short circuit until the cathode heats (cathodes need between 2 and 3.5 volts at 20 to 40 amps, and each different tube has a critical cathode voltage that cannot be exceeded.) Then the cathode is about .5 to 3 ohms when hot.

    Albion Instruments Anesthesia Analyzer

    Here is an example of the use of an argon ion laser tube in an application taking advantage of the possible very high intracavity photon flux when 2 HR mirrors are used instead of an HR and an OC to to produce an output beam. For a laser of this type, the intracavity flux could be 10s or even 100s of WATTs. The air-cooled 60X style tube, resonator, optics, and sensor box are shown in Albion Instruments Anesthesia Analyzer Argon Ion Laser Head and a closeup of the line selector in Wavelength Tuning Assembly using Brewster Prism.

    (From Steve Roberts.)

    The Albion Instruments anesthesia analyzer has 8 PMTs (PhotoMultiplier Tubes) viewing the photon flux in an intracavity cell via interference filters. The intracavity cell is made of metal with its own high quality Brewster windows. Argon gas flowed through the cell with a trace of the patients "exhaust", It looks at N2O, CO2, O2, and the anesthesia agent. It has high reflectors at both ends. I have the cavity from one setting here. We put the tube in my friends 60X head. It's a 150 mW tube with the best Brewster stems I have ever seen on a air-cooled laser - really high grade optically contacted fused silica windows instead of the normal $20 natural silica ones with frit or Epoxy seals. Search for Albion's patents on one of the patent servers. I scrapped one out for a friend. It had a modified Omnichrome 150 power supply with a DAC (Digital to Analog Converter) board built onto the 37 pin connector for setting power level. A Dayton 3C44 fan was used for cooling and it has an extended cast aluminum resonator with no rods, plus an intracavity Brewster prism for wavelength selection. A photodiode monitored the leakage from an end mirror for optical power regulation. The whole thing was automated and on wheels.

    Power-On-Demand Medical Laser Conversion?

    Many, if not all, modern ophthalmic and other medical ion lasers use what's called "Power-On-Demand" (POD) power supplies. Rather than run the tube as long as the laser is on like most scientific and entertainment lasers, it actually turns the tube on only when the beam is actually needed. By carefully controlling operating conditions during the initial starting period, this can be implemented such that tube will last millions of shots without serious degradation. POD technology allows for much reduced power dissipation and cooling requirements under most conditions of clinical use. However, this means that many components in the power and cooling system will be grossly undersized for CW use. So, what about finding one of these in the dumpster and converting it to a laser show laser? Various Coherent models show up on eBay for next to nothing (as these things go).

    (From Steve Roberts.)

    The Coherent Ultima 2000 is a true Power-On-Deman laser. It charges 400,000 uF of capacitors in its base and has a high frequency inverter that builds up the 220 VDC to about 500 VDC.

    The tube is cooled but the magnet is not, and the magnet is only good for a few minutes before it burns up. We tried one. The tube will run on a Lexel-88 power supply. It's small, but you have to let it run a minute, cool for 5 minutes. The tube has sealed mirrors on bellows and uses a Novus Omni type tube core, which is basically a shortened I60. Even the cathode is duty cycled on that one.

    Power-On-Demand lasers are NOT generally CW CAPABLE. These include the Coherent Ultima 2000, Novus 2000, Novus Omni; HGM PC, PC-EDO, Surgica K1, Surgica K5, and all HGM yellow-green krypton lasers.

    Low duty cycle laser may be modded to CW at lower powers. These include HGM models 5, 8, and 20.

    HGM Argon Ion Medical Lasers

    There are some that are unique in that having a tube similar in size to a ALC-60X but with a much larger bore can generate 3.2 watts for a second or two at a very low duty cycle. The computer cranks back the available exposure times to protect the tube.

    Generally HGM models 5, 8, and 20 have CW tubes. All others are questionable on running CW. A 5 is basically an ALC-68 with short Brewster stems, a model 8 corresponds roughly to an ALC-905/909 and a model 20 is close to an ALC-920.

    Some of the HGM yellow-green and the PC tubes, i.e., Surgica K(x), are power on demand tubes and are not really capable of running CW.

    It will be necessary to make a cheater cable to bypass the computer. The PSU will run by itself with the control computer removed entirely. The simplist cheater starts the laser PSU and ramps the tube current up to the upper limit. Install the custom jumpered 9 pin DIN plug, add an on/off keylock switch, and then the high current limit potentiometer on the board becomes your current setting. Getting variable power is complex because the supply curve is nonlinear. You then use the upper limit as a control pot, mindful that strange things happen if you turn it lower then the lower limit pot and that you can usually easily source enough current from the PSU to start frying the tube. The HGM PSUs really don't have a current mode - they are designed for light from the ground up. The upper and lower limits on the control card are soft limits and can drift somewhat. Service mode doesn't get you a current mode either!

    The aiming beam knob is actually the lower current limit on HGMs when in treat mode. There are quite a few variations on the PSUs. Unlike most medical Lasers, HGM's computer is rather deeply integrated into the PSUs, especially on the pulsed power-on-demand tubes. They can't run CW for more then a few seconds. Some units actually have rather complex CPU controlled cooling.

    Note that the control computer also runs the displays, and since its out of the loop, if you try to keep it as a power meter your going to see ERR on the display.

    As an example, the HGM-5 will do 3 W pulsed and is rated around 1 W CW for 2 to 3 minutes. The limiting factor is the size and heating of the electrolytics in the PSU. They're kind of undersized for CW and there is no room to boost them in size or add more.

    It's then a 300 to 500 mW CW laser if you boost the cooling. Running at 1 W will burn off lifetime fast. My favorite way of increasing airflow is to cut a pair of 6" holes in the top lid and add a pair of 60X style patriots. Its a 68B tube with shorter Brewster stems, different ignition, and very very multimode optics.

    The test for medical service is 3.2 watts at 22 A or less. The ones I've set up for friends run at 15 A or so for a nice trade off between power and life. My one buddy got his with 5,000 hours of medical service on it, and we run it at 15 and he got 550 mW.

    At the back side of the tube there is a 4 pin molex hanging free on white wires. light the tube, put in a isolated battery powered plastic cased voltmeter and the reading times 30 is the tube current.



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