The laser head consists of the flashlamp and YAG rod in a sealed box, the KTP doubling crystal (outside the cavity), servo controlled apertures (variable slits) for X and Y spot size, and a servo controlled attenuator to adjust pulse energy. Note that this attenuator approach is much simpler and more consistent than alternatives using adjustable capacitor voltage or pulse duration control.
The YAG rod is probably about 50 mm long by 3 or 4 mm in diameter, AR coated both ends. The mirrors are glued to the cavity box (non-adjustable). The servos are the types used for RC models but work fine in this application. :) Optics are glued to precision X-Y adjustable mounts. A fiber optic light pipe cable introduces a targeting beam for viewing the dimensions of the laser spot into the light path via a 45 degree mirrors which is transparent to the laser beam but reflects an adequate portion of the white light beam.
Energy into the flashlamp is about 28 joules (66 uF at 850 V provided by three 200 uF photoflash capacitors in series). Triggering is via an SCR and EG&G trigger transformer to an external electrode on the flashlamp. I don't know what the exact maximum output energy is but for this application, less than a mJ is adequate once the beam is focused to a spot of a few um.
The controller consists of analog knobs for X and Y aperture and laser power (operating those RC servos, all with digital readout), single shot or 1 pulse/second select, and a knob for the targeting light source brightness via a phase control dimmer. Nothing particularly high tech!
Although the output of the YAG rod is clearly dangerous (it is probably a few 10s of mJ) and the final green output may even be hazardous to vision, the system has a Class I rating (unconditionally safe) because everything is fully enclosed under normal operation. There are two head interlocks: A magnet on the cover and dual reed switches on the optics chassis prevent operation if the cover is removed and a tilt sensor prevents operation if the head isn't within 20 or 30 degrees of vertical. There is also an interlock connector on the rear of the power supply and the firing control is a momentary SPDT (foot) switch. The interlocks interrupt primary power to the high voltage transformer (the rest of the system continues to function) but the firing switch controls logic inputs.
The following specifications have been confirmed by Chris Chagaris (pyro@grolen.com) and amended by Dr. Ed Edmondson (EE0035jr@aol.com). However, there could be other variations with slightly different part values so double checking what you have would not be a bad idea! And, of course, if you replace the PFN with one of higher energy, the values for output pulse energy will be greater (unless the flashlamp explodes). (Refer to Photos of Hughes Range Finder and Home-Built Pulsed Lasers.)
Where "test data" is listed, it's for a sample of this unit I have which included a test data sheet.
These AN/ Numbers are related to the following equipment. AN/VVS-1 is used on the M60A2 Tank, AN/VVS-2 is used on the M60A3 Tank, and the AN/VVS-3 is used on the M1 Tank.
The major contracting firm even made a version designated LAV-105 (Light Assault Vehicle) which was used on a US Marine APC's (Armoured Personel Carrier).
These laser rangefinders were replaced with eye-safe versions using erbium:glass at 1,540 nm and YAG-KTP-OPO at 1,580 nm. This older non-eye-safe version of this assembly is being sold on the Internet by several different surplus companies and individuals on eBay and elsewhere.
Test data: 50 mJ at a PFN voltage of 1,095 VDC.
Test data: 1.4 mW based on 50 mJ pulse energy divided by 35 ns pulse duration.
The top part of the cavity reflector may be taken off by removing the one obvious screw and carefully wiggling the cover loose (if necessary). Take care not to smash the flashlamp in the process. DO NOT TOUCH any of the interior surfaces, especially of the flashlamp! Fingerprint oil may cause it to explode after a few flashes (so they say). The flashlamp does not actually touch any part of the reflector assembly but mounts via its end-caps into a grounded block at the cathode-end and an insulated nylon block at the anode-end The rod is clamped in place in the bottom part of the cavity reflector, possibly cushioned by a layer of indium or other soft heat conductive material.
In normal operation, the motor is spun up on demand just prior to the laser firing. It would be a good idea to implement this feature in any home-built power supply designs to prolong the life of the motor.
Test data: Spinup time at normal operating voltage (?) should be less than 200 ms, 40 ms typical, 70 ms on this unit.
The spec'd maximum voltage is 1,250 VDC and the operating voltage (50 mJ output energy) on the test data was 1,095 VDC.
References:
(From: Doug Little (dmlittle@btinternet.com).)
The pulse generated by the Q-switch's magnetic pickup looks a little like this:
/\
___/ \ ___
\ /
\/
If you build your trigger circuit carefully and make sure you connect the
magnetic pickup the right way around (rising or falling edge) you can
minimize any unwanted delay between pickup and trigger. You can then of
course introduce an artificial and adjustable delay of your own for
optimization purposes. A suitable circuit is shown in
Q-Switch Trigger Circuit for Hughes MS-60 Ruby
Laser and described in the section: Doug's
Q-Switch Triggering Circuit for Hughes MS-60 Ruby Laser (DL-ST1).
There are some important things to realize when you try to set up your own timing circuit:
There are two ways around the second problem. One is to run the motor backwards, giving you a whole rotational period of about 2 ms to play with. Being a mechanical motor, this is a lot of time to wait before discharging the lamp without expecting some sort of speed fluctuation. The longer the delay, the less accurate the prism's final (flash) position becomes in terms of motor speed!!! I have strobed the prism with a super-bright LED using a very short on-time of several us and I can say that a 2 ms delay results in a slightly wobbly prism, instead of a preferred rock-solid one. The second solution involves the motor/Q-switch mounting platform. If you loosen the hex bolts you can rotate the whole unit about 5 to 10 degrees in either direction. This affects timing quite a bit and gives you the opportunity to buy back a few 10s of us.
WARNING! Adjusting the Q-switch platform may kill your laser's alignment and you will have to go through the whole horrible process of adjusting the optics with a reference laser and it can take hours. I know because I did it myself. If your laser is already aligned, you may want to think very hard before you go adjusting those hex bolts!
(From: Randy Smith (randysmith@adelphia.net).)
I too have one of these ruby laser units that I am trying to get running. To start off with, there needed to be some sort of timing control unit to synchronize the flashlamp with the spinning mirror. I built such a device using an 87C552 micro, with a 4 digit thumb switch control to allow for an arbitrary offset from TDC (top dead center), entered in degrees. The jury is still out as to the functionality of this unit, but it does look good on a scope and also, when used to drive a small laser diode, it can be used to view the instantaneous position of the mirror. I will find out for sure this coming weekend, when I test it in operation with the laser.
I finally got the thing to work but I had to step up the power input to the flash lamp. I simply added a second 150 uf cap in parallel with the other to get a total input of about 216 joules. I charged both up to 1,200 volts. I used the Doug Little's Q-Switch Trigger Circuit for Hughes MS-60 Ruby Laser to synchronize the flash lamp discharge with the Q-switch (See the section: Notes on the Hughes Q-Switch. I ran the motor CCW at 36,000 RPM and adjusted the Q-switch prism to be about 1/8th turn past the pickup when the lamp fires. This seems to give the best results. It blows the ink off a page. Next, I'ms going to see what it will do metal. :)
I figure that with only the original 150 uF or so cap producing at most 126 joules, at 1,300 volts max, it is probably just barely at the lasing threshold with an optimally timed and aligned Q-switch. The military techs had a device for this unit that tuned the Q-switch without firing the flash lamp. If one had that device then you could probably get it to work with just one cap. Also if it had a real OC instead of just a clear optical medium I think that would help a lot.
(From: Sam.)
Yes, we know that the use of a dielectric OC reduces the lasing threshold significantly. Wes Ellison actually got the laser operating without the Q-switch using an OC from some other ruby laser.
(From: heru_kuti@yahoo.com.)
The Hughes ruby laser Q-switch mirror block is composed of a beam diverging optic, a spacer, and a "mirror" that is slightly concave. If all three optics are aligned perfectly it will give about 33% reflective power.
The OC for a Q-switched laser is typically 30% while it is 70% for lasers that operate long pulse mode. The one used in the rangefinder is the most durable of all being totally devoid of any coatings whatsoever.
However, if you disassembled it, there is no practical way to realign its optics and you have probably ruined it unfortunately. :-(
(From: Sam.)
The resonant OC in the Hughes ruby laser consists of two think plates separated by a thicker spacer which is hollow in the center so that in effect, the outer plates are air-spaced. It's likely that the coefficient of thermal expansion of the plates and spacer have been carefully selected to make the response temperature invariant, or at least that all of the peaks of the optic move by the same amount with temperature changes. All four surfaces appear to be uncoated and thus have similar reflectivity. The plates and spacer are held in place by a rubber O-ring but the interface between them is either optically contacted or at least ground and polished as it is optically clear.
The problem with the resonant optic is that there is no way of knowing if it is any good by inspection or by any easy tests. The location of the reflective wavelength peaks depend on the spacings of the surfaces in a multiple plate etalon. For these low reflectivity surfaces, the response function results in very broad peaks and multiple peaks will fit within the gain bandwidth of ruby so they don't have to be positioned precisely as long as they match. The thickness of the two plates is what determines the peak location for them and they are presumably matched. However, reflections between the plates with a distance determined by the spacer will also affect the overall response. It is not known (though could be calculated) what the exact effect will be. If someone (before you of course!) was curious and disassembled it, even if all the parts were put back together in the correct order, some change in performance is possible, though it's not known how serious this is likely to be. But, even a speck of dust trapped between one of the plates and the spacer could be significant when dealing with wavelengths of light. Given the general difficulty in getting this laser working with the resonant OC at all, replacing it with a dielectric OC with a known reflectance may be worthwhile especially if there is any uncertainty in the resonant OC's condition. And as noted, this could result in a lower threshold as well.
"I purchased one of the Hughes rangefinders (two, actually, if I can find the other one...), and have been looking at what might optimize the output. It appears that simmer pulse operation, with 600 V square wave pulses with a duty cycle such that one pumps for the length of a rotational period without killing the tube would do the trick. IGBTs would do the switching - the question is how to trigger the tube without a serial transformer in the existing cavity. The best idea I have would be to use an insulated wire externally as the trigger - has anyone tried this and made it work?"
(From: Chris Chagaris (pyro@grolen.com).)
How exactly do you intend to "optimize the output"? I get the impression that you wish to optimize repetition rate by utilizing a pseudo-simmer mode circuit. You must realize that this laser was designed to operate at a low repetition rate and must do so for a number of reasons. The original flashlamp contained in this laser is an EG&G, FX-103C-3 which is the predecessor of their FXQ-1302-3. With the design of this cavity employing only convection cooling of this original lamp, the maximum average power is rated at only 20 watts. At an input energy to the lamp of let's say 100 joules (somewhat above minimum for laser operation) your pulse repetition rate would be limited to one pulse every five seconds. With such a slow repetition rate I cannot see the justification for employing a simmer mode of operation. Since there are no active means of cooling the ruby rod, this could also present a problem, as ruby does not dissipate heat very well and the likelihood of damage from over-temperature is great if this system were to be operated much above its design limitations. With the configuration of this particular laser cavity (semi ellipse) the use of an external trigger wire for successful firing would be highly unlikely. The flashlamp is in intimate contact with the grounded aluminum base of this reflector to aid in the cooling of the lamp. A wire of any kind would interfere with this contact and of course would serve no purpose as the current would just flow to ground. A wire with enough insulation to protect against the very high voltage pulse (10 kV or more) would be very impractical.
(From: Sam.)
I agree with Chris 100% that boosting the repetition rate isn't really viable. As far as triggering, an alternative to series triggering is parallel triggering which can easily be extended to multiple trigger sources. See the section: Basic Structure and Characteristics of SS Laser Power Supplies. EG&G discusses simmer mode in their Design Considerations for Triggering of Flashlamps.
(From: Chris.)
In more detail, there are two points to consider in answering this question:
P(avg) = E x f
Where:
The flashlamps that one may find in the MS-60 rangefinder ruby lasers are either the original EG&G lamp, FX-103C-3 or the replacement EG&G flashlamp, FXQ-1302-3. Since this ruby laser's cavity is not actively cooled (merely convection cooled) the maximum average power rating for these lamps are 20 watts and 150 watts respectively. Consider an input of 100 joules to this first lamp. This would limit repetition rate to one pulse every five seconds. This same input to the replacement lamp rated at 150 watts would give you a safe maximum pulse rate of 1.5 pulses per second. Of course an increase in pump energy to the lamp would decrease the maximum safe repetition rate.
Ruby was never meant to be pulsed at a great repetition rate. Another problem that one would face at high repetition rates is the overheating of the ruby rod, which does not dissipate heat too well (unlike YAG). This can permanently damage the ruby crystal.
The SSY1 laser head used to be available from Meredith Instruments along with a matched pulse forming network (see the section: Pulse Forming Network 1. (Meredith had also been auctioning these and other items on eBay.) SSY1s frequently show up on eBay from various sellers. The going price is in the $100 to $200 range for the laser head. New SSY1s and parts may also be available from Anderson Lasers, Inc. and elsewhere. I constructed a capacitor charger and external trigger circuit. See the section: Sam's AC Line Power Supply for SSY1 (SG-SP1). An alternative design which runs from low voltage DC is described in the section: Sam's Inverter Power Supply for SSY1 (SG-SI1).
For initial testing, figuring it would be real effort to get it lasing, I used my trusty IR remote control tester for detecting the beam. Big mistake. :( The first shot sent the photodiode off to photodiode heaven (or wherever faithful photodiodes go when they die). Its output just stayed on! I should have used the IR detector card available from Radio Shack (and elsewhere).
OK, so go to plan B. :)
I placed a piece of black coated paper in front of the laser and fired off a few shots. No effect except for a bright blotch of white light from the flashlamp. (Maybe I didn't examine it closely enough.)
Next, I tried a small lens approximately focused on a piece of black coated paper. To make sure any effect wasn't just due to spill from the flashlamp, these were positioned about a foot from the laser head. Immediate gratification! The moderately focused output beam easily obliterated the black coating on the paper. This was accompanied by a very nice 'snapping' sound and white or yellow incandescent plume when hitting the black coating, and a more muted sound after the black stuff had vaporized. When carefully focused, it will make nice tiny holes in aluminum foil (the incandescent plume is green-blue in this case) and other thin materials, and mini-craters on thicker objects. I've heard of people driving this laser with much higher energies to blasting holes in razor blades (see below). However, it is all too easy to blow up the laser components when doing this - the flashlamp and Q-switch are most susceptible to damage or destruction.
I don't have any way of actually measuring the energy of the beam but let's just say it is definitely not something to be taken casually, as far as eye safety is concerned! My wild off-the-top-of-the-head guestimate would be at least 10 mJ, probably 20 or 30 mJ, though it may be as high as 50 to 100 mJ. Hopefully, someone will eventually measure the output pulse energy! The Nd:YAG rod is probably capable of much greater energies but that flashlamp doesn't look all that sturdy so I'ms not about to push my luck, at least not yet. :)
The lasing threshold is about 7.5 J - less than the energy of the electronic flash in a typical pocket camera! This low value is no doubt due to both the cavity and optics design - and the optimal pulse length from the PFN. Thus, using one of those cheap flash units (or just its power supply) directly probably wouldn't work at all as the duration of the flash pulse would be way too long with insufficient peak intensity. (The unit described in the section: Micro Laser Rangefinder Using Disposable Flash Pumped Nd:YAG and OPO is based on a much smaller Nd:YAG rod - about 1/8th the volume.)
Here are the specifications, as best I can determine:
The white flashlamp trigger lead is connected to a fine wire that runs the length of the inside of the bore where the flashlamp lives.
The cavity assembly may be detached from the outer casting by removing 4 screws providing access to the inner surfaces of the HR and OC, and the rod ends for cleaning. The flashlamp may then be removed by unscrewing a nylon fastener at the anode/OC-end and carefully straightening the cathode lead. CAUTION: Avoid touching the flashlamp envelope. If you do so by accident, clean it thoroughly to remove all traces of skin oils.
The maximum energy input using this power supply is 15 J (36 uF capacitor charged to 900 V. Nearly 100 percent of the energy in the capacitor is transferred to the flashlamp. An energy of 15 J may not sound like much but it is more than adequate (actually twice the threshold) for pumping the 50 mm rod with the optimal 100 us pulse duration and well designed cavity
WARNING: Despite its small size, this is a Class IV laser. While SSY1 probably won't set anything on fire unless you fire it at an explosive or have a natural gas leak, this laser is quite capable of doing serious damage to vision. Treat it with respect! Cover the HR mirror aperture (I used black electrical tape) since there may be some leakage from there which is invisible and enclose the output beam path so that backscatter can't hit anything of importance (like your eyes).
I've now tested 3 of these babies - 2 that appear to be in original condition and another with the Q-switch removed and the AR coating gone from one end of the rod. (I've also used the mirrors from an SSY1 to construct the resonator for another YAG cavity, see the section: Mini YAG Laser using SSY1 Optics and SG-SP1.) The two intact units produce about the same output energy. The other one lases but probably at slightly lower energy. It still smokes black tape (possibly better than the other ones) but won't penetrate aluminum foil. The sound it makes when focused on a target is also softer. However, I don't know to what extent these differences are due to the lack of a Q-switch versus the missing AR coating It's probably a combination of both but the reduced effect on thermally conductive aluminum foil and softer sound would be consistent with the longer, lower peak power pulse produced without a Q-switch. Perhaps at some point in the future, I will swap rods with an original SSY1 to separate out the effects of the missing Q-switch and AR coating.
I am trying to build a laser rangefinder using this laser.
(From: Ivan (sinebar@bellsouth.net).)
I got my small YAG laser working using the PFN from Meradith Instruments and a power supply based on the SG-SP1 schematic. Even without a lens it will burn a spot on a black target.
(From: Rick (rick@skyko.com).)
I got bored this afternoon and figured I would dig out the SSY1 I bought a few months ago on ebay from Meredith. If that is not the easiest laser to get lasing, I don't know what is. I think it is easier than modifying a green pointer! :-)
I started with two plain old 330 uF, 400 V electrolytic caps in series from my junk box (I have some 1,500 uF 450 Cornell Dublier electrolytics, but I didn't want to take out the Q-switch yet). I then dug out a smallish 12 VDC-powered hene supply (for like a 1 to 2 mW tube and wired that up to the caps through ten 100K 1/2 watt resistors wired in series (for 1M at 5 W). I found a dented old auto ignition coil transformer deep in my junk boxes and I wired up a 4:1 divider using 1M 1/2 resistors off the caps to charge a small 2.2 uF 250 V capacitor. To fire the laser, I turn on the HeNe laser power supply, watch the voltage across the main caps charge (about 20 V per second or so) and then when it is at the desired voltage, I short the 2.2 uF cap across the input terminals of the auto ignition transformer, whose coil is hooked up to the trigger wire of the SSY1. I then took a note from Sam's experience and wound about 55 turns of 14 gauge plain old solid copper wire with thin plastic insulation around an used up plastic speaker wire container bobbin. I measure the inductance of the completed coil with my LC meter and found it to be 199.5 uH. Not bad! Overall though I would say it is the crudest SSY1 power supply yet! :-)
For the very first shot I was not absolutely sure which end was the output, lol, so I put a black electrical tape target about 2 inches from each end. I let the main caps get to 450 V total and then shorted the 2.2 uF cap to the transformer. A nice satisfying flash! and a perfect 3-4 mm white spot on the electrical tape on the end with the red wire (ah, the output end, heh heh).
I then found a 1.5" FL lens and proceeded to de-anodize some aluminum. The thing is loud when it is focused. I am actually adjusting the focal length as I type (while waiting for the cool down of the SSY1 lamp (what is the duty cycle on these things anyway? (Figure about 10 W average power into the lamp. --- Sam ) I am giving it about 3 to 5 minutes between pulses). I think I may be able to make some small craters in the black anodized aluminum, but maybe not until I swap out the series 330 uF caps for the series or paralleled 1,500 uF ones (after removing the Q-switch).
Not a bad little laser for $125. It really deserves a better supply though. :-)
(A day passes.)
I just fired a shot from my SSY1 with 165 uF caps (two 330 uF caps in series) charged to 550 V (so about 25 joules) into a Molectron J25LP-0686 sensor head with a responsivity of 5.0 V/joule at 1064 nm. I measured a 620 mV pulse on my oscilloscope.
This would mean the output power from the SSY1 at 25 joules to the flashlamp is 124 mJ.
Is that even remotely possible?
(From: Sam.)
Might be a bit high, but not out of the question.
(From: Rick.)
It does punch a hole through aluminum foil at this power level, and also it pits a stainless steel razor blade (but does not punch through).
It also left a 4 mm mark on the carbon looking sensor head... whoops. :-(
While making some more power measurements from my SSY1, I heard an increasing snapping sound as I went up in pump joules. Since I have the power sensor head well past the focal point of a positive lens (normally I would hear this snapping sound when I focused the spot on a piece of electrical tape or aluminum foil) I was wondering where it was coming from. I then covered the SSY1 with a piece of cardboard to mask the flashlamp light spillage and fired it up at 165 uF, 700 V (40 J). I saw a bright pinpoint flash of light at 1.5 inches from the lens in mid air! Very very cool (first time I have seen this phenomenon, though I have heard of it). I guess this gives another data point to the output power level... Air sparks at 200 to 400 mJ? :)
I am going to try and capture this on video and stick it on my Web site.
(From: Sam.)
Use a shorter focal length lens and the light show will be even more spectacular and/or occur at lower energy.
(From: Mike Poulton (mpoulton@mtptech.com).)
You can push them really hard. I ran about 1 kW average input power for 5 seconds at a time, letting it cool about two minutes between bursts. I had a small fan pointed at it, but no real forced air. It didn't like this, but I did it quite a few times and it still works fine. The yellowish plastic around the cavity is discolored brown from the heat - it was probably close to 400 °F and it didn't fail.
(From: Sam.)
On another note, the laser described below is the modern version of SSY1 which is similar, perhaps even a bit smaller:
(From: Erbium1535 (erbium1535@aol.com).)
The South Carolina State Museum in Columbia uses a Nd:YAG laser to pop a balloon inside a balloon in their Townes exhibit. (C.H. Townes was born in Greenville, South Carolina.) The laser, manufactured by Kigre, Inc. in Hilton Head, SC is a Q-switched MK-367 unit and is described on the Kigre MK-367 Nd:YAG Laser System Page. The actual laser is approximately 0.6 x 0.8" x 4" in size and emits a 17 mJ pulse pulse with s duration of less than 4 ns. They also offer a frequency doubled green version. The MK-367 was originally developed for the ophthalmic surgical market, specifically as a photo disrupter for posterior capsulotomy. The power supply is approximately 4" x 4" x 1.5" and operates from 12 VDC.
The laser is somewhat unique in that it is permanently aligned, utilizes a ceramic exoskeleton for stability, and a positive branch confocal resonator design for high beam brightness. Kigre has sold more than a thousand of these miniature lasers for various applications including medical, industrial, rangefinding, and pyrotechnic ignition. The MK product line has been around for more than 15 years, so these lasers sometimes find their way to the used laser discounters. New ones are still available and cost about $3,600. If you do come across one of these, be very careful as it is a very powerful Class IV laser! (Yes, but the SSY1 is potentially an even more powerful Class IV laser! --- Sam.)
(From: Shawn West (west007@libcom.com).)
I've taken a different approach than the others and am pumping it with a long pulse, about 2.5 ms. With my long pulse I have put a 0.020 inch diameter hole in a 0.004 inch thick razor blade. I've punched holes through aluminum foil of different thicknesses too. I've back calculated the energy required to punch the holes in the razor blade and the two aluminum foil experiments. The calculations show that it would have taken 1.7 to 1.8 joules to melt and vaporize the metal in each case (if I did my calculations right). When I hit the razor blade with 800 volts on the capacitor (360 joules) I was able to punch a 0.024 inch diameter hole in the 0.004 inch thick blade. My calculations, which again could be wrong, show that it would have taken about 2.5 joules to do this. These calculations do not include the amount of reflected energy or the energy conducted away from the material. I have also sparked the air using a short focal length (about 1.5 cm) lens. I'ms using a 1,120 uf capacitor with approximately a 0.15 ohm ESR.
My inductor is 820 uH with a resistance of about .15 ohms. It is from Parts Express (part #266-760, about $23). The inductor is wound with an effective 12 gauge copper foil and has an air core. I'ms using a piezo-electric igniter from a gas grill to flash the tube.
I have also used a 270 uF capacitor and a 80 uH inductor (ESR of 8 or 9 milliohms). However, the longer pulse PFN put out more energy (more destruction to the target) than the short PFN when the caps were charged with the same energy. This could have been due to the ESR differences of the two capacitors or the higher current density with the shorter pulse PFN exciting the shorter wavelengths of the xenon (i.e. not exciting the 800 nm hues as well to mate with the Nd absorption). I'ms trying to keep the current density in the flashlamp below 4,000 A/cm2 to favor the 800 nm absorption band of the Nd:YAG crystal. I also wanted to pump out a lot of energy. This forced me into a long pump pulse.
I spoke to Jim McMann (sp?) from Perkin Elmer (EG&G) about the flashlamp in mid-December, 1999. His phone number is 1-800-950-3441. At that time, he thought the flashlamp was an FXQG-264-1.4. From what I have found out since then, there are two EG&G flashlamps that could have been used for the SSY1. The first is the FXQG-264-1.4. This flashlamp is made from titanium doped quartz that cuts off UV wavelengths below about 225 nm. The second is the FXQSL-559-1.4. This flashlamp is made from cerium doped quartz that cuts off UV wavelengths below about 320 nm. I don't know which one was originally used.
Both of these have a 1.4 or 1.5 inch arc length, and are probably xenon filled to 500 Torr (though I have not been able to verify the fill pressure). The ID was 3 mm and the OD was 5 mm. If you calculate Ko with a 1.4 inch arc length, you get:
1.28 * (1.4 * 25.4) 500
Ko = --------------------- * (---------)0.2 = 15.5
3 450
Using a 1.5 inch arc length results in a Ko of 16.6 which is what I measured
it to be.
For the more conservative arc length of 1.4 inches with a 3 mm bore, the explosion energy for the flashlamp = time.5 * 90 * arc length in inches * bore in mm = 378 * time.5. (Time is in milliseconds.)
I designed this to run from 300 volts (50 joules) to 800 volts (360 joules). My damping factor (alpha) ranged from 1.03 at 300 volts to 0.8 at 500 volts to 0.63 at 800 volts. I think at about 560 volts the current density in the flashlamp was about 4,000 A/cm2. The explosion energy with a 2.5 ms pulse is about 590 joules and at 800 volts I was running at about 60% of the explosion energy. I normally run at about 560 volts where alpha = 0.76, at 30% of the explosion energy (about 177 joules), and the current density is about 4,000 A/cm2 in the flashtube (the approximate maximum current density for which the 800 nm line is strongly excited). When I was hitting the razor blade and the aluminum foil the capacitor was charged to 700 volts (274 joules - about 46% of the explosion energy). The maximum pulse rate is about once every 45 seconds. Right now my charger is running from 120 Vac but I plan to make this portable and run from 12 volts with a pulse rate capability of about once every 30 to 40 seconds.
I have not removed the Q-switch to see the effect yet.
(From: Sam.)
Well, that's certainly impressive!
I assume that with the Q-switch, you are actually getting a series of short pulses of a few dozen mJ each. My quick off the top of my head calculation for output energy using the Q-switch would be 25 to 50 times 20 or 30 mJ which is in the .5 to 1.5 J range so your calculations of output energy may not be far off. This laser would probably also do nicely with an arc lamp if you could cool it somehow. :)
(From: Shawn.)
My scope is getting calibrated now, but when I get it back I'll check the reflected light to see I am getting a bunch of pulses or a long continuous pulse with a steep front end (maybe even a spike on the front end of the pulse). Does this Q-switch have a self terminating bleaching effect independent of incident power or does it remain bleached as long as the power is above a certain threshold?
(From: Sam.)
I don't know for sure but assume that it returns to its non-bleached state immediately after the laser pulse and until the spontaneous emission (not the incident flashlamp power) exceeds the threshold again. Not knowing the exact composition of the dye used here, I can't say what the exact time is. For the rangefinder, the likely objective would be one intense pulse for each firing of the flashlamp so there would be no need to select one that recovered quickly but they do exist.
(From: Greatest Prime (FishyBill@mediaone.net).)
The nickel complex BDN in toluene has a recovery time of about 1 ns. (Actually, you can make it in a number of ways. One is to dissolve BDN in methyl methacrylate and polymerize it. You have to watch out the active catalysts do not destroy the dye.) This allows for multiple pulsing. Other dyes and solvents tend to shorten the recovery time. That is what makes mode locking possible at a pulse repetition rates of more than 100 MHz. However, repetitive operation of dye Q-switched lasers is more complicated than merely considering recovery time of the dye. There usually are long term thermal effects of considerable importance.
(From: Sam.)
It might be possible to test the SSY1 laser for multiple pulsed operation by firing the flashlamp with a longer than normal pulse. Once the first Q-switched output pulse depletes the upper energy state, the Q-switch should revert to its non-bleached condition. If the flashlamp is still on, the cycle should repeat. Doubling the flashlamp pulse duration from 100 to 200 ns while maintaining approximately the same flashlamp light intensity should be enough and this can probably be done safely (for the flashlamp and dye cell at least for a few shots to perform the test) by doubling the values of the PFN capacitor and inductor. I've heard of rangefinder lasers similar to the SSY1 failing in a way that results in multiple output pulses - this may be a way to experiment with this mode! Diode pumped solid state lasers take advantage of this effect to generate a series of very short pulses with very consistent energy between pulses and a rate determine by the pump input.
One way to determine the pulse shape or pattern would be to fire the focused laser beam at a rotating disk with a piece of black paper or carbon paper glued to its front surface. The shape of the burn mark or pattern of spots should reveal whether it is lasing CW for the duration of the input pulse or pulsing at a regular rate as would be expected if the Q-switch were active the entire time. A 75 mm diameter disk rotating at 3,600 rpm would result in a linear velocity of about 1.4 mm/100 us for this laser oscilloscope. :)
(From: Shawn.)
I noticed that my divergence is significantly greater with the long pulse (2.5 ms) versus the short pulse (approximately 400 us). Do you have any thoughts on why this could be happening? How much more energy do you think I could get out if I removed the Q-switch?
When I was using the short pulse PFN I could discolor a black piece of cardboard about 2.5 feet away with the spot size only growing slightly (perhaps a few mm in diameter). However, with the long pulse PFN, I placed a piece of black cardboard about 3 inches from the output coupler (and hit it) and then moved it back 4 inches (about 7 inches from the output coupler) and the diameter grew by about 2 mm. At about 1 foot from the output coupler I can't discolor the black cardboard with the long pulse PFN.
(From: Sam.)
That's interesting and could indicate that the dye does remain bleached after the initial pulse. Or, the dye bleaches from the center out which would restrict the area of lasing when Q-switched.
(From: Shawn.)
Are you thinking that if the dye bleaches from the center out in combination with the applied pulse duration, then the Q-switch will effectively clip the higher order modes letting only TEM00 to oscillate. However, with a long pulse, the dye possibly remains bleached over the whole rod diameter which permits the higher order modes to oscillate creating the high divergence. Maybe I should pull the Q-switch and insert an aperture into the cavity to clip the higher order modes?
(From: Sam.)
As far as total energy, if the Q-switch is not participating after the initial pulse, than it won't make much difference. However, if the dye bleaches and recovers quickly, then perhaps it could be significant.
(From: Shawn.)
I use a cheap 660nm laser pointer to bore sight the laser. When I get the laser pointer lined up I can see the "orbit" reflections that seem to surround the fundamental spot. However I thought with a plano-plano cavity the reflected spots tend to follow a line from the fundamental or follow a slight curve (i.e., not surround the fundamental spot). Could this cavity be a near hemispherical or a plano-plano cavity? If this is a near hemispherical cavity could that explain why the center of the q-switch would bleach first?
(From: Sam.)
I thought it was a plano-plano cavity but didn't check carefully. Just look at the reflections from the optics of something distant and see if they look flat. :)
Shining a laser pointer into it you also have reflections from the rod ends and the Q-switch to confuse things. I'll have to check...
I just went and used a HeNe laser reflected off the mirrors with a piece of paper to block the reflections from the rod ends and Q-switch (so they wouldn't confuse things). The mirrors appear to be planar as far as I can tell but this still isn't conclusive since I was just kind of holding the thing steady and trying to view the reflected spots.
It does look as if the rod ends and/or Q-switch is ground on a slight angle because without the paper, there is a distinct far off-axis spot.
(From: Shawn.)
I noticed that far off axis spot too when I'ms bore sighting it with the laser pointer. Do you think it would be worth it to put an aperture in the cavity and how big of an aperture do you think would be good to use? What is confusing me is that the output of the side of the rod closest to the flashlamp seems to put out more energy and I am trying to envision the optimal location for the aperture (i.e., should the aperture be placed off centerline toward the flashlamp side).
(From: Sam.)
The fact that you get more energy off-center suggests (at least to me) that the cavity is indeed planar. A cavity with curved mirrors would tend to homogenize the distribution I would think.
What are you hoping to accomplish with an aperture? Obtain a TEM00 beam? That may not be possible from such a short cavity. There's a magic number for a given cavity configuration to determine if a TEM00 beam will be produced (sorry, I don't have the equation or the value for this laser) but I bet it would require a rather narrow beam.
(From: Shawn.)
I was just hoping/dreaming to be able to project the unmanipulated beam further. I think you are right again about the planar cavity. A near hemispherical cavity should have more energy in the center.
(From: Sam.)
Well, you can still expand/collimate it and that will help but if you were after HeNe-like beam quality, not likely. :)
(From: Shawn.)
I fixed my divergence problem. I remember when I got the laser, I illuminated the bore and noticed a slight star-burst pattern that seem to be coming from the Q-switch. Yesterday, I noticed the star-burst getting more pronounced. I guess my higher energy pulse must have aggravated the existing imperfection. So, I removed the Q-switch. My divergence problem has gone away. I'ms assuming that the imperfection in the Q-switch was dampening the oscillations in the center of the laser rod. The beam now grows about 0.1 to 0.15 inches in diameter over a 3 foot distance.
Before, when I charged my capacitor up to 700 volts (about 275 joules) I could only put about a 0.020 inch diameter hole in a 0.004 inch thick razor blade. Now, without the Q-switch I can put a 0.033 inch diameter hole through the same razor blade. If you just ratio the changes in volume the output energy has increased by over 2.5 times.
(From: Sam.)
Yes, I've heard that the dye based passive Q-switch is one of the items that fails most often (the other being the flashlamp). So, it may have been slightly bad to begin with but your super power pulses might have really done it in!
For those who haven't yet begun to abuse SSY1, it is probably best to remove the Q-switch dye cell before attempting to run at much higher energy input than the 15 J max of PFN1. To do this, detach the rod/flashlamp assembly from the resonator frame (make a note of the direction in which it is installed). At one end you can see an AR coated end of the YAG rod (I think there is a screw at that end which holds the rod in place). At the other end is the Q-switch dye cell (slightly larger diameter than the rod) which is held in secured with some tan or brown adhesive which has to be removed to free it. There is a tiny fill hole where some adhesive was forced in on the side - using a drill bit in your hand to remove what's in there may also be needed. Take care to avoid scratching or breaking the dye cell - you may want to replace it at some point in the future (and that dye cell originally cost something like $200!).
Without the Q-switch, the output will not be as short a pulse but may actually result in more total energy (though less peak power).
(Several months pass.)
I have now built everything into a portable self contained unit (including the laser pointer target designator) that could operate from a 12 VDC source. A pushbutton must be held in to charge the caps but there is an overvoltage cutoff to prevent accidental overcharging. There is an LCD readout for capacitor voltage. Of course, the most important part of this rig is my pair of 1,064 nm laser safety goggles!
I've fired well over 2,000 shots with my SSY1 setup and there appears to be no decrease in output power (based on the diameter of hole through a razor blade). The Q-switch has long since died and was removed about 2,000 shots ago. :) My max pulse rate is about 1 shot every 45 seconds. EG&G says that I am driving the flashlamp properly. I bought a couple extra flashlamps just in case.
I've made a sort of hodgepodge laser power meter. I sliced a piece of carbon from a carbon zinc battery anode. The slice is 0.239" diameter (6.071 mm) by 0.065" thick (1.651 mm). I epoxied a thin piece of plastic to the back of the carbon disk to act as an electrical insulator for a Fluke k-thermocouple junction. The thermocouple junction was epoxied perpendicular to the flat surface of the disk. I used an 805 nm laser diode to "calibrate" the disk. The laser diode is calibrated. I set the laser diode to put out 1 watt. I put the carbon disk in front of the laser diode aperture and turned on the laser for different durations as measured by an oscilloscope. I took several measurements while measuring the delta T and time duration for each exposure to the laser diode. Approximately 2 minutes elapsed between each measurement. My data is shown below:
Test Tinitial Tfinal Delta T Pulse Duration MC calculated
# (Deg C) (Deg C) (Deg C) (seconds) (Joules / C)
----------------------------------------------------------------------
1 23.8 30.0 6.2 1.56 0.252
2 24.1 31.2 7.1 1.67 0.235
3 24.2 27.8 3.6 0.92 0.256
4 23.8 28.2 4.4 1.11 0.252
5 23.7 26.1 2.4 0.58 0.242
6 23.5 34.1 10.6 2.50 0.236
Energy into the sensor in joules = time duration in seconds since the power
input is 1 W. The average MC comes out to be 0.246 J per Deg C.
It took about 10 seconds for the temperature to stabilize. I guess that the thermocouple wires were not bleeding away the heat too fast.
I charged up the capacitor for the SSY1 to different voltages and fired it into the sensor which was about 1 foot away. I have a laser pointer with a cross hair diffractive lens that bore sights the laser and is aligned to perhaps 1 to 2 mm. The following are the test results:
Vcap Tinitial Tfinal Delta T Calc Eout Flashlamp Energy Efficiency (Volts) (Deg C) (Deg C) (Deg C) (Joules) (Joules, from Pspice) (%) ----------------------------------------------------------------------------- 350 24.4 27.0 2.6 0.64 57.1 1.1 400 23.7 28.3 4.6 1.13 73.6 1.5 450 23.9 29.9 6.0 1.48* 91.9 1.6 500 23.9 31.7 7.8 1.92* 112.0 1.7 500 24.0 31.3 7.3 1.80* 112.0 1.6 550 24.0 32.2 8.2 2.02* 133.8 1.5 600 23.8 33.6 9.8 2.41* 157.3 1.5* Smoke came from the sensor during these measurements!
The flashlamp energy was calculated by the Pspice simulation. The following are some of the things that were not considered in the measurements:
(From: Sam.)
Cut, file, or grind one of your carbon rods to create some slices length-wise. Sand them smooth and butt the long edges together to form a larger surface area. Yes, I know this will be messy!
You're getting me interested in trying this stunt. I have a pair of 1,800 uF, 450 V computer grade electrolytic caps. Yes, I know, not laser caps, but at with relatively discharge pulse, might survive. With the caps in series, at 800 V, they would provide about 288 J; at 900 V, about 360 J. Or, better yet, I should run them in parallel which would be slightly less efficient but would eliminate any issues of voltage balancing, reduce the stress on the flashlamp, and the air-core inductor would only need to be about 200 uH. I have plenty of thick wire to wind it.
I would remove the Q-switch before the first shot so that it would live to pulse another day. :) I also have some other mirrors with cosmetic defects which I might substitute as well. The same capacitor charger I used originally with SSY1 would work fine here though I might have to beef up the current limiting resistor's wattage a bit. :)
As I mentioned, the air core inductor I used was from parts express. It was about 2.5 inches in diameter and about 2 inches long. It was wound with copper foil 2 inches wide and used insulation between each layer. However, here is a formula for the inductance of a coil whose length is greater than 0.4 times its diameter:
d2 * t2
L (Inductance in uH) = ---------------------
(18 * d) + (40 * b)
Where:
(From: Sam.)
Nah, that's cheating. :) I found a 3 inch diameter form during a walk in the park - from a Hallmark(tm) party ribbon or something - perfect. Extrapolating from the tables above, a 200 uH inductor would require about 50 turns. I actually wound 55 turns in 5 layers using #14 insulated solid building wire. This isn't exactly magnet wire but the insulation is still rather thin so it packs nicely. The 55 turns should yield a bit more inductance - perhaps 250 uH - resulting in a slightly longer pulse. So much the better - it will be easier on the flashlamp.
I located the pair of 1,800 uF, 450 V caps and confirmed that their ESR is still unmeasurable (0.0 ohms) but I will probably need to reform them since they are quite old. I even have a preliminary power supply design. See the section: Sam's High Energy AC Line Power Supply for SSY1 (SG-SP3) and stay tuned for exciting developments.
I successfully fired the SSY1 with a cap bank at 64 uF at 985 V. It made a very clean hole through a razor blade in one pulse with the aid of a focusing lens. I understand that this is running the tube pretty hard at input of around 31 J. I could not find out how long the tube would last under such stress.
(From: Sam.)
That's very impressive since the energy input is significantly lower than that discussed above! I do assume you removed the Q-switch dye cell as it probably wouldn't last long under this abuse. As far as lamp life, it is running 3X or 4X of the energy normally used in the rangefinder application. So, life will be reduced but it would be necessary to calculate the expected life based on the lamp's specifications.
I put together a Microsim Pspice simulation that accurately models the flashtube characteristics (with a given Ko) that agrees with measured results.
Based on the simulation, the amount of energy that actually makes it to the flashlamp terminals is about 75% of the capacitor stored energy for my PFN setup. So for my previous % of explosion energy numbers you can multiply by 0.75 to get the real % explosion values. So, for worst case (800 volt = 360 joules stored on the capacitor) only about 270 joules make it to the flashlamp which gives a % explosion energy of 270 / 590 = 45% rather than the theoretical maximum of 60% as previously stated.
The Microsim Pspice files (ASCII text) for the flashtube follow. You can change Rctrl from 1u to put the reverse diodes in the circuit or a 1M resistor to take the diodes out to see if you would be getting any negative ringing current. Resr is the ESR for the capacitor and Rind is the resistance of the inductor. You can set the capacitance, inductance, Ko, and the initial capacitor's voltage in the PARAMETERS box. You can use Rsense to display the flashtube current. Vtube is the voltage across the flashtube. The energy line integrates the tube voltage x tube current to arrive at the energy that makes it to the flashtube to gauge the efficiency of your circuit. For the energy line 1 volt equals 1 joule. The key for proper simulation is to know the proper C, L, Rind, and especially Resr.
See the OrCad/PSpice Web Site for info - there may be a demo version of Pspice which would have enough capability to run this simulation.
The peak power of SSY1 is something like 16 mJ/4 ns which is 4 MW. I'd expect order of 1 mJ of green without any optics - just put the KTP in the beam and adjust its orientation for maximum green output. The green beam will be almost coaxial with the IR beam with a walk-off of only about 4.5 mR. One problem though is that the beam from SSY-1 is not polarized so you will lose some efficiency there. I don't know how much. But if the KTP is aligned properly, there should definitely be some green photons produced. First try this simple approach to the determine if the green pulse energy and consistancy are acceptable. There is no space inside the SSY1 resonator for a Brewster plate with the Q-switch in place so one of the mirrors would have to be re-mounted externally.
CAUTION: I recommend using an aperture to make sure the IR beam hits only the clear central part of the KTP as at high enough power/energy, it could conceivably damage or destroy the KTP if it hits something that absorbs significantly. (However, as I found out, this is probably critial with SSY1 driven from PFN1. See below.)
Adding optics to concentrate the 1,064 nm beam would boost the energy density significantly. However, this is tricky because the peak power is so high and damage to the KTP is all too likely if the beam waist becomes too narrow inside the KTP even if it is all through the center.
I finally did some very basic experiments.
Using SG-SP1 as the power supply (adjustable from 0 to 900 V, 36 uF capacitor in PFN1, 0 to 15 J, 100 us pulse duration at maximum output) and a 2x2x5 mm piece of flux grown KTP similar to what's available from CASIX and Roithner for use in small to medium power DPSS green lasers. For a mount, I simply placed the KTP on a block of, wood shimmed so the KTP was approximately centered in the beam (very precise!). Here are the results:
The reason of course for the difference in behavior between the two lasers is that although the total energy may be similar with and without a Q-switch, the peak power without the Q-switch is on the order of 1,000 to 10,000 or more times lower (a pulse duration of 100 us as opposed to 4 ns). Since the frequency conversion process is non-linear, it is the peak power which ultimately determines the amount of doubled output.
I would estimate the green output to be in the 1 mJ range (give or take a factor of 5) but have no real way of measuring it precisely - only eyeballs that haven't been calibrated in a few years. :) The consistency from shot-to-shot was fairly good, again as determined by eye. The green version of the Kigre MK-367 puts out about 4 mJ.
Increasing the input to the flashlamp to its maximum value of around 15 J did increase the brightness of the green flashes but not dramatically.
I didn't take any special precautions to protect the edges of the KTP and no damage could be detected after the experiments anywhere on the KTP. So, at these power/energy levels, this concern would seem to be unfounded for a few dozen shots at least. However, your mileage may vary.
So, get out your SSY1s and chunks of KTP and fire away. :)
WARNING: Take care with respect to reflected invisible IR and visible green beams. The KTP and any other external optics should either be fully enclosed or covered with a material that doesn't pass significant radiation at 1,064 nm. Green scatter should be identified and blocked as well.
Just when I thought I had run out of things to point my little Yag laser at I decided to try a tuft of steel wool (no soap please!). The result was surprising! With the voltage cranked up to 900 volts, and the output focused through a simple hand lens the shot ignited a small portion of the steel wool, which then rapidly proceeded to consume the entire pad! This will be interesting to capture on video or digital camera.
Tired of smoking carbon paper with your SSY1? Try steel wool if you dare. Also a great way to blast holes in those pesky free CD rom disks you get in the mail!
Photos of a Quantronix 114 (in slightly better condition) can be found in the Laser Equipment Gallery (Version 1.71 or higher) under "Quantronix YAG Lasers".
Here is a general description though specifications are somewhat sparse:
It looks as though you have got the makings of a nice project. A 'bashed up' laser is better than no laser at all. :-) At least the most important components survived. If you could provide me with the number on the arc lamp, perhaps I could uncover what it actually is. Typically a krypton arc lamp of 70 mm arc length and a 5 mm bore (EG&G, FK-125-C2.75) filled to 2 atmospheres would operate at 100 volts at 30 amps. With this typical input power of 3 kW, coolant flow rate should be at least 120 cm3/s.
The conical and heimspherical electrodes are common. The pointed cathode is to help maintain arc stability.
There is a similar EG&G Krypton arc lamp (FK-111-C3) which has a 7 mm bore with a 75 mm arc length rated at 6,000 W with liquid cooling. Electrical characteristics are 112 VDC at a whopping 56 A. Wall loading is 145 W/cm2 as opposed to the smaller 5 mm bore lamp of 110 W/cm2. However, average lamp life is only 40 to 60 hours, whereas the FK-125-C2.75 should last from 400 to 600 hours with proper cooling.
Sam, where's your sense of adventure? :-) I think an attempt to refurbish this laser as an arc lamp-pumped CW type would be fascinating. Consider the cost of a new flashlamp, the likely necessity to install a new OC of a lesser reflectivity for successful pulsed operation, and the need of a PFN, as opposed to the challenge of building a phase-controlled arc lamp power supply. The design and construction of a PSU such as this strikes me as something that would be right up your alley. I have recently acquired a 6 inch arc length, krypton-filled arc lamp and have considered the construction of such a supply myself. Of course, the lamp that I have will require about 40 amps at 150 VDC! I've got a 10 kW isolation transformer. So there's a start. :-)
Interesting that the OC reflects green. I would tend to agree with you that this laser was not likely doubled. The OC for SHG would normally reflect close to 100% of the fundamental wavelength and transmit about 100% of the harmonic. This being the case, I would doubt such an optic would appear to reflect green.
(From: Sam.)
Geez I dislike even working on the power supplies for little air-cooled argon ion lasers with their current-hog requirements let alone 40 A at 150 V!! :)
It is definitely not a green YAG and I don't even know if intra-cavity doubling had been introduced in those days.
(From: Chris.)
As far as the Q-switch is concerned, I would expect that it was not a simple mechanical system like the one on the Hughes MS-60 ruby laser. I would tend to doubt that a rotating prism Q-switch would be used in-line. Usually if a mechanical Q-switch was going to be used in-line, it would be a rotating HR mirror at one end of the resonator. A roof prism is most often the rotating element in such a system because of its retro-reflecting properties, which assures alignment in one direction, while the rotation of the prism brings in alignment in the other direction.
Mechanical Q-switches tend to be rather slow as compared to electrooptical and acousto-optical Q-switches and judging from the rated pulse width achieved by this laser, I doubt that a mechanical Q-switch would be able to achieve that 50 ns pulse duration.
The power supply/heat exchanger on my 116 requires 208VAC 3 phase to crank the silly thing up. Admittedly, Quantronix did over design the power supply for worldwide use, so the transformers and control circuitry are a bit over-kill. The important point, however, is the fact that a lot of juice gets sucked up generating a clean initial pulse to jump start the krypton lamp and then maintain the 25-35 Amps DC to keep it going. Also, the water for the cooling needs to be kept VERY clean (as you may already know). The micron and de-ionizing filters basically make de-ionized water from store bought steam-distilled, ozonated water. Any particulates in the water stream when the lamp is running is a sure guarantee that the flowtubes and the lamp jacket are going to get coated and cooked!
Be careful YAG rod assembly. Some of the original flowtubes were uranium doped quartz to stabilize the UV into visible wavelengths. Just a word of caution.
The endplates you describe as "polished gold plated brass caps" are now gold plated nickel, since brass has a tendency to contaminate the DI-coolant and turn stuff green. Not good for the flowtubes or the lamp and crystal.
The Q-switch on my 116 is an 25 W RF driven Acousto-Optical model from IntraAction Corp. My guess is the 114 was probably driven the same way.
Anything in the DI-coolant stream should be nylon or stainless steel. No brass, bronze or anything else. The DI-water will pull "tons" of metal ions out of the fittings and put them into the coolant. Also (and this one is a real stretch), under no circumstances should the DI-water be consumed internally! It would literally take the calcium out of your blood-stream and in enough quantities could kill. Sounds strange, doesn't it: Ultra pure water will kill you! Takes the elemental ions right out of your system, or so I've been told. We'll have to leave that experiment untried!
It consists of a power supply unit which attaches to the laser head via a cable with an 8 pin plug, but there are only 6 separate connections as the main power feed is doubled up with two pins for +5 VDC and return. That leaves 4 signals as yet to be determined, 2 of which are fat wires so possibly these are also power related, but measuring them with the connector not plugged into the laser head showed no voltage though the 5 VDC was present. Possibly, the TEC driver is actually in the power supply as there would be just enough wires. Or, perhaps those dead wires are the problem with this laser. :) However, getting inside the power supply box is not trivial so tracing the wiring is something I'll put off to later.
The unit I'm testing is a model DPY305c, no output power rating listed. The laser head itself has quite a bit of circuitry in its squarish rear section, along with a heatsink and cooling fan. The laser/optics are in a cylinder protruding out the front. Unfortunately, gaining access to the laser/optics appears to be a major undertaking, requiring extensive disassembly. The circuitry inside the laser head is on two small PCBs, one above the other, attached via wiring and components with additional parts sandwiched between them. From the similarity in some of the electronic components that are used, it's possible that the same designers worked on the Adlas 300 prior to developing the DPY315M and what later became the other Compass-M lasers. But none of the spiff and polish of the Compass-M construction is evident anywhere in this laser head and some of the soldering is absolutely dreadful. It looks like getting to the laser optics platform may be near impossible without extensive disassembly and unsoldering of wires or worse.
The laser powers up but is extremely unstable with wild and continuous output power fluctuations between less than 1 mW and perhaps 50 mW, though the 5 VDC power is rock stable. The actual rated output power is not known but it has been suggested by a Web search that some versions of the Adlas 300 at least are rated up to 250 mW. (The CDRH sticker lists 300 mW for 532 nm so that can't be the rated output power. Not surprisingly, these stickers are identical to those used on the Compass-M lasers!) Given the difficulty of access and the relative rarity of these lasers, I'm not sure how far I will go. If anyone has additional information on these lasers, please contact me via the Sci.Electronics.Repair FAQ Email Links Page.
According to information that used to be on the Coherent Web site, the C215M and C315M are supposed to be single frequency (single longitudinal mode) lasers and as such, the coherence length should be extremely long and ideal for holography and interferometry. The only reference still present there that confirms this is the "Comparison Chart for Continuous Wave (CW) Solid State Diode Pumped Laser Systems" at the bottom of the "CW DPSS Lasers" page, above. However, unlike the Coherent 532-200, these do not use a ring cavity but a more conventional Fabry-Perot (linear) cavity, though it would support single longitudinal mode operation if the birefringence of the KTP were used in conjunction with the Brewster plate to create a birefringent filter or if the KTP had surfaces coated (or uncoated) to act like an etalon. Both of these appear likely. (See the cavity descriptions, below, and the section: Birefringence or Etalon Effect Used for Mode Selection in C315M?.) Since the spec is no longer present, I wonder if they are indeed guaranteed to be single mode. One current specification in support of single mode operation is the optical noise - less than 0.25 percent RMS from 10 Hz to 1 GHz for the C315M and C415M; and 0.5 percent RMS for the C215M. This would most likely be orders of magnitude higher if these lasers were not single mode. And I did do some tests of one sample of a C315M laser head and indications are that it is indeed single mode under most conditions. See the section: Testing the C315M Laser Head for Single Frequency Operation, which also includes some comments suggesting that under certain conditions, another mode may be present, but at a very low level. However, note that the Coherent chart says the C415M is "broadband" meaning not single frequency, yet it still claims the low optical noise.
Most of the information below is for the C315M since these laser heads have been showing up surplus most commonly, often along with the Coherent Analog Controller (LD and TEC driver unit with analog user interface), and occasionally with the Digital Controller (which plugs into the Analog Controller and adds a computer interface). The C315M is available in power ratings from 20 to 150 mW though the most common one on the surplus market is the 100 mW (rated) version, the C315M-100. The output power is tightly regulated so it generally will not change over the laser's lifetime. The maximum user adjustable power may be set by one of the pots on the laser head itself, and during use by a simple easily constructed control panel. There is no modulation capability though and the time for the output power to stabilize after being changed may be up to a minute or more.
There is also a Coherent Compass 415M which is higher power (versions up to at least 300 mW) but bears much similarity to the C315M. However, it was never claimed to be single frequency. It uses a slightly different and somewhat larger controller (though the same user interface/control panel will work), and the laser head itself is a somewhat different shape. The head PCB which includes the "personality" settings for the laser is more complex and mounted under a cover rather than exposed as with the C315M (see below) but it's possible that the actual internal wiring of the head is the same. At least there are the same number of pins going inside though the interface cable has more pins (37 instead of 25). See the sections starting with: C415M Laser Head for more info. Most references to the C415M have now disappeared from the Coherent Web site so perhaps it is no longer being manufactured.
The other laser in the Compass-M family is the C215M, a lower power version, up to 75 mW. It appears to be much more similar to the C315M than the C415M but the controller is definitely not the same and has a lower maximum rating for power consumption. The overall system is probably somewhat less expensive as some components have been left out compared to a similarly-rated C315M. I have tested a C215M-75 laser head on a C315M controller and it seems to work fine though it is not known if the stability and efficiency will be as good as with the proper controller.
Due to the method of construction, all three of these lasers should retain alignment for their entire life. Everything internally is fastened by glue or solder with no screws anywhere. A fall onto a concrete floor may break internal parts and ruin the laser but normally shipping won't affect anything.
Note that the C315M (and I assume the C215M and C415M as well) were apparently originally developed by a company named Adlas (Advanced Design Lasers) in Germany. Adlas was bought by Coherent but only the newer models have the Coherent part number. They all appear to still be manufactured in Germany. Older C315Ms have a DPY315M model number but except possibly for minor revision differences of the head PCB, mostly artwork related, they appear functionally identical.
And, if you happen across a truckload of junked lithosetters, rumor has it that one machine that contains C315M lasers is the Agfa Galileo, which is an "older" model as these things go. Newer ones are now using violet laser diodes. :)
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".
These laser heads are now showing up on eBay and elsewhere for as little as $300 for the C315M, somewhat more for the higher power and less common C415M but some caution is advised before buying a dozen if they don't come with the Coherent Analog Controller. For the C315M, in addition to the pump diode, there are three (3) sets of TE coolers (a pair for the pump diode, one for the KTP, and another pair for the overall cavity) that need to be controlled independently for optimum performance. It may be possible to power just the pump diode and its TEC but depending on the particular unit, the output power and stability may be substantially reduced. In the unit for which some of the photos were taken, it happened that full output power was produced without even bothering to cool the diode (at least for long enough to take the pics - definitely not advised for continuous operation!). However, getting decent output power is not guaranteed without tuning the temperature of the KTP. In fact, there may be little or no green output at all for some samples!
A smaller number of power units (the Coherent Analog Controller) as well as entire systems have also been appearing on eBay. The price is typically $1,000 for a complete C315M system, possibly $2,500 or more for a complete C415 system.
If buying a surplus C315M, try to get the heatsink and output optics unit that usually goes with it. This includes a 1/2 wave waveplate that may be rotated to select an arbitrary polarization orientation of the output beam. The original complete assembly has many interesting and useful parts including high quality optics and stepper motors for computer control of beam focus, size, and fine alignment (computer not included), but these are only very rarely available. See Photo of Typical C315M Optics Platform from Platesetter for one example. The spinner motor with its 45 degree mirror can operate at 30,000 rpm or more, but the drivers for it as well as the other stepper motors, are generally not provided, or useful if they are since there is no documentation.
But be aware that the C315M uses a small YAG rod (not vanadate) with a separate HR mirror and a very small KTP crystal. The C415M uses a Nd:YVO4 (vanadate) crystal also with a separate HR mirror and very small KTP crystal. None of the parts is particularly useful for a home-built DPSS project so buying one of these lasers just to salvage parts is probably ill-advised. In addition, while the pump diode for the C315M is in a nice package with a GRIN lens on its output, it is not set up for a very small pump beam spot as would be required in a typical home-built green DPSS laser using a (relatively thin) vanadate crystal. The C415M uses external pump beam shaping optics which are mounted separately from the pump diode package itself. The optics in both cases (HR and OC) are also matched to the C315M and C415M cavity configuration. Thus, any home-built laser using these parts would have to retain the cavity design so best to just leave it intact!
The Coherent Analog Controller is a set of programmable drivers that implements an initialization/search algorithm to determine an optimal set of operating parameters based on the selected output power and laser head personality PCB. It can plug into any sample of a compatible Compass-M laser head and find near-optimal operating conditions in under 6 minutes. This might take over an hour to do by hand using lab drivers.
The warmup using the Coherent Analog Controller is similar to other DPSS lasers. It's not as bad as some but significant "fluffing and pulsing" of the output occurs as the unit initializes and goes through its search and optimization algorithm. After a few second time delay, they turn on with a ramp (0 to around 50 percent power), then the fluffing/pulsing until the output power decreases slightly, and then increases to full power and becomes very stable and BRIGHT! :) (There's a Ready status signal that is asserted once the warmup is complete.) However, note that changing power can take anywhere from a few seconds to several minutes for stability to return. It is usually shorter than initial warmup but never instantaneous. Thus, these lasers cannot be modulated in any useful way using the Coherent controller.
Noise in the output in the frequency range of 0 to 20 MHz is very low, probably below 1 percent for the units I tested.
The following includes contributions from Bob (no email), Dave (ws407c@aol.com), and Mike Harrison (mike@whitewing.co.uk).
From an elegance perspective, the C532 might be considered a superior laser since it uses a ring-type resonator with automagical adjustment to optimize the lasing mode location, and enables instant power output adjustment but not true high speed modulation. But it's also a more complex laser in terms of the optical layout, and usually more expensive, new or surplus. The Compass-M lasers use a more traditional Fabry-Perot resonator design with multiple mode selection elements to force single mode operation.
The controller board for the C532 is matched to the laser head so that switching heads (to the extent that this is really feasible) requires complete realignment. Particularly troublesome may be adjusting the mode stabilization circuits. On the plus side, the C532 controller uses mostly off the shelf parts and schematics are available. And, replacement of some parts inside the laser head (e.g., the pump diode) are possible, though not necessarily easy.
The Coherent Analog Controller for each series of Compass-M lasers (C215M, C315M, or C415M) is very reliable and any controller should work with any compatible laser head with no adjustments as head specific LD and TEC settings are read from the laser head "personality" PCB and the controller then determines optimal operating parameters during initialization. Thus, any C315M laser head (e.g., -50, -100, -150) will operate correctly with any C315M controller. Same for the C215M and C415M but except for being able to run a C215M laser head on a C315M controller, they are not interchangeable. However, no service information is available for any of the controllers and except for some simple problems, for all practical purposes, the Compass-M laser heads are not serviceable at all.
In terms of beam characteristics, the beam profile of all samples of the C315M (or C215M or C415M) is virtually identical, nicely circular and Gaussian. This probably derives from the robotic assembly line resulting in a very high degree of consistency from one unit to the next. The beam from a C532 is less consistent varying from perfectly circular to significantly elongated (usually vertically). Both lasers are linearly polarized vertically.
Both lasers should be good for holography and interferometry. However, since the C532 is a unidirection ring laser, it's virtually guaranteed to be single mode and have long coherence length. The C315M is single mode under most conditions, though not guaranteed by the resonator configuration. As a practical matter it probably doesn't matter.
Go to Holography Forum: Coherent Compass 315M Laser used for DCG Holography? for a discussion on the use and characteristics of the C315M in particular.
I found that pressing on the external PCB seems to reduce or eliminate the rattle so I'm now convinced that it is likely an insulator or spacer under the PCB and not something inside the laser compartment itself.
What's the same? Well, the same control panel or autostart board can be used on the analog controller's DB15 interface connector for all three lasers.
The information below applies directly to the C315M analog controller but the C215M and C415M analog controller user interface connector functions appear to be very similar or identical, and the same autostart adapter and/or control panel should operate all types. (However, the higher power C415M - up to 300 mW - requires a different analog controller which operates ONLY on 24 VDC and the analog controller for the C215M operates ONLY on 5 VDC. In addition, there is a 2 pin jumper on the C215M controller which enables the laser to be started automatically without anything attached to the DB15 user interface connector.)
WARNING: The C315M analog controller (at least) is apparently not as well protected against failure from external causes as might be expected from something this sophisticated (and expensive!):
The user should provide current limiting and/or a fast blow fuse to guard against unfortunate accidents. Install a fast acting fuse (IC protector) in series with the +5 VDC. The Interlock input draws about 100 mA to drive a relay directly so the lowest value fuse rating that can be used is probably 200 mA unless the Interlock and logic have their own fuses in which case 125 mA and 65 mA, respectively, should work. I recommend placing the fuse or fuses inside the connector shell of the DB15F you attach to the controller. This will then protect against shorts in the cable as well. Alternatively or in addition, current limiting resistors (e.g., 25 ohms or more) can be installed in series with any circuits using the +5 VDC (except Interlock) if they won't affect functionality of control panel switches and LEDs or other indicators. See the suggestions in the schematic of the control panel I constructed, C315M Laser Control Panel 1.
However, note that low current fast blow fuses may have several ohms of resistance. If the control panel draws more than a few mA (as it would with a bunch of LEDs!), this would affect the output of the power set pot enough to cause a noticeable reduction in maximum output power as a function of how many LEDs are lit! What I recommend is sending a separate +5 VDC feed to the top of the pot either via another fast blow fuse or just a 20 ohm fusable resistor (which would cause only a 0.2% reduction in the power setting). Or, you can get fancy and install something like a MAXIM MAX233 or 555 timer with a voltage doubler to create a boost voltage along with a resistor and 5 V zener or 78L05 to recreate a stable +5 VDC reference for the pot. :)
Adding a "power-on" LED to the power supply if none is already present is recommended so that it will be obvious when DC power is present and non-zero before changing connections.
The C315M Operator's Manual makes no specific mention of some of the above but I know of at least two instances of controller failure for unknown reasons so it makes sense to heed these warnings.
Although this unit is supposed to have a variety of safeguards to prevent damage to the laser diode from overcurrent, or the TECs or what they are attached to from overtemperature, and appears to be fairly robust overall, there is one very fundamental flaw that can result in the destruction of the attached laser head. This is a result of a single point failure in the GAL16V8D PLD which attaches to the 15 pin user interface connector (among other things). The GAL part's inputs are the user switches, status and fault signals from other parts of the controller. Its outputs enable the TEC and LD power and provide the user status signals.
The failure may arise if this GAL is damaged or forgets its programming (these are reprogrammable parts so in theory at least, such amnesia is possible). One scenario which I unfortunately had occur is that an accidental short of the +5 VDC line (pin 11 of the interface connector) to digital ground (pin 9) blew open a trace on the PCB next to the connector and in the process, somehow affected the GAL chip causing it to lose its mind. At this point, the laser diode was turned on (ignoring the associated switch) with no thermal control (despite that switch being on) and no fail-safe protection against overcurrent or overtemperature (contrary to what the operator's manual says). I don't know which part in the laser head died but within a couple of minutes, output power decreased and then there was no lasing at all - ever. With no status indications functional, there was no way to be aware of this sequence of events until it was too late. The PCB trace was easily found and fixed but the GAL remained brain dead. Post mortem testing of the signals on the GAL part showed that indeed, it comp
