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    Ar/Kr Ion Laser Testing, Maintenance, Repair

    Sub-Table of Contents

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    Introduction, Related Information

    Ion Laser Tender Loving Care

    Unlike the ubiquitous helium-neon laser, argon/krypton ion lasers will likely require some amount of attention during their life, even if there are no actual failures. This is due to the high current and power levels at which the tubes operate, and for external mirror lasers, the need to clean and align the mirrors and other optics. Where the current or/and output power is adjustable, the life of the tube and to some extent, the amount of maintenance as well, will depend on the operating point. This chapter deals with ion laser system and ion tube issues; ion laser power supply repair is discussed in the chapter: Ar/Kr Ion Laser Power Supplies.

    Web-Site with Related Information

    Evergreen Laser Corporation has a Web site which includes a significant amount of information on ion laser tube and power supply adjustment, alignment, failure modes, troubleshooting, and repair.

    Unfortunately, most of this did not work with Netscape V3.04. Perhaps, it will work with your browser and/or the problems have since been corrected.

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    Tube Life, Effects of Abuse, Failure Modes, Rebuilding

    Argon/krypton Ion Laser Tube Life

    At the amazingly high power that these tubes use, it isn't surprising that their expected lifetime is a strong function of tube current. While a tube may be rated for '10 A max' - and this is where you get the expected optical power output - the lifetime when running power level may be 1/10th that of the same tube idling at 5 A.

    For example, the following table shows the expected MTBF hours for a 60X tube as a function of beam current:

       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
    Thus, it makes a lot of sense to run at the lowest power that will be adequate for the application whenever possible to maximize tube life (not to mention keep your utility bills within reason!).

    What Happens if the Tube is Run with Excessive Current (>10 A)?

    There are at least three issues:

    Failure Modes of Ar/Kr Ion Tubes

    Also see the section: Hard-to-Start Ar/Kr Ion Tubes - Outgassing and Keeping Your Laser Healthy).

    Aside from obvious physical damage, there are a variety of conditions that can prevent a tube from lasing.

    For a possible way of testing a tube without requiring a complete high current power supply, see the section: Pulsed Operation of an Ar/Kr Ion Tube.

    Ion Tube Rebuilding in Your Basement?

    Well, perhaps slightly more than the average basement will be needed. :-) For additional comments on ion laser rebuilding, see the sections: Caveat Emptor and Britt Pulsed Argon Ion Laser - Description and Gas Adjustment. To get an idea of what is involved in getting started before actually cracking the seal on the tube, see the chapter: Amateur Laser Construction since much of the equipment and techniques that are discussed there will be required.

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    Cyonics Tube, Testing, Hard-to-Start Tubes, Problems

    Cyonics Argon Ion Tube

    This is typical of a well-built small sealed internal mirror argon ion tube. Cyonics is a Division of Uniphase Corporation, 408-434-1800.

    For a detailed description and diagrams, refer to the Cyonics Internal Mirror Laser Patent.

    This tube should do about 5 to 10 mW of 488 blue at 9 A.

    It's designed for a 104 volt drop, so you can get it running with just a 7 to 10 ohm adjustable resistor and a bunch of filter caps.

    The tube wants to see about 2.8 to 3.0 V for the filament at 15 to 25 A, magnetron transformers with the HV winding whacked off are a good start. Adjust for 2.7-3.0 V across the filament after a 30 second warmup. Make sure it does not exceed 3.2 V after it is running.

    Tube current is 9 A maximum, 4.5 minimum. In the past this has corresponded to a 7 to 10 ohm adjustable fan cooled 700 watt power resistor in series with the tube after a 20 A bridge, with 2,200 uF in front of the resistor and 2,200 uF behind it in a pi configuration, resistor goes in the positive side. Cathode connection is via a center tap on the filament winding so the plasma is evenly balanced.

    This will do for a few hours until you can figure out how to do a more sophisticated PSU, but you will need to watch it like a hawk and have a good current measuring shunt in series with the tube.

    The recommended fan is 225 cfm minimum, a bit less then for a 60X. The patriot P2B3s are $22.95 from Marlin P. Jones and Associates quantity 1. Make sure flow is sucking air out of the vee shaped aluminum shroud around the tube.

    Testing a Small Ar/Kr Ion Tube

    This was written for the Cyonics but portions apply to others like the tube used in the ALC 60X or Omnichrome 532.

    Also see the section: Pulsed Operation of an Ar/Kr Ion Tube.

    For initial tests, it can be run at a low duty cycle. Power up for say 4 to 5 seconds and power down, just don't rush, unlike HeNe tubes, these do not stablize instantly.

    You want to fire it up and run it, say 1 minute on or so. When all other causes of failure are accounted for, the igniter pulse becomes the biggest factor in the life of the tube. It (a) buries gas, (b) blows chunks out of the cathode until the cathode spot forms and (c) deposits junk on the internal optics. I'd say don't worry about it, as this one is really overbuilt, it has the same cathodes and anodes as a 5 watt, and has so much metal that you need some time to heat it up, unlike a 60X which is critically cooled, this one will take some punishment, and no amount of energy you can dump into it with a 110 line will rupture or implode or melt it. I have one sitting in the garage that contained all the fragments when the tungsten disks and cathodes melted, it held the vacuum as well.

    You could run it at 14 amps for short periods of say 1 minute and do no damage at all, provided you have cooling and the cathode is no higher then 2.8 V Its when you log hours and hours of operating with a improper supply that you will damage this, not during short term testing, in fact they are sometimes ran without cooling to recondition them a little.

    It's a good idea to power up and see what is going on then switch off, but it may take 5 to 7 seconds after ignition for the cathode to come up to final temp as pressure rises from the discharge heat. Hook a voltmeter across a current shunt in series with the tube, if it goes way out of range, just switch off, don't panic, this thing has a much larger thermal mass then a HeNe tube, and the metal will desorb the pure gas over time, unlike the HeNe glass which destabilizes the discharge when it heats up enough for ion migration and the bore conducts/outgasses CO2-N2.

    I do my initial checks for 30 to 60 seconds with the lid and fan off, often taking my time to stick the voltmeter leads across the tube and then the cathode, and my DVM is slow. Then I simply slide on the fan and cover and run it at 4 to 5 A for a while or shut down for 5 mins, which is about a 1% duty cycle. :-)

    You will pop a weak ceram to metal seal long before you ever risk a catastrophic failure. The main safety hazard is reaching in and touching line side. If it fails the discharge will go out when the cathode opens up.

    There is a BeO (beryllium oxide) warning on the tube for those who would actually grind or smash a tube. Recent discussions with a shop that laser machines BeO have revealed that the hazard is only for a certain particle size. They cut it with CO2 lasers every day and set it up so the particles are outside a certain range without wearing space suits and using only a simple HEPA filter to recover the BeO dust for remelting as it's an expensive material with resale value.

    Hard-to-Start Ar/Kr Ion Tubes - Outgassing and Keeping Your Laser Healthy

    If you don't run them periodically, gas is released from the tube walls and then it can build to such a level that the tube won't start, so you have to spark them in the right place with an Oudin coil of the neon sign shop/vacuum lab variety till they start and bake them at full current for a few hours to drive the pressure down by burying gas into the metal parts of the tube. This is easy when you have a plasma core that is hotter then the surface of the sun and has 170 nm extreme ultraviolet photons bouncing around from every ionized atom in the tube!

    An Oudin coil is a sort of hand-held Tesla coil that is adjustable in output and runs off AC line. It may also be called a Tesla coil or spark coil. A new one costs about $160 from neon sign equipment suppliers but these are often found in high school physics labs and other places working with vacuum systems. You may be able to borrow one for for an afternoon (or longer). A home-built alternative is also possible. See: Home-built Substitute for Oudin Coil.

    (The Oudin coil is fun as sometimes you get all the energy stored in the PSU caps to flow through the air in a inch or two spark, pzzzzzap-flash-bang! Using it around plastic bags of pure noble gasses also is fun. You get 6" long sparks or glows. The only glitch is if you turn it up too high it can puncture the brewster stems. The high voltage low current RF field doesn't damage the PSU or support electronics, even when you see corona off the leads of solid state component (though I wouldn't press my luck with MOSFETs or CMOS devices!), yet it will light a nearby fluorescent lamp to the point you can read by it.)

    Plug the coil into the wall and set the knob on the coil for a 2 to 4 cm spark to metal. Then hold it at the cathode-end of the tube where it can hit the air riser (metal box below the fan) and the bare ceramic of the tube with the power supply on (and trying to start). This will totally ionize the whole tube volume and may allow enough current to flow for the discharge to start.

    WARNING: Beware of turning up the spark coil so high you punch holes in the glass to metal seals on the brewster stems.

    If it succeeds, running the laser for a few hours will eventually bury enough gas that the igniter will handle starting on its own the next time.

    This problem results when you don't run the laser for a long time and it outgasses, I shudder to think how many perfectly good tubes have been thrown away that just needed a little assist.

    A cure for this is to run these lasers at least 30 minutes every three weeks, like HeNe lasers, they like to run, it takes a couple of months of setting there for the pressure to really build. On some tubes this doesn't happen but it is a common problem even with the large frame water-cooled units and is worse with a krypton fill.

    The Krypton red and yellow lines share the same upper state, and what determines where they fall to ground state is the gas pressure. If your tube is on the low end, you get a lot of yellow; if it is high you get a lot of red. Large krypton lasers have a pressure control pump built into the tube. Two solenoid valves and a gas reservoir form the pump.

    Lower tube pressures cause more gas to be buried, leading to a runaway cycle. An argon tube with clean multiline optics installed that only emits the 488 blue line is a dead giveaway for low pressure and end of life. However, it could just be dirty optics as the 488 is the highest gain line, it will lase with a 90% reflector, so try proper cleaning before throwing in the towel.

    Problems with NEC Argon Ion Tubes

    The comments below were prompted when the NEC-3030 tube in the laser head Ben was using with his version of the home-built SG-IL1 power supply described in the section: Ben's Linear Ar/Kr Laser Power Supply (BJ/SG-IL1) refused to start for no apparent reason.

    (From: Steve Roberts (osteven@akrobiz.com)).

    Sometimes for seemingly no reason at all, NEC tubes go south and need a little starting help. However, first check all the zeners and SCRs on the ignite board as well as for shorted/leaky caps. Since the NEC laser head generates its own boost voltage, something may be wrong on the ignite board. Make sure the filament volts/amps are adequate - cold filaments won't ignite!

    If this doesn't turn up anything, just getting some Tesla type HV near the glass bell may help.

    Leaking a spark across to the metal fins on the body may do it as well. Set the laser on a wood board along with a battery powered Tesla coil. Snake a single conductor lead over to the laser with the top cover off, then let the wood act as the return to the HV, if conditions are right, you might get some faint sparks to the fins to pre-ionize the gas.

    As soon as the tube starts, you have about 30 seconds to get the hood on (to provide cooling), so don't panic. It needs to run to get the pressure down, DO NOT interrupt the current flow for any reason. (Well, at least not anything less than life or limb threatening!)

    More on Oudin Coils

    (From: Steve Roberts (osteven@akrobiz.com)).

    There is a lot more to a Oudin coil then a simple interrupter driven ignition coil circuit (though this is what it may appear to be from the outside). They have some unique windings for enhancing resonance and some caps in critical places. Mine is from the 1930s, insulated by wax. A few years ago I had to rewax it - paraffin doesn't work, only pure beeswax for some reason (so I had to rewax it again). I wish I had drawn out the details, but that is a true resonating Tesla coil in every sense, not just 60 hz pulsed AC.

    I've never got a start boost off anything but a pure RF spark, not a sharp pulse! - hence the confounded wood block setup I suggested.

    Oudin coils source so little current they generally do not harm the start boards, even the UJTs. Its not uncommon to stick one in a laser for 30 to 45 seconds with no harm to the electronics, its extremely high peak voltage, but NO current, but its broad spectrum RF that seems to contain (and perhaps adapt) to the natural frequency of whatever its trying to excite, gas wise.

    Home-built Substitute for Oudin Coil

    Oudin coils or hand-held Tesla coils aren't the sort of thing to turn up at your typical garage sale (although you might find one at a high-tech flea market or hamfest). However, it should be possible to construct something suitable for dealing with hard-to-start Ar/Kr ion tubes at minimal cost. The typical hand-held Oudin coil (may also be called a Tesla coil or spark coil) from external appearances would seem to be just a line powered high voltage transformer driven like an ignition coil with a buzzer type of primary interrupter. However, from the section: More on Oudin Coils this is not the case (at least for some types) and produces significant radio frequency (RF) energy - not just a stream of pulses at 60 or 120 Hz.

    A high voltage high frequency source can be constructed from a TV or monitor flyback transformer using a transistor or two for drive. Alternatively, an interrupter (relay/buzzer) or low pulse rate transistor driven circuit using a flyback or ignition coil can be enhanced with suitable resonant components (additional capacitors) to promote the generation of RF energy. Note that if a flyback is used, it must NOT have an internal rectifier as that would simply result in voltage building up on the stray capacitance of the wiring, tube, etc., until a discharge took place inside the tube or an arc to something occurred outside - which isn't what we want.

    Some of these home-built devices won't have as great a maximum voltage as the genuine article but should still be adequate for our purposes. This may actually be an advantage as there is less likelihood of damaging the tube or seals with lower voltages.

    What Could Prevent an Internal Mirror Tube from Lasing?

    If it has good pure argon at the right gas pressure, a working fan, the proper stable current down the bore and reasonably aligned clean mirrors it will lase. The 488 nm line will lase from .01 torr to 2-4 torr even with dirty mirrors, so it's almost guarenteed that you will get something. I doubt there is dirt or scratches in the optics as those would not leave the factory.

    Where these conditions are met, the most likely explanation is very slight mirror misalignment.

    It would not surprise met that just applying pressure in the right direction to one of the mirror mounts by slipping a steel tube over it without loosening the setscrews and it will pop on. It should operate once the mirrors are somewhat aligned, and even if somewhat misaligned, tubes like the Cyonics have a curved from mirror and a flat high reflector. This configuration is set up to tend to correct itself optically from small misalignments.

    Mirror alignment can be tested and corrected using techniques similar to those described for sealed HeNe tubes. See the section: Checking and Correcting Mirror Alignment of Internal Mirror Laser Tubes in the chapter: HeNe Laser Testing, Adjustment, Repair as well as the section in this chapter for external mirror Ar/Kr ion lasers: Argon/Krypton Ion Laser Cleaning and Alignment Techniques.

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    Tips for Potential Ar/Kr Ion Lasers Enthusiasts

    Caveat Emptor

    A local 17 year old paid $700 for a NEC 3030 with 20,000 hours on it, it was missing the hour meter when he got it, it was rated for 40 mW and doing 10 when he got it, it took all of two months of intermittent use for it to die, and that was half a summers's spare wages for him, that could have been avoided by putting a voltmeter across the tube and measuring its drop, which is a function of pressure etc. WARNING: Lethal voltages and currents around the tube.

    A spark coil became a permanent resident at his house so he could somewhat prolong its life, by assisting it in starting. We got the 20,000 hour figure from a second hour meter buried on the start board. The higher the tube drop voltage the better, you measure it at the high and low current ranges and compare against a known good unit.

    There is a certain popular laser surplus company, that buys these units, chops the seal off, fills them with a fresh argon fill at a lower pressure for more power, doesn't bother to bake them out or replace the cathodes or brewster windows or drill the bore out so its clear of metal migration. then sells them for $3,800, after setting the PSU upper limit to 11 amps or so, so you get a 175 milliwatt laser that lasts 3 months and dies from a dirty tube and bad cathode that was not reprocessed right in order to save time. While I can't print the name in a FAQ for fear of being sued, I can say: Ask your seller how your tube was processed. He should say: Oven bake out for under vacuum for 24 hours, flashed the getter assembly, reactivated the cathode, lased it while on the pumping station or after reprocessing to check it, replaced the brewster angle windows and possibly the cathode. IF THESE STEPS WERE NOT DONE, think twice or get a good warranty!

    With a complex laser - especially where the resonator mirrors are external (not inside the sealed tube) - it is extremely important to get the needed equipment and a manual so you can maintain and clean the laser. The air in a home environment is not really as dust-free as in a typical lab - and they still have problems with optics needing cleaning periodically even if not being used. However, don't overboard cleaning everyday just because you think the power has decreased - every cleaning is an opportunity to accidentally scratch the optics - and that will mean permanently lower performance!

    Above all, understand the safety implications of a higher power laser of this type. Furthermore, when first started, an ion laser may operate at or above its maximum rated power for a short amount of time - regardless of the control settings. Therefore, additional precautions are a good idea. A set of eye protection goggles for the range of wavelengths that your laser produces is highly recommended. Can you place a price on each of your eyeballs?

    Hands-On is Best

    Actually having access to one of these small ion lasers is definitely the best way to learn but keep in mind that what you will likely get your hands ON is a 20 year old design so you need to stock a few things (assuming a laser with an external mirror tube) like gas chromatograph grade (or better) acetone and methanol for cleaning the 5 active optical surfaces. Swabs in individual sterile packages with wood sticks are generally best for Optics.

    Cleaning. Not all lens tissue is created equal, and some of it has hard particles that wouldn't hurt a camera lens, but could spell death to a soft coating on a laser optic. High grade Kodak tissue in the sealed envelopes is insisted on by most people in the laser refurb business but they prefer sealed sterile swabs for all cleaning. You use acetone first and then methanol, and you never dip the swab or tissue in the bottle, always pour it out so you don't contaminate the fluid. Always keep a lid on the bottle. If these fluids pick up water from the air it forms a white film on the optic that really zaps laser power.

    The Coherent Laser Group provides a nice tutorial on Cleaning of Laser Optics.

    Argon Laser Anonymous

    A big warning here is these are addictive, suddenly you find yourself trying holograms and illuminating low clouds and looking at the ramen scattering spectra of beer.

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    Argon/Krypton Ion Laser Cleaning and alignment Techniques

    Lasers for Which These Procedures Apply

    The following was developed for a typical 100 mW external mirror argon laser (henceforth referred to as 'Argon'). It will, of course, also be suitable for a krypton ion or mixed gas laser.

    (Note that this includes a more sophisticated version of the general procedure described in the section: Major Problems with Mirror Alignment for sealed internal mirror HeNe laser tubes. However, the basic principles are similar.)

    This technique also works for copper vapor, CO2 and 2 mirror YAG. If you already have a working argon laser or *green* HeNe laser, you can do red HeNes. HeNe lasers are aligned at the factory using the 488 or 457 lines of an argon laser. Hold up an unpowered (red) HeNe tube so a light shines down its bore and you will usually see a deep blue light transmitted through the mirrors. This is what we're going to use to our advantage. This technique was developed the hard way after discovering the techniques described by those who write laser books are not exactly tested in the lab and often written by someone who has graduate students to do it for them. The 'Cards with Crosses' technique only works on lasers that are nearly aligned. The approach described below works on anything including newly installed tubes that are not yet centered in their cradles as well as for very short lasers.

    My (Steve Roberts) thanks to Dale Harder at H&H Laser Refurb for teaching me this neat little trick for initial mirror alignment. Dale is the ultimate prefectionist. His lasers exceed their specs.

    It's also the only technique for low gain lasers short of an autocollimator or factory computerized search mode alignment jig.

    This technique works on the fact that the optimized dielectric mirrors used in laser cavities are often largely transparent or only partially reflective at wavelengths at least 100 nm away from the design wavelength.

    Mirrors within a milliradian of parallel is a hard angle to achieve, and that's what you are shooting for.

    Moral: Once it's aligned and tweaked, don't mess with it!!



    Chemicals and Supplies

    These assume hard coated dielectric mirrors for HeNe or ion lasers. (Those of you with CO2 and excimer lasers with salt optics or group III-IV semiconductor optics will will need other materials, consult your laser supplier or optics manufacturer for details.)
    1. Ultra high purity methanol - gas chromatograph grade or spectroscopic grade.

    2. Ultra high purity acetone - gas chromatograph grade or spectroscopic grade.

    3. For really bad fingerprints, hydrogen peroxide (3%) and high purity laboratory distilled water.

    4. Sterile Throat Swabs in individual packages on wood sticks without glue (Puritan(tm) and Qtips(tm) brands don't seem to have glue when purchased from a pharmacist in sterile form. Unwind the cotton carefully to look for glue. (Have your friendly Pharmacist order you a case - it's cheaper!)

    5. Lens tissue from a lab supplier in sealed envelopes. Using generic photographic tissue is not recommended. When I did a survey of laser refurb techs and light show techs, most recommended the Kodak tissue as it is sold in sealed packs. Many inexpensive tissues have impurities that can scratch soft laser optics.

    Required Equipment

    1. Alignment jig or optical table (described below).
    2. A 5 to 7 mW HeNe laser with clean beam (henceforth referred to as the 'HeNe').
    3. A fluorescent orange (preferred) or yellow sticker (get these from office supply companies).
    4. A long thin sewing needle.
    5. A lab jack or custom made HeNe mount to set the beam height of the HeNe (see below).
    Alcohol or acetone sold over the counter in the United States will contain denaturing agents or tracking chemicals to prevent unintended uses such as drug making. These show up as a white film on the optics. Get these chemicals from a lab supplier who can attest to the purity. (Kodak or Omnisolve is preferred). Keep the lids on the bottles so as not to pick up moisture from the air, and always pour out what you will need into a smaller container to avoid back contamination of your primary source from swabs or lens tissue. I flow warm filtered argon gas into my bottles before resealing. This aids in shelf life of these expensive fluids. DO NOT attempt to use common isopropyl (rubbing) alcohol for optics cleaning as it will leave a thin polymer film on your optic that is hard to see but really decreases lasing power. You can tell if you have water in your fluids as they will 'ball up' into droplets that hang around after the film of cleaning fluid evaporates.

    The Alignment Jig

    I can't afford optical benches on my budget. I needed a long bench to work on my lasers. The solution was to go to the local aluminum company and see what they had in the computer as leftovers from a larger sheet that was cut to order for a customer. A 1/4" thick 3.5 foot long 14" wide piece of T6061 polished on one side was $40. A 16 foot piece of 1" x 1" finished square stock was 25 dollars.

    Initial Bore Alignment

    Watch out for stray beams that come off the Brewster windows!!!!!

    DANGER: Class III and Class IV power levels, use laser safety precautions appropriate for your laser. When your Argon is lasing, terminate its beam on a proper beam stop where you can't see the blocked beam. A cumulative exposure to a medium power reflection can cause eye damage.

    DANGER: DO NOT stare at diffuse or specular reflections!!!!

    You are now ready to deal with the Argon:

    I am assuming you have the output end of the Argon facing the HeNe.

    Optics Cleaning Procedure

    These two steps will be repeated for each optic surface:
    1. Take a swab, wet it with acetone, let it set a moment, then flush it with more acetone. Wipe it from the top of the Brewster Window down-word with a rolling motion. You want to just scrub the optic. This takes some pressure, but be gentle. You are rubbing quartz, so it can take some punishment. Do exactly one pass with each swab and then discard the swab. Break it in half so you do not accidentally reuse it. Do this 2 to 3 times and then let it dry and examine the window with a bright light for residual dust and films. Use lens tissue if very dirty, otherwise use swabs. Use gravity to your advantage to get the contaminants to flow downward.

    2. Repeat the above with Methanol till clean. Acetone kills grease, methanol cleans the surface.
    Now repeat the same procedure on the high reflector mirror until clean and let dry. Clean the back side of the optic as well as the face, as contaminants migrate.

    DO NOT clean any optics mount rubber O-rings as these will contaminate the optic with byproducts after the laser heats up if cleaned with other then distilled water.

    Aligning the Rear Mirror

    Install the high reflector (HR, rear mirror) into the far end of the Argon making sure the coatings face inward.

    Slowly adjust the optics mount screws so that the weak REFLECTED HeNe beam from the front coated surface of the Argon HR mirror is visible on the face of the HeNe, then carefully walk the HeNe beam so it is right back into where it came from. If this is really good, the HeNe will flicker from you canceling out lasing with a third mirror, but this is rare. Note that each mirror will reflect 2 beams, 1 from the front and one from the back of the optic. You want the one off the coating. Note also the Brewsters may generate more reflections as well. You will end up with a bunch of dots dancing on the HeNe, keep track of the one you want. Keep working until you have a stable tight mirror mount with the beam centered in the hene beam. then back off some slack to leave room for adjustments of the screws, and recenter the beam. Your WORKING with FRESNEL reflections, or about 2-3 PERCENT of the HeNe beam, a few hundred microwatts at best, so turn off the room lights to see the weak beams.

    HeNe laser with reflected dot shows the HeNe alignment laser on its 3-point adjustable mount. The reflections of the HeNe beam from the mirror being aligned on the argon laser (off of the lower right corner of the photo, not shown), can be seen on the fluorescent sticker. In this case, the mirror still needs some more work!

    Aligning the Front Mirror

    Clean the FRONT Brewster Window using STEPS 1 and 2, above.

    Carefully install the FRONT mirror (nearest the HeNe) after cleaning both sides using STEPS 1 and 2, above.

    Carefully align it using the rear mirror procedure, above.

    Powering up the Argon - Final Alignment

    Making sure you have proper cooling for the argon laser, leave the HeNe on and switch the Argon ON and turn the tube current up to the upper limit, then back it off a little. This should be About 9 amps if you have a 10 Amp maximum laser. Let the laser warm up. DO NOT EXPECT IT TO LASE AT THIS POINT. If it does laser you are very lucky or you have a large frame (e.g., 1 meter long) laser.

    VERY SLIGHTLY loosen the rear mirror mount, NOT THE MIRROR ITSELF!! and slowly press it against the Mount holder or REAR PLATE of the laser, rock it back and forth slightly while doing this, you will see a small flash of laser light on the HeNe face.

    WARNING: THIS PROCEDURE IS FOR LASERS LESS THEN 250 MILLIWATTS ONLY. For bigger lasers, see the vertical search procedure in the laser manual or use the fine adjust screws or search bar!!! This flash will tell you which way you have to move the mirror mount screws, usually opposite of the way you have to hold or tilt the mirror.

    Once you have it steadly lasing, see the sections on Tweaking, Walking the resonator for maximum power, and Cleaning and general maintenance!!! (some still under development.)

    If you don't get a flash, repeat the alignment procedure until you get it!. This take patience and time. Commercial laser optics have a small amount of wedge to avoid creating ghost beams which interfere with the lasing process. This wedge may be what is messing you up as you may be trying to align the incorrect reflection!

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    Additional Alignment Information

    Markus's Comments on Argon Laser Alignment

    (From: Markus Hakes (mah@josquin.pc.rwth-aachen.de)).

    Instead of dismounting both mirrors, aligning the plasma tube to an HeNe laser, adjusting the rear (HR) mirror followed by the front one as described in the procedure starting with the section: Argon/Krypton Ion Laser Cleaning and Alignment Techniques, I took off only the HR. After that, I put my HeNe in from the rear and aligned it so that the dot was centered coming out of the front mirror.

    Then I used the procedure from the section: Aligning the Rear Mirror but first on the front mirror, followed by the HR, which I aligned very carefully till the lasing began. I then adjusted both the front mirror and HR for maximum power.

    I found this to be an easier way especially because the front mirror was not that easy to remove (the photodiode light sensor is on it) and the adjusting of the argon and HeNe was a bit easier (the bore for the outgoing laser beam is much smaller than the end of the plasma tube, so it was easier to get the HeNe beam in the middle). Now at least I had only to remove one mirror. The mirrors are hygroscopic (they love water), so it is not good to remove them for a longer period than absolutely necessary.

    Multi-Line Tuning

    (From: L. Michael Roberts (NewsMail@laserfx.com)).

    Theoretically, an argon laser should produce 5 wavelengths: Two greens, two blues and a violet. Try passing the beam through a diffraction grating or a prism to observe if all these lines [colours] are present.

    If you laser is 'out of tune', the violet line will be the first to disappear so you could try adjusting the resonator screws ONE AT A TIME (and without adjusting the apex screw/nut) to see if you get more/less violet.

    DISCLAIMER: If this causes any problems/damage or reduced output - don't come crying to me Adjust the tuning screws/nuts at your own risk.

    Disks for Laser Alignment

    A Spectra-Physics tech just showed my friend at Sea World a neat trick. He had round disks with rims that slip over the optics without touching the face of the mirrors. Each disk has holes that are centered in the face of the mirror. You peak the laser with a disk with a large hole and keep switching to disk pairs that have smaller and smaller holes until the beams are centered on the mirrors, which results in maximum beam quality and power.

    This only works with medium and large frame lasers that have snap in optics in bayonet mounts that maintain alignment, air-cooled lasers with mirrors held by a clamp and "O" ring can't use this because they don't have repeatability in the optics mount, although if the laser is peaked you can take one mirror at a time out and it has a 90% probability of coming back lasing at least weakly, though its position will need to be adjusted.

    More on Mirror Adjustments

    Moving any adjustment more then 1/2 turn will probably kill lasing unless you have the nice scientific unit with the 80 pitch adjustment screws, so be extremely careful to remember what you just did, it doesn't hurt to take a pencil and mark a mark on the knob/nut so you can see where it was before you started adjusting. Bigger lasers have dial turn counters installed so you can see where your at. One of the tricks is to push on an end plate with a finger to see if the beam gets brighter or dimmer - you are changing the mirror's angle by a milliradian or so, and it makes a difference. This is a good quick check to see if something is messed up. If the setting is just slightly off, a small touchup on the adjustments should send the output power way up. It won't need much - just a fraction of a turn, as you move the screw back and forth. Power should peak and then go down again - that is the time to stop and go the other way, loss of alignment is a real pain to correct!!!!

    Looking at the laser you should see something like this:

           @ Vertical adjust
           X---------------------@ Horizontal adjust
    The pivot is never touched once the laser mirrors are being walked for best cavity alignment relative to the tube bore during initial alignment. Then, all the user ever has to do is slight touchups on the vertical and horizontal, a rule being:

    DO only BOTH verticals then do BOTH horizontals. Then do BOTH verticals again!!!!!!! (repeat the sequence).

    If you start playing with adjustments like "OK, I'll do the front vertical then the rear horizontal then the rear vertical then the front vertical again" you are not moving the axis of the lasing cavity relative to the tube, you are just misaligning your mirrors at random.

    Now it is not uncommon to just have to slightly touch just one vertical or horizontal adjustment on the laser from time to time, for a small fraction of a turn to get around a power loss from drift in the mounts or from vibration, slippage, settling in the mount springs etc. Because the brewster windows are oriented vertically, they tend to act as a non dispersing (not splitting light into its spectra, meaning beam bending) prisms inside the cavity, which means most often a vertical adjustment will be what is needed, in fact most large lasers have a knob or other means for rapidly scanning the vertical adjustment to find to reestablish lasing if its lost, cause the vertical alingment of the mirrors is the harder then horizontal.

    So most of the time all an air-cooled laser needs is a touchup on the rear vertical mirror. If there is an intracavity (line selecting) prism, that complicates things so you might try the front vertical screw for maximum, once you get the feel, do the horizontals. It has to be learned by practice.

    Your eyeball is not a good laser power meter, there are usually jacks provided on the laser head to hook to a analog voltmeter, or in the case of the Spectra Physics or NEC lasers, the front panel meter on the power supply, to see when the >output is peaking. If you don't have a meter, the trick is to use a diffraction grating or prism to get the spectrum of the laser on the wall (assuming you have a multiline laser) and peak the lowest gain lines lasing, which are the deep blues and violets, since they have so little gain, you can see their intensity go up and down due to small movements of the mirrors.

    Another trick is when the laser is in constant light mode, to monitor the tube current, as the cavity is peaked, the current will go down.

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    Spectra-Physics Argon Ion Laser (SP-162) - Discussion

    (All replies from: Frank Tompkins (frank@Uakron.edu)).

    "Hello, I recently picked up a Spectra-Physics Model 162A-07 Argon Laser head and 262A power supply. I It was a pull from a Hell Drum Scanner after the scanner was hit with a forklift. The problem is is the umbilical that hooked the laser to the scanner. I have no idea what the wires do. There is no output power control pot on the front of the panel, just a hole with the markings for it."

    (Pulling out my manual) The power control is a 50 ohm linear pot, going to points N, O, and P on the circuit board (N is the wiper, O is the ccw extreme, and P is the cw extreme)

    "Inside there is a binding block where the pot was and wires leading to the umbilical. The umbilical terminates in a AMP plastic twist lock connector. Is this standard on these supplies? Where might I be able to find a schematic?"

    My 162A has an "older style" rectangular connector that has flat pins oriented either horizontally or vertically to provide polarization. Sorry I don't know the brand, but it's definitely not an AMP twist lock. The manual shows the type of connector I see on mine, so you must have an OEM version.

    "When I got it, the laser would put out maybe 2 or 3 mW at 488nm at about 3.5 A tube current in light feedback control mode. If you move your hand in front of the beam you can see that the light is pulsed. When I switch to current feedback mode the laser goes to full power to about 12 mw at about 10 A.

    I adjusted the mirrors. I am now getting about 22 mW (so says the meter on the front) of the blue/green line. It just needed some adjustment of the vertical adjustments (It just has allen screws. One thing though, If I move the rear vertical adjustment I get the other lines. I guess it is a beam selecting prism. I called SP today and they are looking up the info on the multiline optics. Since I can tune the laser to the desired line, does this mean I will only need to get a different high reflector and not a different output coupler?

    What I am thinking is that the guy that had it before me (who knew next to nothing about lasers or electronics) played with the pots and variable capacitors on the control board. Does anyone know the procedure to reset these settings to normal?"

    There is some calibration and alignment info in the manual.....

    "What is the maximum tube current the head can handle?"

    Depends on the cooling fan on top. The small fan units are limited to 9 A, the large fan unit is limited to 12 A.

    "Last but not least is the optics themselves. In place of a high reflector there is a wavelength selecting prism. Where can I find multimode optics to replace these?"

    The manual does list part #s for the optics, but considering it is a 17 year old laser, the parts are probably discontinued by now.....

    "What is the maximum output I can expect from this head (label says .12 W, yeah right...) It was manufactured in '79/'80."

    The manual is not real clear on this, but from my experience, about 25 mW (all lines) seems about right. That .12 W (120 mW) seems high.

    "FWIW, I only paid $50 for the set, so I am not too worried about doing to much to it."

    Good deal - I paid $90 for a head, and had to build my own supply! Got any more???

    P.S. Watch your eyes - 25 mW of argon laser light can do damage quickly. Also, watch out around that power supply, 140 V at 10 A can kill!

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    Britt Pulsed Argon Ion Laser - Description and Gas Adjustment

    (From: Frank Roberts (Frank_Roberts@klru.pbs.org)).

    I have come across a Britt medical argon laser which appears to be a pulsed design. It is about 1 meter in length, runs off 240V single-phase AC and is air-cooled. The power supply is integral to the laser head itself, and appears to be a flashlamp type design. What I mean here is that the 240 VAC is rectified to about 340VDC and applied directly across the tube and a considerable capacitor bank for energy storage. The laser is fired with a high voltage trigger pulse applied to a trigger electrode wrapped around the tube bore itself. All the electronics appear to be functional in that indeed the potential measured from anode to cathode is about 340 volts DC. A string of 3 100W lamps burns brightly across the supply, so it is sourcing current as well as voltage. The cathode filament does light up as expected and a high voltage pulse of several hundred PPS is applied to the trigger electrode. This pulse easily jumps an air gap of about 1/4", so I assume it's healthy also. Only one problem, no discharge. I applied a 7.5 KV neon sign transformer across the tube to check tube integrity and the tube did light up with a characteristic argon color. I know the difference between nitrogen purple and argon lavender, and the discharge color was indeed the color of argon. One added note: The getter deposit inside the tube is shiny, indicating no air leaking into the envelope.

    There is a gas recharge system built into this laser and the vacuum gage on it seems to indicate an overpressure condition. This may be the only thing keeping this laser from firing. Methinks that somebody got a little punch-happy with the recharge button. Each time it's pressed, a tiny amount of fresh argon is metered into the tube.

    I'm sorry to be so wordy, but I'm trying to come to some kind of decision as to what to do with this laser, and would appreciate some input from my fellow laserists out there. I do not have a vacuum system readily available to me at this time. I do have a three-stage pump capable of pulling a vacuum of 6 microns. The decal on the tube indicates the operating pressure of the laser to be 40 microns. It's theoretically possible to pump the tube down below it's operating pressure and then refill the tube from the recharge supply.

    (From: Steve Roberts (osteven@akrobiz.com)).

    You'd need a 10E-8 Torr vacuum or better.

    Attempting to repump a ion laser with getters without rebuilding the tube is a no-no. The getters will powder and end up in the bore, destabilizing even pulsed lasing, then it will find its way to the most electrically neutral spot in the tube, the brewster windows via Murphy's law, thus suppressing or killing lasing. You really need a turbo pump to clean the water vapor out of the tube, else lifetime is nil and the cathode will be shot. The Britt is pulsed but it has a 2-3 mW quasi cw simmer mode for aiming for 15 seconds at a time. The pressure in a pulsed argon is much much lower then the similar sized CW argon which would have a pressure of around a torr and needs a much cleaner gas fill.

    Hint, hint: Might I suggest building a styrofoam reservoir around the gas bottle and ever so slowly chill it down with LN2, then pulse the solenoid till the pressure goes down using Mr. Boyles and Mr. Charles's laws for gases in a constant volume enclosure. You are not trying to freeze the argon, only to do sorption pumping onto the glass and volume reduction per basic physics. Otherwise you'd have to do a thorough the reservoir repump, as soon as you cracked the seal on the tube you'd suck in enough dirt from the atmosphere to fog the windows big time even in a clean room.

    Note I'm leaving out details of how you'd have to go about reprocessing a cathode if it ever sees nearly atmospheric pressure in argon etc because I work closely with a refurb shop and can't give away all the secrets etc. I will say the LN2 trick works however.

    (From: Frank Roberts (Frank_Roberts@klru.pbs.org)).

    My god Steve, the LN2 idea's so simple I'm ashamed that I didn't think of that. A neat little variation on the cryo pump. According to Merck, Argon's normal boiling point is 87.28 K while LN2 boils at 77.36 K. LN2 should cause the argon in the reservoir to sweat the inside of the metal tank. (Now to calculate the vapor pressure of liquid argon at 77.36 K.) To add insult to injury, I was a physics major the first time around. I should have caught that one, thanks for the tip. I'll let you know if it works.

    (From: Dean Glassburn (nitelite@concentric.net)).

    Actually the Britt laser comes with a cryo attached to the tube on the end of the copper pipe. pull it away from the tube while the head is suspended upside down and place the cryo into a cup of LN. wait until the copper tube frosts up, about 10 minutes, then open the large valve connected to the assembly, about 1 turn. This will pull the tube below the operating range. on the end of the laser put the switch into the standby range and turn on the system. this will outgas the cathode. remember to keep the cup filled with LN. if you have a gauge it should be reading zero. Shut the valve, remove the LN, and allow the temp of the cryo to come up to room temp. At that point you can turn on the system, add gas back to the tube using the refill button until about 25 milliTorr is there. You should get the tube to ignite. Keep and eye on the pressure as it will use gas very quickly initially. Once stabilized, you can fire off the getter using a 12 volt transformer and a variac to heat up one of the getter filaments to a dull orange. Do this only if the gas has a pinkish discharge around the cathode neck and anode.

    (From: Steve Roberts (osteven@akrobiz.com)).

    I have only once seen a Britt on a trip to a conference in Canada, now I know why they last so long. A built-in pump, a built-in fill system, and a built-in cleanup system, and not to mention some of the most beautiful glasswork ever done. Too bad these features don't come on CW systems. :-)

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