The LCM-DTL-347QT is made by the Russian company "Laser Export", and is rated to put out 20uJ of 355nm (UV) laser light at up to 2KHz (ie, 40mW average power), and is rated to deliver the 20uj in 5ns (ie, 4KW average power).
The construction quality of these lasers is amazing, you can really see the Russian engineering in these things. The overall idea of operation is fairly complex, and can be broken down into 4 main components, a power supply which powers the system, an 808nm diode laser, which is used to power a q-switched 1064nm solid-state Nd:YAG laser, which is then tripled in frequency using a set of nonlinear optical crystals.
The power supply for this laser is quite complex, it has 2 TEC controllers, 2 laser diode drivers, a Q-switch driver, and an ATMega microprocessor that ties everything together. The TEC controllers are pretty straightforward buck converters based around a TL494 chip, the laser diode drivers are simple linear constant current sources, and the Q-switch driver is based on a M67743H hybrid RF amplifier (7W 75-85MHz) in its own CNC'ed aluminum box (to prevent RFI I would guess). The manual notes that there is a RS232 serial port available to communicate with the system in the DB9 connector, however in the power supply I examined the pins related to the serial port in the manual were not connected to anything internally. Other than that the supply seems rock solid, however it does need some heatsinking or forced air cooling because the q-switch driver and current source for the diode put out a fair bit of heat.
One final note, the manual for these lasers quite sternly says that you cannot mix and match supplies, which judging by the variation I observed between power supplies (some have a fair amount of rework done internally), I would be very cautious mixing and matching supplies. However, in a pinch I have been able to power mismatched head/power supply combinations, and it appears the temperature settings are set in the laser head (note the 3 pots near the power connector), and the laser produced the full expected power.
This is the module furthest from the output end of the laser (to the left of the image at the top of the page), and is a fairly powerful laser on its own. I have not measured the power directly myself, but I have heard powers at high as 10w of 808nm laser radiation being quotes for these lasers. There are actually 2 laser diodes in this module, which are mounted such that they have orthogonal polarizations, and a polarizing beam splitting (or combiner in this case) cube is used to combine the 2 laser diodes into a single beam. The whole assembly is mounted on a TEC to keep the temperatures, and thus wavelengths, of the lasers stable. The output of this module is then sent to the 1064nm laser module.
This is module in the center of the laser, which turns the multimode and continuous wave (CW) output from the 808nm pump into a singlemode (presumably judging by the beam quality) pulsed output. This works by having a Nd:YAG crystal (roughly 3x3x5mm, hidden to the left of the q-switch in the bulge in the side of the metal box in the above picture) between 2 mirrors, which forms a laser resonator. Additionally, there is an acousto-optic modulator (AOM) inside the cavity, which is the crystal in the center of the above picture. The AOM can be used to ruin the 'Q' of the laser cavity, which prevents lasing action, and allow the crystal to store up the optical energy from the 808nm pump source. The AOM is then turned off, which allows the laser cavity to resonate, and all of the energy stored in the Nd:YAG crystal is then released as a short burst of 1064nm light. This short burst of light has an extremely high power (approximately 50KW), which can be used for the frequency tripling process in the laser.
This is the last major component in the laser, and is the box furthest to the right in the picture at the top of this page. It consists of 2 nonlinear optical crystals (probably made of KTP and KDP) which serve to combine 2 photons of light, and emit a single photon at twice the energy (and thus frequency). The crystal to the right takes 2x 1064nm photons and combines them into a single 532nm photon, and the longer crystal on the left takes a 1064nm photon and a 532nm photon to make a 355nm photon--which is what the laser is designed to output. This module also has a power sense beam pickoff and photodiode to monitor the lasers output, and there is a small prism (the black module next to the frequency tripler module on the image at the top of the page) which separates the 355nm UV light from the residual 532/1064nm output from the 1064nm pump source.
The easiest way to get this laser to run is to make an interlock defeater (which connects pins 6, 3, and 8 of the DB9 on the power supply), feed it with 24v (it will draw up to 3A so you need a pretty decent supply), and while you wait for the 5-10 minute warm-up period grab a ne555 and build an oscillator that will go from about .1-10khz The key with getting these lasers to run is that the rise time of the trigger needs to be under 100ns, so cheep function generators or other slow sources won't work. If the source you have is to slow, run the output through a logic gate of some kind (preferably one with hysteresis) which will clean up the signal and keep the laser happy. When the status light on the power supply switches over to double yellow flashes (the difference between yellow/green is subtle, but if you watch for it quite noticeable). Then you can hook a 1khz trigger into the trigger in, and in a few seconds the light will go to solid orange, then solid green, and ultimately solid yellow at which point you should have a nice UV dot on the (preferably florescent) target you set up. After you get it lasing turn up the frequency, I found the power will increase up to a sweet spot at about 5KHz, and then slowly die off after that when the pumps can't keep up with the pulse rate. I have found on average that you can get about 70mW of 355nm power at 5KHz! The complete manual to this laser was available from Laser Export, but it was taken down, and can now be found on Sam's laser faq, here, and more general information about the laser can be found here.
Note however, you absolutely must have proper eye protection with these lasers, due to the extremely high peak powers (4kW!). One hit to the eye could be last thing that eye sees, or at least increase your chances of cataracts. Furthermore, because the radiation is UVA, it is fairly unwise to shine it on skin (imagine a 4m^2 square magnifying lens focused at a 1mm spot size on a sunny day at the equator, and that is about how intense the light coming from this laser is), although since 355nm is not really ionising it shouldn't give you cancer any more effectively than normal sunlight. Finally, the beam is quite dim, if you can see it at all, (even on a fluorescing target it it does not seem as bright as it really is), which may give you a false sense of security.
One of the most amazing aspects of these lasers is how easily they can be modified to produce different wavelengths. I have summarized a few possible actions below:
It is possible to easily, and reversibly, get the 532nm green output from the laser by unscrewing the knife edge after the harmonic separating prism (See the section on the frequency tripler). Simply remove the 2 small screws holding the piece of stainless steal right before the output aperture, and slide it out of the way. This does not affect the 355nm output at all, but lets a roughly 100mW 532nm green form, which exits the laser at about a 5 degree angle relative to the 355nm UV beam.
Additionally, you can remove the front panel from the laser by removing the 2 screws holding it to the front of the laser, which will allow the 1064nm residual beam to exit, which is approximately 120mW.
It is also possible to remove the harmonic separator prism completely, but it would be a little bit tricky to reinstall it and align it properly. This changes the 3 separate beams of about 100mW each into a single 300mW beam, which has 355nm, 532nm, and 1064nm all in one beam. As a side note, if you plan to use this laser for etching/burning, this would be an ideal mode because between the 3 wavelengths almost any material will absorb some light, and after the material begins to char all 3 of the wavelengths will be absorbed and you will have a solid 1/4watt of q-switched power available. Just make sure that you use an acromat for focusing, or you will only have one beam focused at a time.
One last experiment you might try is disconnecting the q-switch, and replacing it with a 50ohm dummy load (make sure it is sized to handle a few watts of power at 100MHz) to keep the driver happy, which will cause the laser to run continuously instead of pulsed. I did not measure the power operating in this mode, but it is much lower, and very little 355nm is produced, however there is a considerable amount of 1064nm.
A final modification that could be made in the search for higher powers is to remove the 1064nm laser module, and using the output of the 808nm pump module directly. This would be a fairly complicated process, you would need to remove the 1064nm laser module and empty frequency tripler module, and convince the power supply to run with the q-switch and temperature monitors for the removed modules--which I have a feeling would involve cutting some traces. Alternatively, you could run the diodes off of pair of good current mode supplies, which would also let you run them at higher powers that the supply is set to. Furthermore, the output from the 808nm pump diodes will be a very poor quality laser beam (because the 808nm diodes are multimode), and would not have any of the desirable characteristics that the 1064nm laser head produces (pulsed output, better beam quality). Unless your laser is very sick, I would recommend buying a different laser if you want raw 808nm output.
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