(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.
(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:
(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.
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.
Note that Zapit paper is great for this but there are many common materials that will behave similarly including common packaging (anything with dark ink on a light colored cardboard base). Even 5-1/4" diskettes (either the envelope or the diskette itself) can be pressed into service. Now you have a use for those cartons of antiquated storage media. Other magnetic media like data and video tape may also work because they consist of a very thin dark layer on a clear base material. Don't toss those crinkled VHS tapes! :)
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.
"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.
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.
"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.
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.
(From: Clive Washingtron (clive.washington@astrazeneca.com).)
A section of the jewelry/lapidary community, those who facet gemstones, may be able to do this - and there are several thousand such people worldwide. Some cutters just turn out 'ordinary' stones but a few of us aspire to high optical quality. Do you have any idea of just how flat the end faces need to be? On a quarter inch facet I could easily achieve 3 or 4 rings so perhaps it would be possible!
(From: Sam.)
Yes, I think it may be possible to achieve sufficient flatness to get it to lase, but probably not to achieve the original performance. By "3 or 4 rings" I assume you mean interference fringes where flatness specs are often in 1/10 or 1/20 lambda. However, all that is needed is a stable round trip path in the resonator to get it to lase, but that could still result in a messed up beam and reduced output power. There is also the issue of applying an antireflection or mirror coating to the rod ends.
One purpose of my strong comments is to discourage people who think they can simply buy a cheap unpolished or damaged rod on eBay and all it will take is a few minutes to grind it flat free-hand using 1,200 grit sandpaper and then polish it (also free-hand) with jeweler's rouge! :)
Stock KTP crystals generally have a cross-section of 2x2 or 3x3 mm, though some commercial DPSS lasers use 1x1 mm crystals. The typical maximum beam diameter at the crystal faces is typically less than 200 um, possibly less than 100 um. To minimize diffraction losses, space equal to the beam diameter should be allowed surrounding the beam so a 100 um intracavity beam would require a 300 um area on the crystal. So, if there is a small undamaged area remaining, repair may be possible using the existing KTP with minimal rework.
The main challenges are:
Where the original holder has no adjustments but assumes the beam is in a particular location, it may be necessary to grind down the sides of the crystal so the clear area can be positioned precisely where it is needed. KTP is relative soft and files designed for metal actually appear to work reasonably well. But make sure to protect the very delicate coated ends.
Two part Epoxy or UV cure adhesive can be used to attach the crystal to the holder. Depending on how many degrees of freedom are available for the holder, the orientation of the KTP may be critical, both in terms of rotation about the optical axis of the laser and the angle with respect to the optical axis.
I've successfully remounted the damaged KTP crystals in a couple of DPSS lasers. On one, the KTP crystal was chipped at one end with about half of the face totally missing. The entire mount had to be fabricated from scratch as the previous owner had lost the original parts (though the mount wasn't very good to begin with, which is perhaps why the crystal got damaged). See the section: Reconstruction of an 80 mW Green DPSSFD Laser. The other was a Uniphase SLM uGreen DPSS laser where the 3x3x3 mm KTP crystal had been shattered by threading a screw too deeply into its mount. Only a small area in one corner was still usable. The KTP had to be filed down so the good area could be centered in the beam but the original holder then could be used.
(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.
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.
"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.
With prices as low as $1.00, 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 over $150, 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, though not all models are quite this complex. However, even the new generation of green pointers using Multiple Crystal Assemblies (MCAs) rather than discrete crystals and optics, still have 2 or 3 times the number of parts and the need for very precise alignment.
Fortunately, 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.
The following photos and diagrams apply to the two typical approaches:
The detailed disassembly procedure will depend on the exact model. A combination of screw, press-fit, and glue holding things together is likely. Non-destructive disassembly may not be possible for some components. See the section: Disassembly and Reassembly/Alignment of the Edmunds L54-101 Green DPSS Laser Pointer for the detailed procedure for the L54-101 model and Disassembly and Reassembly/Alignment of an MCA-Based Green DPSS Laser Pointer. Lower cost models will be more along the lines of the second type, but may be even more difficult to disassemble if it's possible at all. And, as regulatory agencies discover the potential dangers of boosting the power of green laser pointers, manufacturers may be required to assure that they can't be disassembled non-destructively!
Here are the most common adjustments/repairs:
If anything actually got inside the DPSS module, or its components are not even partially sealed, repair may be hopeless since any contamination will likely render it totally inoperable and may result in permanent damage even if cleaning can be performed quickly. In any case, total disassembly of all the crystals and optics would be needed. This will necessitate partial or total realignment of the laser diode, crystals, and optics. In the unlikely event that the laser diode is in a hermetically sealed package (not many, if any, models do it this way due to cost), a total cleaning using proper laser mirror cleaning techniques - followed by realignment - may permit the pointer to be salvaged. However, with most or all units using bare laser diodes, any contamination that reaches the laser diode chip may be bad news indeed. In the latter case, very careful cleaning with pure alcohol or acetone may save it but this has to be done before attempting to power the diode - anything on the facet while powered may be terminal. For pointers using composite vanadate/KTP crystals, much less alignment is needed but access to parts will still be a challenge.
Here are possible problem areas for a pointer that is weak or dead and hasn't been run over by a Sherman Tank:
Of course, if not using the pointer for a few days, remove the battery. Leakproof batteries have been known to leak!
Finally, although the typical green pointer is very well constructed with remarkably precise machining and the use of generous amounts of adhesive, they are still susceptible to shock and impact. And, as the technology matures and costs come down, corners may be cut as well.
I was given one unit that was totally dead after falling onto a hard floor (material not known). The pump diode was butt-coupled (almost touching with no relay lens) to what looks like a CASIX DPM0102 composite crystal, which was secured in place with RTV silicone (essentially bathtub caulk). What must have happened is that the inertia of the crystal and mount at the time of the fall caused the crystal to move ever so slightly, impacting the diode and breaking it into two pieces, the larger of which was still attached to the two bonding wires hanging in mid-air.
If not totally ruined by mechanical shock, alignment may be affected resulting in decreased output power and degradation in beam quality.
So, as preventive maintenance, dump the fancy wooden box that so many of these green laser pointers arrive in and use a well padded case instead. In addition, it might be wise to fasten a lanyard to the pointer so it can be attached to a belt and won't fall on the floor when you bend over. Your pointer will thank you. :)
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:
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:
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:
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:
Since it is unlikely I can find a replacement pump diode unless from a similar pointer that died for some other reason, I put everything back together, aligned the DPSS module for maximum output (what of it there is!) and a clean TEM00 beam, but didn't touch the alignment of the output optics, which appeared to be close enough. So now I have perhaps the World's weakest green DPSS laser pointer producing about 0.2 mW on a good day. Not knowing the ratings of the pump diode, I don't dare increase the current beyond the 400 mA peak produced by the driver at the original setting of the pot (about 200 mA average current at the 50 percent duty cycle). Even at 0.25 mW, the pointer is quite usable since 0.25 mW of 532 nm green has about the same brightness as 2 mW at the typical 650 nm red pointer wavelength. And, it's guaranteed eye-safe. :)
A detailed diagram of the internal construction of a typical MCA-based pointer is shown in Typical Green DPSS Laser Pointer Using MCA. The procedures below are based on this pointer.
The first set of steps deals with basic disassembly of the case:
Note: There is no need to actually detach the driver board if you aren't going to remove the laser diode but there is less risk of damaging the diode's leads while working if there is no bulky PCB hanging off of it.
The next set of steps deals with removing the pump diode:
A spacer and the pump focusing lens may come out as well. There may be an orientation (front-to-back) of the focusing lens. On the pointer in the diagram, the aspherical side was facing the diode.
The next set of steps deals with removing the output optics and MCA. For the following which require unscrewing three pieces, it may be necessary to use a vice and pliers, or two pairs of pliers to get enough torque to break the glue bond locking them in place. As above, use some soft material to prevent damage to the brass and don't squeeze too hard! I found that pieces of rubber from a bicycle inner tube worked well as cushions.
CAUTION: The mirror coatings on the MCA are very fragile and will peel off if given the slightest excuse. It is really best to leave the MCA in its holder with the expanding lens mount screwed in place.
At this point you have a box of green pointer parts. :) However, if the disassembly operation was done with reasonable care, it should be possible to restore the patient to perfect health. A new diode (5.6 mm can with cover would work) or replacement MCA can be installed if needed. Here is the procedure for reassembly and alignment - confirmed to work. A means of powering the pointer guts outside the case will be needed. This can be a battery holder for the 2 AAA cells or a regulated 3 V power supply.
WARNING: Without the IR-blocking filter in place, there can be enough IR leakage at both 808 nm and 1,064 nm to be a vision hazard. This is most significant for the 1,064 nm which is both invisible and collimated like the 532 nm green. The use of proper laser safety goggles is highly recommended especially for those procedures like the expanding lens alignment requiring the beam to be pointing vertically and where it's direction can change suddenly while adjusting the lens position.
Pump diode installation:
Note that while the original pump diode may have been a 5.6 mm type without a cover, it should be possible to use a commercial 150 to 500 mW 5.6 mm can diode with a cover in its place. Roithner has one that might be suitable.
There are now two possible procedures for aligning the MCA with respect to the pump diode and optical axis. The first doesn't require fancy equipment but may not result in best performance. It may also result in maximum frustration. The second requires a 5-axis micropositioner (X, Y, Z, yaw, pitch) with some sort of gripper to hold the MCA mount but should result in maximum power and nearly perfect beam pointing direction. A laser power is desirable when peaking output power. (It can just be a photodiode and multimeter on its mA range.)
Basic MCA alignment procedure:
MCA alignment procedure using micropositioner:
Output optics installation and alignment:
Final assembly:
I was given a very dead green laser pointer to analysis, autopsy, or anything else I pleased. It is described in the section: Anatomy of an Inexpensive Green Laser Pointer. A detailed diagram of the internal construction of a typical MCA-based pointer is shown in Typical Green DPSS Laser Pointer Using MCA. I used the basic procedure I developed for this type of pointer (see the previous section) to nearly completely disassemble and reassemble it. However, there was a very significant complication in that the pump diode was severely mangled and was definitely beyond life support.
The original owner had decided that the beam shape wasn't perfect or fuzzy or something (you'll see why shortly) and the beam was erratic so he removed the driver PCB and DPSS module from the case for inspection and cleaning of the switch. Being unable to find any obvious cause of the poor beam shape, he attempted to reassemble the pointer but the driver PCB caught on the power button or something along these lines causing one of the feed-through leads of the naked 5.6 mm can pump diode to be yanked loose. A similar diode is shown in Laser Diode With No Cover. The post to which the feedthrough lead was attached (the far one in the photo) ripped the bonding wires from the top of the laser diode chip resulting in a certifiably dead laser diode. But this represented an irresistible challenge as the diode appeared undamaged otherwise. So, here's what I did:
The first thing was to stabilize the diode lead so the same thing wouldn't happen while working on the pointer. So, I positioned the damaged lead in approximately the correct position and used slow curing Epoxy to secure it in place.
Had the bond wires been ripped from the post rather than the diode chip, it might have been possible to reattach them to the post either with solder or silver Epoxy. Unfortunately they ripped from the top of the chip. So on to Plan B. I installed the diode package in an IC socket along with another pin which had a single strand of #36 wire soldered to it. This was then positioned so it could be soldered to the post with its end just touching the top of the 0.5x0.5mm laser diode chip.
At first, I positioned the tip of the wire expecting to use a dab of silver Epoxy to attach it to the diode chip. But my first attempt resulted in a mess and a totally shorted diode. So, after wiping away the mess and soaking the diode in acetone, I applied the tiniest amount of silver Epoxy to the tip of the wire and then pressed it down so the blob of silver Epoxy just contacted the diode chip. This appeared to work and the diode lased at low current so I let it sit overnight.
Note that in addition to the original trauma, this diode has been dropped more than once, stepped on, fingered, covered in silver Epoxy and cleaned off, and still survived at least somewhat. Some laser diodes are tough. :)
(For more photos and other approaches to this sort of miracle repair, see Colin Kaminski's 808 nm Laser Diode Dissection Page.)
Next day, I soldered some thin hookup wires between the diode and driver so it could be tested without applying excessive stress to the diode leads, which despite the Epoxy were still not that sturdy. Even with the Epoxy securing them, it's still not the same as the original glass to metal seal!
For initial testing, the diode was installed loosely with its retaining ring so it could be positioned for optimal orientation (recall that vanadate is polarization sensitive). Applying power, there was immediate green light, though no where near the power it should have been. Adjusting orientation helped some but it was obvious that during manufacture, alignment must have been done on the output optics only after the diode was secured. Adjusting the diode position - just given the tolerances of the machining - resulted in the beam going all over the place and there was no way of doing this precisely. Even if a good setting was found, tightening the retaining ring messed it up. But even if accurate positioning was possible, it wouldn't have helped. When the brightness was best, the beam was way off to one side with a bad shape. This was probably how the pointer was shipped. Great quality control! When centered, it was barely visible.
At least the diode was working so I reinstalled the driver board, added some additional Epoxy to stabilize it with respect to the diode, and reinstalled and aligned the output optics. I had to increase the current to around 300 mA to get the output to settle down. At 260 mA, it was erratic until the DPSS module warmed up, from use or body heat. The beam wasn't too bad but the expanding lens had to be way off center to get the beam through the output aperture, and then it shot off at a 5 degree or so angle to the pointer. This was unacceptable!
Further disassembly - unscrewing and removing the brass cylinder with the expanding lens - revealed the underlying cause: The MCA had been glued into its holder at a significant angle so the raw beam was shooting at an angle rather than parallel to the optical axis. To compensate for this, the expanding lens had to be offset so far that the beam became distorted, and this was probably the cause of the original messed up beam shape. The MCA (which appears to be seomthing between a CASIX DPM0101 and DPM0102 (what might be called a DPM0101.5 if there were such a thing as a standard product) held in place with some white RTV silicone (bathtub caulk!) so it was easily pried out. Reinstalling with some 5 minute Epoxy resulted in a somewhat better aligned beam, though the power was still very low (maybe 0.2 mW). So, perhaps the MCA wasn't quite positioned correctly.
I wasn't yet happy so I totally redid the alignment one more time. This time I removed everything after the pump focusing lens including the MCA and MCA mount. First, the MCA mount was glued in place aligned with the pump diode facet so that the crystal could be correctly oriented for optimum pump absorption. Then the MCA was carefully placed in the mount, and with power applied, carefully positioned for maximum green light while confirming that the beam was reasonably well aligned with the optical axis. Then, with the pointer clamped in a vice pointing up, the mount for the expanding lens was installed and the expanding lens was pushed around until the beam shot out perfectly straight up, and glued in place with 5 minute Epoxy. While the Epoxy was curing, the collimating lens assembly (essentially, the rest of the pointer) was screwed in place to confirm that the beam was fairly well centered in the output aperture.
While the power is still low but adequate (maybe 1 mW), the beam shape is now quite good and the output lines up well with the pointer optical axis. A bit of instability is back but the increased power and better beam shape makes up for that. :)
The only major casualty was the IR-blocking filter which got crunched when it popped out and landed on the floor. :( If anyone has a spare kicking around, please contact me via the Sci.Electronics.Repair FAQ Email Links Page..
(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.
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!).
In all honesty i think an amateur may be better suited to align a Laserscope than a someone who has been working on argon ion or other non-SHG lasers for a few years. Most guys who know lasers, but don't know Laserscopes, go to tweak everything they can for most green out. This is DEFINITELY NOT the way you want to do things. The IR laser must be adjusted for maximum performance and the doubling crystal must be adjusted for maximum conversion into the green. These are two separate fundamental systems, and need to be treated as such. There is, of course, interaction. But the two objectives must be kept in mind throughout the entire process.
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.
Next, 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.
(From: Kevin Criqui (usenet@kce.com).)
I've been collecting information about what is required to modify a Laserscope KTP/532 for show use. What I have so far is:
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.
(From: Spankey Lee (spankey@eece.unm.edu).)
"Here is the wiring cheat sheet for the ALE power supply originally drawn by some cool guy but redrawn by me so it is easier to read. I built this from my drawing and it works except I added a bunch of led's to mine to show the states of the switches. See Control Panel 1 for Laserscope ALE Power Supply.
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.
The Laserscope KTP oven pin-out is as follows:
Most Laserscopes are not wired for both TEC operation. They usually use only the top TEC and the thermister is on the bottom TEC for temp feedback. One unit had a TEC get damaged and switched the leads to lower TEC to allow for continued operation with no apparent issues. It is still smoking the skies somewhere in Asia. (-;
To replace the original controller, use an Oven Industries 5C7-350 driver and follow their wiring and set-up instructions included. The driver was $80.00 or so last time I ordered any.
As a general rule of thumb, L-folds are capable of around 30 W max, maybe a small bit more if you get a good one. Z-folds are capable of as much as maybe 65 W, but to be realistic expect 40 W or so max. Air-cooled units can NOT be run in all locations continuously (obviously this is dependent on your ambient temperature - January in upstate NY on an outdoor gig is obviously a much easier environment to dissipate heat than a rave in an un air-conditioned warehouse in Florida during August.
L-folds have about 40% lower divergence than Z-folds. If you buy an older unit (before 1988) make sure it has an ALE power supply in it. (It's quite obviously marked, and it's at the end of a 10 foot long 1/2" diameter piece of RG8-U that goes to the head). If you can't find an ALE, but rather what looks like an OEM power supply covered with Plexiglas, BEWARE: Those are ihouse built switchers and they not only fail left and right, but you have to be able to fix them yourself - they can't be sent out for repair.
Also you should NOT really pay attention to high power outputs. There isn't a lot of difference between 60 W and 24 W in most venus. There was a time when I figured 'hell yeah, crank it up till she can't do no more', and as a general rule of thumb optics do NOT last long when run like that. A 20 W'er will be a lot cheaper than a 60 W'er, and a unit run at 'spec' powers will last a lot longer than one cranking out max power. That might not mean much if you're going $50 kilobuck gigs and want as many photons as you can get, but if you aren't doing big shows, $1,200 for KTP and $600 for a mirror adds up pretty quick.
Cooper has been out of business for years.
Once you've seen one Lasersonics YAG laser, you've seen them all. If you pop the top cover you'll find a sticker for who really made the laser head for Cooper under contract, but most of those companies have been bought out and died by now.
The best you can do is pop the side door and find the nameplate on the 19" rack arc lamp PSU, and try and get a manual from that company, usually its ALE. Unless for some odd reason you have one made with a external igniter module not built into the PSU, once you master the small terminal strip on the side of the PSU you have the laser under control.
Here is the typical rundown on an older Trimedine or Cooper YAG laser:
The YAG rod is kept at optimal temperature without cavity corrosion by a deionized (DI) water system flowing through a heat exchanger. The cooling water flow to the heat exchanger on the hospital side of the loop is regulated by an on/off solenoid to keep the cavity at about 90 °F to 95 °F. It has one arc lamp pumping a very short short extremely multimode high divergence cavity, usually with a intracavity Risley prism to ensure mode control. The cavity is not Q-switched and is too short to do anything useful with except produce unbelievable amounts of highly dangerous 1,064 nm CW power, often at levels up to double what its rated for by the manufacturer. It gets launched into a 300 micron or larger diameter fiber.
The lamp current is from 1 A to maybe 35 A at about 165 volts.
Make sure you buy a pair of OD9 or better goggles designed for surgery use at 1,064 nm and use them whenever you have this beast energized watch out for the lamp ignite pulse, its on the order of 35 kV at couple of amps. I cannot stress enough how major a eye hazard a yag laser is!
Keep the DI cooling loop and flow and cover interlocks, they are there for good reasons, just using tap water will ruin a rod and pump cavity faster then you think, often on the order of a few seconds.
For God's sake if you do not have previous experience with high power 1,064 nm light at these power levels, get some guidance and professional help.
A big medical service company like Laserlabs or East Coast might be able to sell you a manual, but expect to pay dearly for it. Lamps and some parts might come from Kentek lasers or another similar YAG parts specialist.
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.
Photos of the C315M and C415 construction (and dissection of the C315M) can be found in the Laser Equipment Gallery (Version 1.94 or higher) under "Coherent Diode Pumped Solid State Lasers".
Pin 1 is at right facing the laser head near large amber LED. This agrees with the cable numbering.
Connection Signal Direction
Pin Internal Function X for yes PCB Function <- or -> Controller
-------------------------------------------------------------------------------
1 LD Current Control <-
2 Shorted to pin 6 (some units) LD Current Set/Limit ->
3 LD anode (+, case of LD) X Protection/LED Enable <- LD+ drive
4 LD cathode (-) X <- LD+ drive
5 LD thermistor X 10k pullup to +5 -> LD temp sense
6 Shorted to pin 2 (some units) LD Temp. Set-Point -> LD temp ref.
7 Common, jumpered to pin 23 RES Temp. Set-Point -> RES temp ref.
8 RES thermistor X 10k pullup to +5 -> RES temp sense
9 Lower LD TEC+ X <- L LD TEC+ drive
10 Lower LD TEC- X <- L LD TEC- ret.
11 * Heater under Stop 4
12 Upper LD TEC+ X <- U LD TEC+ drive
13 Upper LD TEC- X <- U LD TEC- ret.
14 * Heater under Brewster Pate KTP Temp. Set-Point -> KTP temp ref.
15 KTP TEC+ X <- KTP TEC+ drive
16 KTP thermistor X 10k pullup to +5 -> KTP temp sense
17 KTP TEC- X <- KTP TEC- ret.
18 RES TEC+ X <- RES TEC+ drive
19 RES TEC- X <- RES TEC- ret.
20 LEDs Return -> LED control
21 * Heater 2 under Output Lens LED Power <- DC input
22 BP thermistor X -> BP temp sense
23 Common, jumpered to pin 7 X Temp sensors, set-point + PD circuitry.
24 PD Anode X Output Power Set-Point -> Power sense
25 PD Cathode X +5 for pullups, set-point circuitry, PD.
26 * Heater under Stop 3.
27 * Heater 1 under Output Lens.
28 * Heater common (except heater 2 under Output Lens).
29 * Heater under Turning Mirror 2.
30 * Heater under Turning Mirror 1.
* Factory use only: Allows for installation. removal, and alignment of
associated optical component.
Note: The C215M laser head is very similar to that of the C315M but lacks the RES TEC (pins 18 and 19) and the upper LD TEC (pins 12 and 13). The RES thermistor is also not present but its pin (pin 8) is connected to the LD thermistor (pin 5) internally. The P3 pot and associated components for the RES temperature set-point (pin 7) are also missing on some samples, though the PCB pads are there.
Versions of the C315M made after some time in 1997 have what is almost certainly a digital running time meter consisting of a PIC12C508, 24C021 EEPROM, and (32,768 Hz probably) crystal. Info on the PIC and related parts can be found at: PIC Programmer 2, 16C84, 12C508, etc. Page. Older versions are functionally identical in other respects but lack the time meter. The PIC and EEPROM keep track of how much time the laser diode is actually driven. A continuously repeating serial bit stream with the time code is output via an IR LED when the laser diode is off. The present challenge is to decode this 8 to 10 character data! It doesn't appear to be standard RS232. See the section: Deciphering the C315M Laser Head Serial Datastream.
Schematic of C315M Laser Head PCB (Common Wiring) shows the circuitry associated with laser operation and Schematic of C315M Digital Running Time Meter shows the circuitry only present on newer versions of the PCB. The two PCBs are shown in Older Version C315M Laser Head PCB (Note: PCB in photo is rotated 180 degrees from normal orientation) and Newer Version C315M Laser Head PCB. There may be minor differences in component values depending on PCB revision. It wouldn't surprise me if some resistors are select-on-test. It would be useful to compare values on a few units. The letters on the presets seem mostly fairly obvious but some may be German as Germany is where these are manufactured:
The wiper on P1 represents the LD current set by the controller with a calibration of 1 V/A. But the controller doesn't drive it directly. Rather, it provides a voltage between 0 and about 4.7 V to pin 1. As a result of the resistance network on the head PCB of which P1 is a part, 0 V from the controller results in some small current to the diode while 4.7 V from the controller results in the current limit for the diode.
Note that the settings for P3 and P4 are not the actual values used once the laser stabilizes but only the center values for the operating point search routine of the Coherent Analog Controller. However, unless they have been changed from their factory settings, the reference voltage and sensor voltage corresponding to these pot settings will probably be fairly close once the laser stabilizes. DO NOT touch the settings of any pot other than P6 unless you know for sure someone has already messed with them!!!
The LD, KTP, and RES temperature sensor pullups are on the PCB. It is almost certain that the set-points are where the output voltage of each temperature sensor with pullup is equal to the output voltage of the corresponding adjustment. So, if we know the behavior of the thermistors, we can predict the correct temperatures for each components. Even if we don't, the settings will enable the correct temperatures to be maintained either manually by comparing the adjustment and sensor output, or with a closed-loop controller. Based on experience with the Coherent 532, the temperature for the sense or set-point will be approximately: 20*(2.5-V)+25 °C.
Pads U and O - may be "Under" and "Over" for KTP temp - adapting to cases where the optimal KTP temperature is outside the standard range?
It should also be applicable to C215M and C415M systems (including running the C215M laser head on a C315M controller) subject to the differences in their DC input power requirements and laser head wiring. The user interfaces are the same. See the section: Differences Between the C215M, C315M, and C415M.
It is assumed that a suitable startup sequence is used including a delay between Power On and Laser On, and pulsing of the Power Set line. If all you have is a gooped reset widget with a pushbutton, build or otherwise acquire a proper control panel or autostart adapter. Though no damage is likely to result, behavior can be somewhat random and strange without proper startup sequencing.
A multimeter and/or monitoring PCBs are desirable to be able to check some of the voltage levels on the User Interface Connector and laser head, though most tests can be performed without electronic test equipment. However, a laser power meter capable of handling at least 125 percent of the maximum output of the laser is highly desirable if there is an issue with the power level and stability. It should be set for 532 nm if wavelength sensitive. As an example, with the Coherent LaserCheck, make sure the attenuator is in place (1 W range) and hold the wand about 1 foot (0.3 meter) from the laser output oriented so the small reflection of the beam back to the laser just misses the output window. Press the button for several seconds to get an accurate reading.
Of course, a complete Compass Diagnostic Unit (CDU) would make troubleshooting much easier. The most important things to monitor would be the TTL status signals on the User Interface Connector, the laser diode control voltage and current signals on the laser head, and the laser output power. The LEDs will instantly identify any fault conditions and the laser diode signals enable the health of the laser head to be easily determined. See the info near the end of the section: Compass-M Laser Control Panels. If all you have to test is a single laser head and controller, such luxury may be excessive, but if you're testing multiple lasers, the benefits of a CDU are self evident.
Here are a list of the common symptoms, more or less in the order in which they may occur, with possible causes and solutions:
Initial power up problems: When DC power is applied to the controller, it should immediately generate +5 VDC available at pin 11 of the DB15 User Interface Connector and pin 25 of the laser head connector (either end of the cable). (For the laser head, pins are numbered starting at the far right.) The interlock (pin 1 of the Interface connector (a relay on most C315M and C415M controllers) is normally wired to run off of this source but could also use an external source of +5 VDC with its return to pin 9 of the Interface connector or pin 23 of the laser head connector.
Remove anything attached to the DB15 User Interface Connector and check for +5 VDC between pin 11 (+5 VDC) and pin 9 (Digital Ground).
Check for +5 VDC between pin 1 (Interlock) and pin 9 (Digital Ground) of the Interface connector.
It's conceivable that pin 7 could be high without pin 12 being high and correcting any cooling problems will take care of that. Power cycling the laser should then result in correct operation once it has cooled sufficiently.
Lasing problems: There should be green output shortly after the yellow LED on the laser head comes on, and then gradually increase to approximately 50 percent of the selected output power within another 30 seconds to 1 minute. Then, power will fluctuate as the controller goes through its seach routine. After another 2 or 3 minutes, the power will drop back to a lower level and then gradually ramp up to somewhat above the selected power with the Ready LED/signal coming on. Finally, during the next 30 seconds or so, the output power is fine tuned and should stabilize at the selected power to within 1 or 2 percent. Ready may flicker a bit before the power fully stabilizes.
No green output at all is usually bad news. The controller should attempt to ramp up pump current until it sees about 50 percent of the specified output power or hits the current limit, whichever comes sooner.
Look for a green glow *inside* the laser head via the output window. (Turn the lights out if necessary.)
WARNING: Do this from an oblique angle to prevent the possibility of being hit in the eye with the beam should it decide to come on suddenly or if there is invisible IR emission.
