Sam's Laser FAQ, Copyright © 1994-2001, Samuel M. Goldwasser, All Rights Reserved.
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    SS Laser Testing, Adjustment, Repair

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    System Maintenance and Adjustments

    Cleaning a Laser Rod

    There are two issues: The sides of the rod and the (probably) AR coated ends.

    (From: heru_kuti@yahoo.com.)

    Two words VERY GENTLY!!!!! The best way to clean laser optical surfaces is not to dirty them. If you inevitably get it dusty blow it off with compressed dry air. If some dirt remains it is best to clean with a piece of REAL lens tissue and very pure acetone being very gentle on it. If you get it badly dirty, like as if some yahoo from the machine shop touched it before washing his hands, the only course is to really wash it. To do this, first dust off as much as you can, next rinse in warm clean water. follow up by washing with a cotton ball and warm detergent/water solution. (Clean water with CLEAR dish soap works well.) Finally rinse with distilled water followed by pure acetone.

    Laser Rod Mounting and Removal

    The ruby, YAG, or other rods in solid state lasers are often mounted in holders using some adhesive, probably Epoxy.

    (From: Klaus Dupre (dupre@fee-io.de).)

    Sometimes the rod holders are fixed with epoxy glue or other glues. Than you may have problems removing the holders without damaging the rod and/or the holders. You may chip the ends and thus require regrinding, polishing and coating.

    There are two methods to remove the glue:

    1. Heat the rod to about 250°C, this will destroy the glue (or make it softer) and you can pull the holders from the rod. With some luck the end-faces will have no or only small damages, but you will have to clean them very carefully.

    2. Put the rod with the holders in hot acidic acid, this will destroy the glue, the holders and the coating, but its the best way to protect the rod.

    (From: Elliot Burke (elliot@nonamehitide.com).)

    Before using aggressive means to undo an Epoxy bond, you might try methyl bromide. This is available as "Milsolve" from Summers Laboratories. It dissolves Epoxy. If you use this stuff, be sure to obtain and read the MSDS. Methyl bromide is much more aggressive than methylene chloride on Epoxy.

    Good practice for use of solvents is to soak things in them in a covered container. Rags with solvent in them should be disposed of carefully. At least bag them before they releast all the solvent into your local air. I leave a few windows open.

    (From: Josh Halpern (vze23qvd@mail.verizon.net).)

    Believe it or not many Epoxys can be "rotted off" if left overnight in methanol, which is somewhat safer to use than methylene chloride.

    Aligning a Solid State Laser Where there are adjustments, the wide bore, planar mirrors, and high gain of the typical solid state laser make alignment quite straightforward for once. :) The same basic principles apply as with HeNe and Ar/Kr ion laser alignment (see the sections starting with: External Mirror Laser Cleaning and Alignment Techniques for details) but due to the orders of magnitude more gain, you only need to get close for the system to start lasing. Disks with small holes will be useful to center the alignment beam in the cavity and for IR emitting lasers (like YAG at 1,064 nm), some means of detecting the beam such as an IR sensitive camera and Zapit paper will be needed. Once the cavity is roughly aligned, the Q-switch (if present) is installed and aligned to the beam path. This can still take a long time - hours - especially if you haven't done it before.

    The following applies to a typical medium-to-large ruby or YAG Q-switched pulsed laser:

    (From: Christopher R. Carlen (crcarle@sandia.gov).)

    Typically to align a laser, you set up a reference beam from a HeNe laser through the cavity. With a solid state laser, you may want to ensure the rod is centered on the high reflector and output coupler optics. Then with the OC removed, align the HR to aim the HeNe back into itself. The same is done with the OC.

    With a Q-switch (Q-sw), the situation is complicated. What might be advisable if the assembly is simple enough, is to remove the Q-sw from the cavity and align the laser without it. Than, if it lases, at least you know that there is no optical problem with the rod, HR, or OC. And you may be able to just pop the Q-sw components back in while retaining the non Q-switched cavity alignment. Note, this may not be possible unless you at least align the cavity first with the polarizer, as some translation and perhaps off axis pointing of the beam will result from it. So that means: align initially without the Pockel's cell and 1/4 wave plate, but with the polarizer.

    It is then a matter of figuring out if there is something wrong with the Q-sw, polarizer, or 1/4 wave plate (Note all of this assumes that the Q-sw uses this common topology).

    If the Q-sw optics look OK and are clean, (DO NOT clean them unless you really know how to clean laser optics), then reinstall the Q-sw components. (Also note if the Pockel's cell is filled with liquid. It should be. If it is dry, it is probably no good.) Since the HeNe beam is of a different wavelength from the ruby laser, it may be difficult to verify proper operation of the Q-sw using the HeNe. However, if you have access to a 670nm, 680nm, or ideally a 690nm laser diode, that wavelength would be close enough to the ruby's 694nm to use the diode laser as an alignment reference (assuming you can get a reasonably circular and collimated beam). Then you can align the polarizer to Brewster's angle by orienting the polarization of the ref. beam for a P-bounce off the polarizer. Adjust the angle of incidence for a minimum reflection. At that point, with the Pockel's cell still out of the cavity, the beam that passes through the polarizer will pass through the 1/4 wave plate, reflect off the HR, bounce back through the 1/4 wave plate, and be reflected out of the cavity by the polarizer. That is because the two passes through the 1/4 wave plate caused a 90 degree polarization rotation, resulting in a S-bounce off the polarizer, which is a high reflectivity incident condition.

    Now there are complexities to getting the Pockel's cell aligned that are deeper than what we are into already. But assuming you can get it close, the situation described above should not be altered by its presence. However, if you can arrange to apply a constant voltage to the Pockel's cell that is identical to the voltage applied by the power supply to produce an output pulse, you should find that the reflection off the polarizer (the reflection of light out of the cavity of the beam that has bounced off the HR) no longer appears, or is effectively gone (it will be very faint). So that is the proper operation of the Q-sw: No voltage on the Pockel's cell=strong reflection of light from the polarizer. Voltage on Pockel's cell=minimal reflection off the polarizer.

    If you can get this far, there is a good chance you can run in Q-switched mode now.

    If you can get some YAG or ruby laser manuals from other lasers with alignment procedures, do so. Of course, the holy grail would be the manual for your laser.

    Comments on High Power SS Laser Alignment

    The following were in response to questions about misalignment of a particular model Q-switched YAG laser but apply in general to many similar systems.
    "Oops... it happened. We moved into our new lab space and one of the Quanta-Ray DCR-11 Nd:YAG lasers got bumped."

    Realignment from scratch isn't a trivial job if you haven't done it before. It involves removing everything from the optical cavity and subsequently lining components up with the aid of a HeNe laser. If you hare a real budget, letting the laser manufacturer or a reputable laser service company do it may be best since aside from the safety issues, damage to the laser crystals and optics are quite possible if alignment isn't perfect. But, here are some comments and suggestions along with the risks if you want to play:

    (From: David Demmer (ddemmer@physics.utoronto.ca).)

    And a word of caution here: These DCR's tend to operate pretty close to the limit of what their guts can handle. Operation anywhere near full power with a misaligned cavity is almost guaranteed to blow up some internal component, such as your YAG rod or Pockels cell.

    If you attempt this yourself, you must remember to do all the alignment with the laser "free-running", i.e., not Q-switched. Use some diagnostic aid like Zapit(tm) (burn) paper or a CCD camera to look at your beam profile. Don't Q-switch the thing until the beam profile is perfectly symmetric, and therefore well-aligned.

    (From: Joshua Halpern (jbh@IDT.NET).)

    One of the things that may be wrong is the Q switch. Turn the laser to long pulse (Q-switch off) and see if you get full power or near. If so the Q-switch is the problem. The easiest thing would be if the Q-switch delay is set incorrectly. It's just a matter of turning a dial.

    Next you need to look. First get some exposed Polaroid film and put it in a baggie. Then hold it in front of the laser for one shot. Hopefully you still have enough power to get a burn pattern. This should be symmetric (I forget whether the JK had a near Gaussian or a "doughnut" pattern). If you see ugly striations, you either have a very badly adjusted laser or some burned optics.

    You have probably "misaligned" the laser when you tweaked the lamps. YAGs, especially the oscillators, are better tuned for beam shape than pulse power.

    Next remove all the beam tubes and look carefully at the mirrors, rod ends, polarizer and Q switch for burns. TURN THE LASER OFF FIRST!

    You probably will need a dentist's mirror and a small flashlight. Maglites(tm) are great for this. The best way is to hold the light at a high angle to the perpendicular and look directly at the component, but you may have to move the light and your head to see this.

    Then put a piece of white paper behind the laser head and use the flashlight to illuminate it, while looking through the rod with your dental mirror. This should give you a much better idea about what is going on.

    JK was represented in the US by Lumonics, who appear to have gone out of the scientific laser business, but they may know someone who still does servicing (They are in Ottawa, Canada). Finally there are lots of folks in chemistry and physics at NIST who operate YAGs and can give you some idea of how to proceed.

    Issues of Liquid Cooling in SS Lasers

    (From: Steve Quest (Squest@cris.com).)

    Why do you think we charge so much to setup and tear down a YAG laser. YAGs have external and internal cooling water. The internal water is deionized (distilled, without minerals or salts that cause ions) also called anionic water. Anionic water doesn't conduct electricity, so it CAN come into contact with electrodes and cause no harm. Our system flows water across the pumping lamp, and YAG rod to cool them, then circulates the heat-bearing water across a stainless steel heat exchanger which couples the heat to cold tap water, and dumps the heat-bearing tap water down the drain. Efficient, eh? I want to get a chiller someday.



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    SS Laser System Repair

    Discussion About Nd:YAG Laser Repair

    "I and a colleague tried several things: replacing the flashlamp, tweaking the front and back mirrors, replacing the deionized water in the closed-cycle cooling system, but none of these steps improved the output."
    (From: Jim Cavera (jcavera@alcnet.com).)

    Try checking the crystal. Nd:YAG crystals are prone to heat-induced, microscopic fractures. Enough of these can drop the output energy considerably or even extinguish lasing altogether. CAREFULLY remove the crystal (no dust or oil, please, or even fingerprints, and be particularly careful of the AR coating that most crystal manufacurers add) and put it under a good optical microscope. What you would be looking for are site inclusions and fractures that cut across the axis of the crystal.

    NOTE : this is probably the last thing you would want to check. Try everything else first. Nd:YAGs are simple creatures though, and it sounds like you have everything else pretty well covered.

    (From: Rick Fletcher (fletcher@news.uidaho.edu).)

    Definitely the last thing! Check the cavity condition (corrosion, algae, etc.) before doing this. Also, make sure you do not have a damaged optic, like a cracked quarter wave plate, etc.

    (From: Joshua Halpern (jbh@idt.net).)

    One of the things that may be wrong is the Q-switch. Turn the laser to long pulse (Q-switch off) and see if you get full power or near. If so the Q-switch is the problem. The easiest thing would be if the Q-switch delay is set incorrectly. It's just a matter of turning a dial.

    Next you need to look. First get some exposed Polaroid film and put it in a baggie. Then hold it in front of the laser for one shot. Hopefully you still have enough power to get a burn pattern. This should be symmetric (I forget whether the JK had a near gaussian or a "doughnut" pattern). If you see ugly striations, you either have a very badly adjusted laser or some burns.

    You have probably "misaligned" the laser when you tweaked the lamps. YAGs, expecially the oscillators are better tuned for beam shape than pulse power.

    Next remove all the beam tubes and look carefully at the mirrors, rod ends, polarizer and Q-switch for burns. TURN THE LASER OFF FIRST.

    You probably will need a dentist's mirror and a small flashlight. Maglites are great for this. The best way is to hold the light at a high angle to the perpendicular and look directly at the component, but you may have to move the light and your head to see this.

    Then put a piece of white paper behind the laser head and use the flashlight to illuminate it, while looking through the rod with your dental mirror. This should give you a much better idea about what is going on.

    The hint about looking at the cavity is also good.

    JK was represented in the US by Lumonics, who appear to have gone out of the scientific laser business, but they may know someone who still does servicing (They are in Ottawa, Canada).

    Finally there are lots of folks in chemistry and physics at NIST who operate YAGs and can give you some idea of how to proceed.

    Grinding and Polishing a Laser Rod

    The bottom line is that unless you have no choice, refuse any offers of low cost 'slightly chipped or otherwise damaged solid state laser rods, or those that just need a little finishing! :) These show up regularly on eBay and elsewhere. The general consensus is that they should be avoided despite the attractive prices.

    Note, also, that rods sold in this condition may have failed other preliminary quality tests including not having the proper percentage or uniform doping or being cut from a portion of the original crystal which had optical defects. Presumably, the manufacturer would not have gone to the trouble to cut them (to the rod shape) if they were total garbage but who knows?

    The first comments are for ruby and the second for YAG.

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

    Sadly, ruby is about the second hardest mineral on the planet to polish. Most commercially available abrasives don't even scratch it and you need a optical grade finish or the rod ends will blow off. A less then perfect finish greatly increases the lasing threshold. A flat takes two lapping rigs (one slightly spherical, one slightly concave) made of a material of slightly less hardness then the ruby, and a lot of different sizes of abrasives. You could start at a local lapidary shop and have them saw the ends, but they must be parallel to within 1/2' or so, then try to lap it down following the instructions in an amateur telescope making book.

    By the time you go through all this, including buying a optical flat to check the ends and a HeNe laser to measure parallelism, plus practicing on a couple of glass rods, you could buy the whole head from say Meredith or Midwest in working order for less cost. Your other option is to have one of the laser rebuild companies that do ruby and YAG, such as Kentek, repolish it for you.

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

    I called a friend. He makes YAG optics. I think most are smallish. Here are some notes for YAG in particular:

    He said to get a book on polishing YAG in a lapidary store.

    I would bet though, that normal lapidary techniques won't yield anything like 1/4 wave optics. Perhaps the addition of keepers to normal lapidary practice would get you into that realm.

    Multiple Pulses From Rangefinder Laser

    Lasers designed for time-of-flight ranging usually use a Q-switch to produce a single intense pulse. A failure where multiple weak pulses are generated instead points to a problem with the Q-switch. There are several types of Q-switches:
    1. Mechanical Q-switch (Hughes M-60 tank rangefinder). Most failures would result in no output at all but if the motor wasn't turning and the mirror or prism just happened to line up, there could still be some weak or erratic output pulses. However, such a failure of this type of Q-switch is highly unlikely.

    2. Passive Q-switch using a bleachable dye cell, Hughes M-1 rangefinder). Damage to the dye cell could result in it bleaching prematurely resulting in a series of weak pulses. If the dye cell was broken or missing, the output would be similar to an non-Q-switched laser.

    3. Pockels or other electro-optic Q-switch (medical, scientific, and research lasers). Misalignment or electrical problems could result in improper operation.
    Modern compact rangefinders are likely to use the passive Q-switch technique of (2) and problems with the dye cell are common (probably just behind flashlamp failures). Dye cells can be replaced. For example, the Hughes M-1 unit shown in SSY1 Laser Head Assembly uses such a dye cell attached near the end of the Nd:YAG rod. See the section: A Small Nd:YAG Laser - SSY1 for more info.

    Problems with Beam Quality of High Energy Pulses

    The following comments were prompted by a complaint that a particular pulsed laser produced a TEM00 beam only when it was run well below its published output power/energy specifications.

    (From: billyfish@aol.com).)

    One of my pet peeves in laser specification is the attempt to get maximum energy out instead of maximum peak brightness (radiance). In most cases, although not all, high brightness is preferred over gross energy. Once energy is stored in the rod, there is only a certain amount that can feed into forming a TEMOO mode. As the energy that can support that mode is used up, there is a remainder that can only couple efficiently to higher order modes. The result is more energy, but with more beam divergence and greater pulse width, ore even multiple pulses.

    Single mode performance can be achieved by using a dye Q-switch. Optical quality of the system has to be good. Otherwise, you end up with a single mode, but it may be a poorly shaped one.

    The dye Q-switch does its magic by keeping gain slightly above threshold for a long time. This gives the lowest order mode a chance to grow at the expense of higher order modes that have lower gain. Finally, when the dye bleaches, that mode grows in energy, excluding off-axis modes.

    A similar effect can be achieved using electro-optical Q-switches. Use a two step process where the first step gets you barely above threshold thereby selecting the highest gain mode. The second step reduces resonator losses to where the selected mode seeds the laser to produce a high energy lowest order mode.

    Inconsistent Energy from Forth Harmonic Pulses

    (From: Bob.)

    How much variation is there?

    Please keep in mind that you are going through two nonlinear processes to get to the 4th harmonic. it is the least efficient process of the the other nonlinear processes, and any instability in pulse to pulse power/energy is magnified by the time you get to the 4th harmonic.

    The easy way to tell if this is the case would be to have a dual channel oscilloscope look at the fundamental and the 4th harmonic. If you notice dips in the fundamental at the same time as the dips in the harmonic, then that's the problem. If you are seeing very large fluctuations only in the 4th harmonic in this case, it's probably due to improper phase matching. Try adjusting the angle of the crystal (quadrupler) or the temperature. Comments on Nd:YAG Laser Power Instability The following discussion was prompted by the question below on the USENET newsgroup alt.lasers:

    "There is a 15% power instability when the laser is working CW, single transverse mode and polarized. The crystal is new, and so are the mirrors and polarizer plate. The power level is OK (13 Watts) but stability is terrible. This is so even without the Q-switch and mode-locker acousto-optics modulators in the cavity."

    I measured the optical power variation of the pump lamp. Its instability is well below 1%, with frequency components of 60 Hz and harmonics, as expected. It is a new EG&G lamp, clean, and properly installed with correct polarization. The problem is not there."

    (From: David Demmer (ddemmer@physics.utoronto.ca).)

    My best advice: lasers are simple machines so don't panic, approach it systematically and it will work. Finicky: simple and finicky.

    There's only going to be three sources of instability: electronic, optical, and mechanical. Rule out the easy ones - get your electronics shop to have a look at the current to the lamps. If it is steady and the lamp is not in backwards you are OK. If the optical and other mounts are steady, you are OK - they almost certainly are, even if they have crummy adjustments they won't go anywhere unless the system is vibrating.

    Optical problems. These usually arise in YAG because it has strong thermal lensing and there are always small fluctuations in the cooling water flow. The trick is make sure that the flow is as laminar as possible and that the intracavity beam is centered in the rod and not too large.

    Check the flow tubes around the lamp and/or rod: Are they in good condition? no cracks? held firmly in place? Cracks are hard to see when the tubes are wet.

    Are the ends of the rod clean? Sometimes leaks around the rod end seals cause mineral deposits on the faces. This is very tough to check properly without disassembling the lamp/rod housing, but here is a quick-and-dirty.

    With the lamps off (!!!) shine a flashlight through the rod while looking through it along the laser axis using a small dental mirror. It should look PERFECT, absolutely NO indication that there is something there. ANY flaw, haze, or whatever which is visible under these conditions will kill you.

    Is the rod aligned? Make small (1 to 2 mm aperture) alignment apertures that you can place on the cavity mirrors, and align the laser so that the beam is centered on them. Make similar, though smaller (0.5 to 1 mm) apertures that you can place on the "pot", i.e. the assembly that holds the rod. You must make absolutely sure that the beam is centered on the laser rod. The laser may stop lasing with these in place: this would be a good sign, since it should not if the rod really is centered.

    If necessary you will need to do a HeNe alignment of the whole works: mirrors and rod. Don't be afraid to move the pot around to align the laser: it is the only way, and with a HeNe you can always recover from any alignment disaster.

    If the beam really is centered and there are still problems, try restricting the size of the intracavity beam: it may be "trying" to go multimode and need a bit of help to keep it TEM00. You may need to reduce the power by 20 or 30% to get it stable, but use the largest you can. Also, if the lamps are driving too hard the thermal lens may be just too strong and the cavity may be getting close to an unstable resonator configuration. Try backing off the lamp current. I know of one laser (Coherent Antares) that will actually stop lasing with too much lamp current.

    Above all, there is no point in putting in the mode locker etc. until the laser works really well as an unpolarized cw laser.

    (From: Roland A. Smith (see@www.lsr.ph.ic.ac.uk).)

    We found cooling water fluctuations to have BIG effect on the system. It originally had the mode locker cooled from the flashlamp supply (ugly) and running a separate small cooler on the mode locker helped quite a lot. In addition we added our own control electronics to the existing temp control. We actually stuck a central heating system heater in the main water bath coupled to a programmable differential controller. This adds heat as necessary to keep things more stable. Do you hear the cooling water controller go "thump .... clunk woosh.... wait .... repeat. If so you're going to have problems.

    The water circulation to the mode locker is currently removed. We do have this "thump .... clunk woosh...." system. (Very nice sound effect :) ) However, by changing the secondary water pressure I can have it run almost continuously (only woosh). There doesn't seem to be a correlation between the CW Nd:YAG noise (1 kHz range) and the water temperature control system.=

    These systems can be a real bitch. Ours now provides useful service as a door stop. :) Believe NOTHING they tell you.

    (From: Mattias Pierrou (mp@optics.kth.se).)

    Since your laser components are all new, I suggest that you take a look at the flashlamp and/or your power supply. Some time ago we had stability problems with one of our high power lasers (different kind though - Ar+) and we tracked it down to the old, worn power supply.

    (From: Ralph Page (Ralph.Page@Prodigy.net).)

    Reading these comments brings back some pretty horrifying experiences from my past. I am not sure I saw the original post but all of the suggestions you noted were consistent with my thoughts. I am really suspicious of the water flow within the pump chamber. Is it possible for you to alter the flow rate/pressure of your cooling source? If you have an alternate to the existing water source or you can alter it simply (flow rate pressure, etc.) you may get a hint about minimizing the instability.

    Repair of DPSS Laser Pointers

    The following applies to laser pointers based on Diode Pumped Solid State (DPSS) laser technology. The most common by far is the green variety. However, a few very expensive blue DPSS laser pointers do exist. For common red pointers, see the section: Repair of Diode Laser Pointers. And, for older style helium-neon laser based laser pointers, see the chapter: HeNe Laser Testing, Adjustment, Repair.

    With prices as low as $4.95, serious troubleshooting and repair of a cheap red laser pointer probably isn't worth the effort, time, and expense. However, with the average price of a green DPSS laser pointer still around $300, there could be significant motivation if the warranty has run out, is void due to damage or abuse, or never really existed in the first place. :( But, if there is still a useful warranty, I highly recommend that you take advantage of it!

    From the Comparison of Red and Green Laser Pointer Complexity, it is quite obvious that there is a lot more "stuff" inside a green pointer. However, the most common problems are probably still external to the DPSS laser module itself. Better hope so - doing anything inside there is at best a royal pain and probably justified only by its educational experience or laser parts salvage value.

    Refer to Edmund Scientific L54-101 Green DPSS Laser Pointer for a general idea of what to expect. The detailed disassembly procedure will depend on the exact model. A combination of screw, press-fit, and glued construction is likely. Non-destructive disassembly may not be possible for some. See the section: Disassembling a Green DPSS Laser Pointer for the detailed procedure for the L54-101 model. Lower cost models will be slightly simpler using composite crystals and simpler optics, but may be even more difficult to disassemble if its possible at all.

    Here are possible problem areas for a pointer that is weak or dead and hasn't been run over by a Sherman Tank:

    Disassembling a Green DPSS Laser Pointer

    Here is the complete step-by-step procedure for non-destructively disassembling the Edmund Scientific L54-101 green DPSS laser pointer. (Compare this to the simplicity of a Typical Red Laser Pointer!) For a description of this unit, see the section: The Edmund Scientific Model L54-101 Green Laser Pointer.

    The construction details are shown in Edmund Scientific L54-101 Green DPSS Laser Pointer. This should help make sense of the procedure below.

    The L54-101 uses the same DPSS module as the unit disassembled somewhat destructively in the Laser Equipment Gallery (Version 1.74 or higher) under "Dissection of Green Laser Pointer" and probably many other models. See Internal Organs of Green DPSS Laser Pointer for an annotated photo of the major components.

    Here is a detailed procedure that should provide access to everything inside with at least the possibility of reassembly, though putting everything back together with any chance of getting back to a working state with good beam quality will require quite a bit of care, determination, and the prolific use of selected four letter words (see below). :) It would probably be a good idea to have the sequence of photos in front of you while embarking on this adventure. A warning to the squeamish: some of these pics are a bit gory and you may want to send any working green pointers you own to another room for the duration. ;) The case and laser diode driver of the L54-101 are different than those shown in the dissection but all the actual DPSS laser parts are absolutely identical.

    The first set of steps deals with basic disassembly of the case:

    1. Unscrew the battery retainer cap and remove the lithium cell.

    2. Gently, bend and twist the two chrome sections back and forth at the gold plated ring until the longer section enclosing the battery compartment and laser diode driver can be removed. Don't lose the rubber push button pusher.

    3. Wrap a fine wire around the two terminals of the laser diode (to protect against ESD, etc.) and unsolder the driver board. Then solder a jumper across the two terminals. Make a note that the driver terminal marked LD+ goes to the grounded terminal if you ever intend to put this thing back together!!!

    Note: There is no need to actually remove the driver board if you aren't going to go inside the cavity itself and will only be dealing with the front optics but if there is a need to remove the inner brass barrel of the DPSS module, it's easier without the bulky circuit board in the way.

    The next set of steps deals with removing the "rear cavity" components including the pump laser diode, vanadate (Nd:YVO4, and KTP:

    1. Make some scribe marks on the inner brass barrel or use some other means so you will know the exact orientation of the copper disk/LD terminals. This is critical in achieving proper alignment if all you do is remove and replace the rear cavity components and expect the pointer to have any chance of working properly without a lot of risky fiddling. The reason is that since as a result of normal manufacturing tolerances, the pump diode isn't perfectly centered and the vanadate isn't perfectly perpendicular to the optical axis, the original orientation must restored to match up with the alignment of the OC Mirror, Expanding Lens, and Collimating lens. Even then, a bit of lateral jiggling will be needed. :)

    2. Using a pointed tool (Xacto knife, dental pick, tiny screwdriver, etc.), scrape all the visible dabs of adhesive from the joint between the aluminum retaining ring and the brass barrel, and between the ring and the copper plate (which is the back of the laser diode mount).

    3. Firmly grasp the case in one hand and with a suitable retaining ring adjustment tool, attempt to turn the retaining ring counter-clockwise just the smallest amount. (My custom retaining ring adjustment tool is a piece of thin sheet steel with a pair of projections filed to fit the slots in the ring with a cutout to clear the LD terminal posts. Other possible "tools" for this and the other retaining rings include reworked oversize paper clips and hose clamps.) It is best if the copper plate not rotate with the ring. Thus, as soon as the ring can be turned, attempt to break it free of the plate and prevent the plate from rotating as you unscrew the ring all the way. As the ring is removed (don't lose it!), keep the case upright as this will expose a few loose parts.

    4. The copper plate on which the laser diode is mounted can now be pulled free. If the diode is still good, store this assembly in an antistatic bag.

    5. With a soft lint-free cloth or preferably, some lens tissue, turn the case over. The following parts should fall out one by one. Make a note of the orientation (top/bottom) of the vanadate and KTP assemblies:

      • Aluminum spacer ring.
      • Copper plate with the 2 x 3 x 0.5 mm vanadate crystal.
      • Three-quarter round brass disk with 2 x 2 x 3 mm KTP crystal.
      • Indexing pin.

      Note that the indexing pin goes through the hole in the vanadate plate that's closer to the outer edge and into the center of the three holes in the KTP plate. The outer end of the indexing pin also fits into the laser diode mounting plate so all three components remain more or less aligned (though there is a lot of slop).

    At this point, if the problem (if any) was with the rear cavity components (and not the OC mirror), then there is no need to go further and reassembly may be possible without complete realignment - but probably only if all you do is look at the parts! Any replacement or even just regluing of vanadate, for example, will almost certainly result in a large enough change that this won't be possible.

    The next set of steps deals with removing the OC mirror and front optics:

    1. Unscrew the front gold plated bezel.

    2. It is best to remove the inner brass barrel at this point. This is done using a press (e.g., a drill press but not for drilling) and suitable scrap wood or aluminum. Force needs to be applied between the very outer brass casing (there should be two visible - one is part of the casing and can stay in place) at the cavity end and the inner aluminum cylinder (the one with the collimating lens glued to it) at the output end. It shouldn't require very much force and the entire assembly will slide out. Take care that no force is applied to the collimating lens.

    3. Unscrew the long aluminum cylinder holding the collimating lens revealing the green IR filter. If desired, scrape off the bits of glue and remove the disk on which the lens is glued.

    4. Similarly, unscrew the aluminum assembly which contains the IR filter and expanding lens.

    5. Using a large flat blade screwdriver, remove the retaining ring securing the OC mirror. Take care not to scratch the mirror!

    6. Finally, use a tiny flat blade screwdriver to slightly loosen the set screws holding the OC mirror and remove it.
    That's everything! Admire your pile of green laser pointer parts. :)

    CAUTION: If both the rear cavity components and OC Mirror are moved, a complete realignment will probably be required as described below. However, if only the rear cavity components or the OC Mirror are moved (but not both), then only they would need to be realigned.

    The procedure for reassembly (or original assembly at the factory) and alignment would be something along the lines of the following:

    1. Install KTP, Nd:YVO4, spacer ring, indexing pin, and Pump Laser Diode assembly. Tighten the retaining ring.

    2. Install the OC Mirror and its retaining ring loosely.

    3. Power the Laser Diode and adjust OC Mirror radial position until cleanest and strongest TEM00 beam is obtained. Carefully tighten the 3 set-screws (only 1 is shown) and retaining ring to lock OC in position.

    4. Install Expanding Lens/IR Filter mount. Adjust its position to center output beam and add dabs of glue to lock it in place.

    5. Install Collimating Lens mount. Adjust Collimating Lens radial position to center output beam. Add dabs of glue to lock it in place.

    6. Adjust Collimating Lens axial position for desired focus in far field and add dabs of glue to lock it in place.

    7. Install completed DPSS module in laser pointer body.

    If I can obtain a replacement pump diode, I will report back on how well this procedure actually works!



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    Laserscope Double YAG Maintenance

    Laserscope Description

    If you've never seen one of these up close and personal, check out the Laser Equipment Gallery (Version 1.78 or higher) under "Laserscope Green YAG Laser Systems" for the inside story.

    (From: Steve J. Quest (Squest@cris.com).)

    The Laserscope is a frequency doubled Nd:YAG - 532 nm (green) quasi-CW at about 57 watts maximum average power (kick ass power, eh?). It is a Q-switched, krypton arc lamp pumped, deionized water cooled system. Arc lamp pumping energy is about 4.5 kW average. It sucks 30 A of 208 V 3-phase. It uses a KTP doubling crystal.

    My major application is commercial advertising. "Just follow the green laser beam in the sky to the XXYY company". When we take it to cities that have never had an outdoor laser operate there before, it causes GREAT excitement! The collimated static beam is visible literally for hundreds of miles! The worlds largest bug zapper, mosquitoes are attracted to the beam, and are instantly dessicated for at least the first few hundred feet of beam length I'm aware of, possibly farther. :) Very buggy areas it literally rains dry/dead mosquitoes for a time when it first comes on.

    For $53,000 you too can own a Laserscope. They've come way down in price. :) The lowest you can get one for (non-working, severely in need of repair) is about $5,000.

    Laserscope Cleaning and Alignment

    (From: Bob.)

    There is nothing special to be done during cleaning. If you are familiar with laser optics, the standard once over swipe method works fine. The only problem is the KTP - the crystal is so small, it's a real pain to clean. Also, when taking the KTP out, be very careful when putting it back in the mount as it is very easy to put too much torque on the mounting screws to damage the crystal. A rule of thumb: When you see the spring loaded screws get tight, turn them no more than an addition 1/4 to 1/2 turn, then pick up the mount, and turn it on it's side over a soft clean surface (optic gloves are best) and lightly tap the mount to make sure the KTP doesn't move around (the last thing you want is the KTP to work free of the mount and end up loose on the base plate - I have seen this happen to a laser before!).

    To perform a complete alignment from scratch, take out the KTP and Q-switch. If you have an alignment optic, things will be made much easier on you. A typical optic is a 5 to 10% OC. If you have one, put it in place of the mirror closest to the Q-switch (hence forth referred to as mirror 'A') and walk the laser to provides the most output using the adjustments on mirror 'A', and the other back mirror (mirror 'B').

    At this point, install the Q-switch and turn on its voltage but not the gate pulse. This will put the laser in hold off mode. Dip the Q-switch as usual to get minimum output from the optic after this is done, I'll normally go back and readjust the mirrors a bit with power (24 V) turned off to the Q-switch, as some are manufactured with no parallel/perpendicular faces, causing the beam to distort some.

    At this point, you replace the output coupler mirror with the normal flat HR. It is convenient to have a second power meter for the rest of the alignment. Leave one power meter behind optic A and place a second one in front of the green output coupler. Once the KTP crystal is in place you will not adjust any optic mount other than the KTP crystal and the 'B' optic. I normally turn back the angle alignment screws on the KTP most of the way, then use my hand to physically move the mount till I see a flash of strong green light (at about 28 amps of lamp current, and NO Q-switch), then use my other hand to drive the screws in till the mount stays stable. At this point you should double check the beam's position on the KTP by looking in the small orange filter glass on the mount, center the beam as necessary. The alignment procedure is similar to walking in an laser, a movement you make on the 'B' optic will require a complementary move on the KTP angle adjustment mount. Before you adjust the 'B' optic, adjust the KTP as you have just put it in the laser for max green out of the green OC adjust one axis of the 'B' mirror mount for max IR from the power meter behind mirror 'A'. Go back and make a complimentary move on the KTP looking for max green from the green OC. Repeat till no more gains can be made in green power, then move to the second axis. Repeat this overall process two or three times.

    if you would like to REALLY tune in the laser, it is permissible to make small adjustments to the 'A' mirror after this initial alignment is complete, but in no case should you turn the optic adjustment more than about 1/4 turn (on the 'A' mount) - otherwise you may hit the side of the KTP, causing thermal fracture.

    Keep in mind that when ever you move a cavity optic you are always looking for gains in IR, when you adjust the KTP you look for gains in the green output.

    Next step would be determining the fold back point, in Z-folds this may be as high as 40 something amps, keep in mind than you can go this high, but it will obviously decrease the life time of the lamps, and optics (not catastrophicly, but none the less, a reduction in life will be seen) the power supply on these lasers is capable of 6 kW continuous, as long as this is not exceeded (and it shouldn't be) you should not have any problems. Normally, after maxing out the current, I'll go back and do a quick realignment.

    The only other adjustment that are left at this point are the crystal temperature and Q-switch. Use a multimeter to monitor the crystal temperature (the test points read in ohms). It should be around 700 to 800 ohms, but I have seen some lasers that are hundreds of ohms off. Turn the trim pot on the crystal temperature controller in small increments while monitoring the power.

    At this point it is safe to turn the Q-switch on: Apply 24 volts, and enable the gate pulse generator. on the systems with the external gate pulse generator, RA3 controls the RF power, this may need to be adjusted up slightly if the laser is putting out a lot more power than it did originally, RA2 controls the repetition rate. With doubled YAGs (or any other Q-switched laser) there is a definite sweet spot where maximum average power will be reached at a certain Q-switch repetition rate. This is normally arounds 20 to 25 kHz. However most laser light show guys turn this pot all the way up to get maximum repetition rate, so that the beam looks that much more continuous when scanned at high speeds. This will cause a reduction of no more than about 10% of average power.

    Notes on Medical Conversion Laserscope Adjustments

    (From: Bob.)

    All Z-folds are capable of putting out far more than what they are rated for. They should put out at least 30 W, and a decent one should do around 40 W. I have personally seen one that was really smoke'n - doing 55 or 56 W.

    These lasers are adjusted for medical use so that their output is just enough to achieve rated power, although all of the components are capable of much more. Basically, all you do is turn the current up on the lamp until you don't notice any significant increase in optical power (you need a power meter for this, you can't do it visually). this is called the fold back point, and it is safe to run your laser at this threshold. This normally happens at around 36 to 40 amps for a Z-fold (a few amps lower for an L-fold). Generally, these lasers are set at 28 to 32 amps at the hospital. If you use a multimeter to look at the voltage on the two test points on the lamp power supply (it unbolts and swings out to you, the test points are normally blue and red, and on the same side of the supply as the lamps cable, power in and all that good stuff) if memory serves me correctly 100 mV is 1 A of lamp current. If your unit is putting out, say, 6 W, it should be doing in the ball park of 30 W when Q-switched. And, if your laser came straight from a hospital and hasn't been adjusted for maximum power, it should do a fair bit more than that when cranked up...

    The Q-switch module does all the first pulse suppression. Its control input only tells the Q-switch driver circuitry to supply gate pulses, and allow the laser to turn on.

    CAUTION: On the Laserscope or any Q-switched high power YAG (or other SS laser), don't think you can get away with blanking it (as you might want to do for laser show applications) by using the gate to the Q-switch. Some people have tried and quickly found out that it's the quickest way to destroy your optics. If you blank very quickly with your Q-switch, there is no way to get effective first pulse suppression, and you end up with giant first pulses drilling all your optics, starting with the most expensive ones. :( Use other means for blanking like an external galvo.

    The first pulse suppression on the early mods of the Laserscope Q-switch drivers (the units with the gate pulse circuitry mounted on a board exterior to the RF section) is kinda iffy. If you are thinking of using it for blanking, don't do it very fast (i.e., don't use it for blanking during a show, but turning the beam on and and off during a show, or for alignment, etc., is OK). The newer Q-switch modules have far better first pulse suppression, and I have been told by the manufacturer that you can blank up to a few hundred hertz with the Q-switch enable input, but I have not done this personally in a Laserscope type system.

    There is a heavy transformer in the bottom of the unit which is an isolation transformer to keep electrical noise from the laser and power supply from getting into the hospitals electrical system. I have seen many of these fail, and they are about 140 pounds of dead weight. First thing I normally do after bypassing the computer on a Laserscope is tearing out that tranny. It's not needed unless you plan on doing laser light shows in a hospital surgery. :)

    The schematic for modifying the Laserscope DPSS medical laser to run without the use of the built in computer can be found on Skywise's Laser Reference Page.

    A WORD OF CAUTION!!!!!!!!!: DO NOT adjust this laser when it is operation. Major adjustments while the KTP is in the cavity can damage optical components, even if the Q-switch is off. Adjusting the cavity while the Q-switch is on is tantamount to playing Russian roulette with your KTP and optics!!!!! I have seen many a system damaged by such actions, especially by someone who is new to lasers, or new to frequency doubled YAGs.

    FYI there isn't much reason to look for a manual for this beasts, as they were all designed for hospital technicians. no real info that is of any use. The service manual did have some information about changing optics, etc., but someone with some familiarity of laser shouldn't really need this. There was also some info on the working of the recirculator, and other items, but again, nothing to write home about. The laserscopes are designed fairly well, you should basically just be able to 'turn em on' after they have been in storage for a while.



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    Coherent Compass 532-200 Green DPSS Laser

    These sections include notes on the testing and repair of this series of Coherent DPSS lasers. For information on their organization and operation, and specifications, see the section: Coherent Model 532-200 Green DPSS Laser. Detailed photos of the cavity can be found in the Laser Equipment Gallery under "Coherent Diode Pumped Solid State Lasers". For the procedures, below, it may be best to open that page in a new browser window for reference.

    Pump Diode Replacement in the Coherent Compass 532

    The following procedure provides step-by-step instructions for replacing an old, tired, worn out, or totally dead SDL-237x laser diode assembly in the Coherent Compass 532 series of green DPSS lasers. It assumes the rest of the cavity is in good condition and the power supply/controller is operational.

    Symptoms of pump diode failure:

    Of course, first make sure the beam shutter is open! :)

    Three main settings determine the operation of the laser: Pump diode current (LD current), pump diode temperature (LD Temp), and KTP temperature (KTP Temp). The rest of the electronics is for fine mode optimization but this should not have a very strong effect on output power. Thus, if the readings at the interface connector show proper diode and KTP temperature (settings written on the cavity cover) with the diode current maxed out, diode failure is very likely. However, it is possible for there to be other problems like internal cavity damage or bad electronics. The actual diode current should be checked if possible to confirm that it is indeed excessively high. If/when the cavity is opened, measuring the actual pump diode optical output power would also be desirable. The cavity should be inspected for obvious damage as well. (See instructions below for opening the cavity.)

    Availability of replacement pump diodes:

    Replacement pump diode assemblies (including the TEC cooler but probably NOT the collimating lens) may be available from Coherent at prices you don't want to think about. However, the original manufacturer is SDL, now part of JDS Uniphase. Complete specifications can be found on their Web site. Go to "Products", "Commercial Lasers", "Laser Diodes, and finally "SDL-2300 4.0 W CW High-Brightness GaAlAs Laser Diodes". Or, here's the direct link but in all likelihood, it won't remain stable for long: SDL-2300 4.0 W CW High-Brightness GaAlAs Laser Diodes.

    A pump diode assembly which appears to be compatible is available from Sony as well and may be much less expensive. This diode is rated 3 W instead of 2 W so it may be possible to increase the output power of the laser (but no guarantees and electronics modifications may be needed). Go to: Photonics Products UK, "Products", "Infrared Laser Diodes", "Sony", "SLD327YT".

    CAUTION: This is as yet a mostly untested procedure. Use at your own risk. I would appreciate comments or corrections via the Sci.Electronics.Repair FAQ Email Links Page.

    Special requirements/tools:

    CAUTION: All work should be done at an ESD protected workbench with high impedance ground connected wrist strap. The cavity should only be opened in a dust-free environment.

    Preparation prior to pump diode removal:

    1. Remove the two screws at the top-front and bottom-rear of the case. Separate the two halves of the case flip the top with the power supply over so that the unit can be powered with access to the adjustment pots.

      WARNING: Avoid contact with the exposed line voltage on the power entrance of the power supply!

    2. Confirm that the Light/Current mode jumpers are set to Light Mode.

    3. Power up the unit and allow 15 to 30 seconds for the LD_OFF LED on the controller board to go out. If the old pump diode still had some life left in it, there should now be some green output.

    4. Monitor the appropriate test points on the interface connector and set the following pots on the controller board:

      • LDIM (Laser Diode Current Max): Set to slightly above threshold for the new pump diode (from the test sheet that came with it). I suggest around 1.5 A. This should be high enough to produce some green output assuming the pump spot is reasonably well aligned and the LD and KTP Temps are approximately correct. The test point sensitivity is 1 V/A.

      • LD Temp (Laser Diode Temperature): Set so the peak of the new diode is at 808 nm assuming 0.3 nm/degree C based on the test sheet data. The test point reads Temperature = (25-V*20) in degrees C. Thus if your diode spec is 810 nm at 25°C, the LD Temp should be set at 19°C or 0.3 V. This will need to be tweaked later for maximum power but should be close enough for initial tests.

      • KTP Temp (KTP Temperature): Confirm that it is set at the value printed on the cavity cover. The test point reads Temperature = (25-V*20) in degrees C. Thus if the KTP Temp spec is 37°C, reading should be -0.6 V.

    5. Power down the unit, disconnect and set aside the top half of the case with the power supply/controller.

    Cavity access and pump diode removal:

    1. It is possible to do the swap with the cavity installed in the lower part of the case but removing it will make things more convenient (3 screws from underside).

    2. Remove the cavity cover by unscrewing the six (6) security hex screws.

      CAUTION: The YAG assembly metal block near the center of the cavity contains a powerful magnet. Any metal object that gets in its vicinity will be sucked toward this block likely smashing the 1/2 wave plate or other optics and rendering the laser useless. Take extreme care to keep ferrous metal tools away from this area.

    3. Use a hex wrench to remove the two (2) visible screws holding the pump diode package. Set these aside with their lock and flat washers.

    4. The entire resonator is on a glass and metal frame held to the main body of the cavity with two hex nuts from the outside of the heatsink. While holding frame, use a 1/4" nut driver to remove these two nuts and set them aside. The resonator is now only attached by its wiring. CAREFULLY tip the cavity on its side and free the resonator from the cavity placing it on a clean smooth surface. At the same time, it will probably be best to remove the plug to the pump diode and replace it with a shorting jumper for at least the 2 LD connections (if you care about its health). Be especially gentle with the wiring to the photosensor and KTP piezo/heater which is particularly thin and fragile.

    5. Use a hex wrench to remove the remaining 2 screws holding the pump diode package and set these aside with their lock and flat washers. There are channels cut in the aluminum frame to allow access from underneath.

    6. Most likely, the pump diode package will still appear to be firmly attached to the heatsink due to the indium foil pad underneath it. Use a plastic or wood stick between the package and aluminum frame to free it taking care not to let the collimator hit the optics! The indium foil pad will probably stay with the pump diode package. If it can be freed intact, it may be reused. Otherwise, a replacement will be needed. The purpose of the pad it so facilitate heat transfer. A substitute may be possible but it must be something that won't outgas and contaminate the optics.

    7. If the replacement pump diode doesn't include the collimator, carefully free it from the old one by chipping away at the adhesive attaching it to the old diode. The collimator consists of two parts - the lens barrel itself which is glued inside the flange thing that is actually stuck to the diode package. Hopefully, these two sections won't need to be separated but at this point I don't know. If the actual laser diode chip on the new pump is mounted deeper inside the package, it will be necessary to free the two sections to be able to achieve a collimated beam. Set the collimator aside in a cushioned box for now. Store the old pump diode in an antistatic bag if you care about it.

    The procedure now diverges based on whether the new pump diode has a collimator pre-installed:

    Installing the new pump diode - Case 1: Collimator is already installed and aligned. This would also be the situation when swapping a diode in from another identical unit. Note that I really don't know if the alignment between diodes is close enough to obtain full power by only adjusting the diode position; it may be that there really isn't any interchangeability guaranteed and the full alignment procedure will be needed. The one data point I have isn't entirely conclusive.

    1. Install the new pump diode with an indium foil pad using the 4 screws. If your new diode already has the collimator (or is from another similar laser and has the collimator), then it may be necessary to fine tune the diode position to center the spot on the YAG rod. In this case, tighten the screws equally snug but not so tight so as to prevent some movement.

    2. Remove the shorting jumper (or whatever is present on the new diode) and attach the electrical cable. Make sure it is aligned correctly and pushed on completely!!!

    3. Install the resonator assembly into the cavity casting using the 2 1/4" hex bolts and tighten snugly. Its position (which only affects beam centering at the laser exit aperture) will be fine tuned later once there is a beam.

    4. Attach the cavity, laser diode, and cavity fan cable to the power supply/controller.

    5. The moment you've been waiting for has arrived! With your multimeter on the LDI test point, power up the laser. After the 15-30 second warmup period, the LD_OFF LED should go out and the LD current should read what you set it to before removing the old diode. If it isn't close to this value or the LD_OFF LED doesn't go out, immediately power down and figure out what is wrong!

      Assuming the current is correct, there should be some dim red light visible from the diode, through the beam shaping optics, and hitting the YAG crystal. With any kind of luck, there may be at least some, perhaps a lot, of green light. With the resonator frame not aligned in the cavity casting, the output beam may not make it out of the cavity so there may be no output beam or one that is totally messed up. In this case, loosen the 2 1/4" nuts very slightly and shift the position of the resonator until the beam is centered in the output aperture, then tighten the nuts securely.

      CAUTION: In order for there to be efficient heat transfer from the pump diode to the heatsink, the diode mounting screws and 1/4" resonator nuts must all be reasonably tight. It is possible that since the diode is now not firmly secured (so it can be moved slightly for alignment) that the TE cooler may be incapable of maintaining the selected temperature. It is important to watch out for a thermal runaway condition by monitoring the LD Temp test point and switch off power if things get out of hand.

    6. Adjust LD Temp to center the pump diode's wavelength around 808 nm based on the test data received with the diode and a sensitivity of 0.3 nm per °C. If it was far away from 808 nm, output power may increase significantly. Note that this setting is somewhat dependent on pump current so it should be checked and adjusted as needed.

    7. CAREFULLY locate the pump spot on the YAG crystal. It should appear reasonably centered. Move the diode around within the freedom of the screw holes to see if output power can be maximized within the available adjustment range or if it appears to be maxed out at one extreme. If the "sweet spot" appears to be within the adjustment range, there is a good chance that no further pump alignment is needed. However, again, I don't know how good the consistency is among these lasers and their collimated pump diodes.

    8. Power down the laser. Without disturbing the pump diode position, securely tighten the 4 screws in several increments using an alternating X pattern (like tightening wheel lug nuts).

    9. Power up the laser and confirm that the output power hasn't changed significantly after tightening the diode.

    10. Once the best position is found, slowly increase the LD current to to more than 2 to 2.5 A. Readjust LD Temp in small increments (e.g., 1/4 turn) to see if power can be increased - wait a minute or so between adjustments to allow time for the LD Temp to stabilize.

      Note that since Light Mode is enabled, if other electronic adjustments haven't been touched, the feedback loop will start regulating once the output power exceeds the set point (200 mW for this model unless it was adjusted for something else). In this case, the LDIM pot will not increase current above what is required. Should you be very lucky and hit this before 2 A, don't keep turning - for now, set the pot to a point just beyond where the current stops increasing (1 turn or so).

      CAUTION: Under NO circumstances should the LD Current be set above the diode's rated value. If that is needed to achieve adequate output power, something else is wrong. Check the LD and KTP temp settings and that the feedback loops are working, the cavity for damage, etc.

    11. Replace the cavity cover, tightening the 6 security screws in small increments in an alternating pattern (like an engine cylinder head). If possible, the cavity should be purged with dry nitrogen (I assume) using the two ports provided for this purpose.

    12. Perform the electrical alignment procedure to optimize cavity mode stability. (Yeah, like we know how to do this!)

    Installing the new pump diode - Case 2: This set of steps deals with the situation where the new diode didn't come with a collimator and you are reusing the collimator from the old diode.

    1. Install the new pump diode with an indium foil pad using the 4 screws. If there is no collimator, then the diode position isn't as critical since fine alignment will be done with the collimator. In this case, just center the diode (there isn't that much clearance in any case). Tighten the 4 screws securely in 3 or 4 increments using an X pattern (like wheel lug nuts!).

    2. Remove the shorting jumper (or whatever is present on the new diode) and attach the electrical cable. Make sure it is aligned correctly and pushed on completely!!!

    3. Install the resonator assembly into the cavity casting using the 2 1/4" hex bolts and tighten snugly. Its position (which only affects beam centering at the laser exit aperture) will be fine tuned later once there is a beam.

    4. Attach the cavity, laser diode, and cavity fan cable to the power supply/controller.

    5. It's time to apply power! With your multimeter on the LDI test point, power up the laser. After the 15-30 second warmup period, the LD_OFF LED should go out and the LD current should read what you set it to before removing the old diode. If it isn't close to this value or the LD_OFF LED doesn't go out, immediately power down and figure out what is wrong!

      Assuming the current is correct, there should be a widely divergent beam of dim red light from the aperture of the pump diode. Confirm with a white card if it isn't obvious. DON'T look into the diode aperture from the front! Turn the LDIM pot down until the red light is just barely visible, just above threshold for the diode. This will permit the collimator to be positioned without risk of burning/damaging anything.

      CAUTION: In order for there to be efficient heat transfer from the pump diode to the heatsink, the diode mounting screws and 1/4" resonator nuts must all be reasonably tight. It is possible that since the diode is now not firmly secured (so it can be moved slightly for alignment) that the TE cooler may be incapable of maintaining the selected temperature. It is important to watch out for a thermal runaway condition by monitoring the LD Temp test point and switch off power if things get out of hand.

    6. Adjust LD Temp to center the pump diode's wavelength around 808 nm based on the test data received with the diode and a sensitivity of 0.3 nm per °C. If it was far away from 808 nm, output power may increase significantly. Note that this setting is somewhat dependent on pump current so it should be checked and adjusted as needed.

    7. Position the collimator assembly you saved from the old diode in front of the diode's output aperture and center it by hand. Using a white card as a screen, check to see if the resulting beam is diverging, collimated, or converging.

      • Diverging: A three-axis positioner will be needed to align the collimator and hold it in position while the adhesive sets.

        A diverging beam means the new diode chip is a little closer to the surface of the diode package than the old one. Set up a 3-axis (XYZ minimum, 5-axes with pitch and yaw may be better) so that the collimator assembly can be located in the center of the front of the diode's output aperture and adjusted over at least 1 millimeter or so in each axis.

      • Collimated: It may be possible to position the collimator by hand for maximum green output power and/or to center the beam on the YAG. If you have a positioner handy, this may be easier but not essential.

      • Converging: A three-axis positioner will be needed to align the collimator lens barrel itself and hold it in position while the adhesive sets.

        A converging beam means the diode chip is further inside the diode package than for the one the collimator was originally on. Separate the lens barrel and flange and set up a 3-axis (XYZ minimum, 5-axes with pitch and yaw may be better) so that the lens barrel (loosely installed inside the flange can be located in the center of the front of the diode's output aperture and adjusted over at least 1 millimeter or so in each axis.

    8. Power up the laser and set the LD Current so that the output of the diode is just visible (no more than 1.5 A). There should be some dim red light visible from the diode, through the beam shaping optics, and hitting the YAG crystal. Use the positioner (or your hand in the case of the already collimated case) so the beam is centered on the input face of the YAG crystal and for the smallest spot diameter. With any kind of luck, there may be at least some, perhaps a lot, of green light. With the resonator frame not aligned in the cavity casting, the output beam may not make it out of the cavity so there may be no output beam or one that is totally messed up. In this case, loosen the 2 1/4" nuts very slightly and shift the position of the resonator until the beam is centered in the output aperture, then tighten the nuts securely.

      Note: I'm not absolutely sure that minimum spot size is the optimal setting. Try it on either side of this if possible and leave it in the position that results in highest output power.

      CAUTION: It's possible that too good a focus isn't good for the YAG crystal either so there is some risk here.

    9. Once the best position is found, slowly increase the LD current to to more than 2 to 2.5 A. Readjust LD Temp in small increments (e.g., 1/4 turn) to see if power can be increased - wait a minute or so between adjustments to allow time for the LD Temp to stabilize.

      Note that since Light Mode is enabled, if other electronic adjustments haven't been touched, the feedback loop will start regulating once the output power exceeds the set point (200 mW for this model unless it was adjusted for something else). In this case, the LDIM pot will not increase current above what is required. Should you be very lucky and hit this before 2 A, don't keep turning - for now, set the pot to a point just beyond where the current stops increasing (1 turn or so).

      CAUTION: Under NO circumstances should the LD Current be set above the diode's rated value. If that is needed to achieve adequate output power, something else is wrong. Check the LD and KTP temp settings and that the feedback loops are working, the cavity for damage, etc.

    10. Assuming output power is satisfactory, use some dabs of quick setting adhesive to fix the collimator (and lens barrel if separate) in position taking care not to drip anything on the optics! DO NOT use cyanoacrylic (Super Glue)! 5 minute or UV cured Epoxy is probably best. Check that the output power is about the same after the glue cures.

    11. Replace the cavity cover, tightening the 6 security screws in small increments in an alternating pattern (like an engine cylinder head). If possible, the cavity should be purged with dry nitrogen (I assume) using the two ports provided for this purpose.

    12. Perform the electrical alignment procedure to optimize cavity mode stability. (Yeah, like we know how to do this!)



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    Tid-Bits from Laser Mainenance Land

    When You're in Continuous Panic Mode

    (From: Bob.)

    I remember a story from a friend of mine in the laser show biz. They bought an old medical doubled YAG laser, chopped it down and put the optics in a head enclosure, the power supply in a cabinet, then put everything in road cases. It sounded like the neatest little green YAG at the time. Well, they were setting up for a show in a large convention hall for some corporate big wigs (GM, Boeing, something like that). They needed the power of a YAG because the customer wanted the laser to be seen without the room lights turned off. The day before the show during set up they fired up the laser and they were only getting a hand-full of mW out, and very unstable at that. Apparently, something had bumped the KTP mount in transit and when they first turned the laser on, the intracavity light hit the side of the crystal shattering it. So they had a replacement Fedexed to them for next day delivery. After delays with Fedex, they finally got the crystal like 45 minutes before the laser was needed. This was already DURING the time of the performance, seminar, whatever, was was going on. So I was on the phone with this guy, for the whole time, as they had never fully aligned a KTP before. The heated conversation went like: Me: OK, check IR power then realign the back optic, then tweak the KTP for max green. Him: !@$# only 3 W and we're on in 10 minutes! I don't know what it is about laser light shows that seem to bring out the largest assortment of Murphy's laws. But they sure do! If you're going pro, you'll have stories of aligning optics in the field and the like for us, sure enough. :)

    (From: Steve J. Quest (squest@att.net).)

    I'd say I probably am (an) expert on making the laserscope laser lase at full power given no replacement parts and no time to get it done. :) I've done shows at full power using cracked KTP by realigning the crystal and ringing through the largest chunk that was left. I've overdriven and chipped the Q-switch quartz, and fixed it in the field by flipping the crystal upside down (luckily the beam doesn't go dead center but a few mm off to one side). I've burned tuned dielectrics and realigned off to the side to "get her going for the show". I've cracked mirrors.... OK, enough said about the trials and tribulations of this business. :)



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    Aligning the Hughes Ruby Laser

    This applies specifically to the Hughes M-60 ruby laser assembly found on the surplus market and described in the section: Hughes Rangefinder Ruby Laser Assembly. However, the same general methods can be applied to other similar solid state or other wide bore lasers.

    Here are two approaches assuming that a small HeNe laser (the A-Laser) is available. The first one uses the ruby rod exit holes as the initial alignment axis and then reference everything else to those but requires the removal of the HR prism and aiming the A-Laser in from each end. The second can be done without removing any optics or moving the A-Laser but aligning to the ruby rod may be more difficult.

    Both require a means of aiming the A-Laser precisely through the RLA optics. Usually, this would mean bolting or clamping the A-Laser to your lab bench and mounting the RLA on a three-screw lab jack or something similar so that its height, side-to-side position, and pitch angle can be adjusted. A basic design can be found in the section: Simple Adjustable Optics Platform. The distance between the A-Laser and closest end of the RLA should be no less than 12" so that the alignment of the reflected spot from the HR/OC can be accurately set.

    Sam's Method

    1. Drill some 1 mm holes in a couple of pieces of opaque cardboard and tape them to the ends of the ruby cavity with the holes precisely centered to act as a bore sight for aligning the A-Laser to the optical axis of the assembly. Prepare another piece of cardboard with a 1 mm hole which is taped to the OC mount precisely centered.

    2. Remove the HR prism reflector.

    3. Using whatever means you have at your disposal to mount the Ruby Laser Assembly (RLA) and A-Laser, aim the A-Laser beam in through the HR mount so that its beam passes cleanly through the holes in the bore sight (and thus down the axis of the ruby rod) and reflects off the 45 degree mirror to the Q-switch prism.

    4. Rotate/adjust the Q-switch prism so that the A-Laser beam passes through the exact center of the OC or its mount. Adjust the OC alignment so that the reflected beam returns along the same path back to the A-Laser's aperture.

    5. Aim the A-Laser in from the OC end so its beam passes cleanly through all three pieces of cardboard. Install the HR prism and adjust it so that the reflection returns along exactly the same path to the A-Laser's aperture.

    Wes's Method

    The following is based on discussions with Wes Ellison (erl@sunflower.com).
    1. Drill some 1 mm holes in a couple of pieces of opaque cardboard and tape them to the ends of the ruby cavity with the holes precisely centered to act as a bore sight for aligning the A-Laser to the optical axis of the assembly. Prepare another piece of cardboard with a 1 mm hole which is taped to the OC mount precisely centered.

    2. Using whatever means you have at your disposal to mount the Ruby Laser Assembly (RLA) and A-Laser, aim the A-Laser beam in through the hole in the cardboard on the OC mount so that it reflects off of the Q-switch prism and 45" mirror to the bore sight on the ruby rod.

    3. Rotate/adjust the Q-switch prism, 45 degree mirror, and A-Laser so that the A-Laser beam passes through the exact center of at both ends of the bore sight to the HR prism.

    4. Adjust the HR prism so that the beam reflects back to the center of the A-Lasers' aperture (ignoring the reflection from the OC).

    5. Adjust the OC so that its reflection is also centered in the A-Laser's aperture.

    Aligning the Q-Switch

    To set up the Q-switch without powering the flashlamp, proceed as follows: This requires a high speed photodetector circuit and dual trace oscilloscope. A suitable circuit would use a silicon photodiode and trans-impedance preamp to channel A, the pulse from the magnetic pickup to channel B.
    1. Install a beam splitter (e.g., microscope slide at 45 degrees) in the HeNe beam so that some of the reflected beam can be diverted to the photodetector.

    2. With the Q-switch motor running at the proper speed, adjust the angular orientation of the Q-switch platform/magnetic pickup so that you have the desired delay between channels A and B.



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