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Coherent Compass 315M-100 laser module

   Compass family lasers from Coherent are considered to be one of the best solid state lasers ever manufactured. They are known for very good beam quality and stability. Green Compass lasers were available with power range from 10mW to 150mW. They have been commonly used in industrial machines, like for example CTP, thus those lasers are often sold used at eBay. Due to parameters stability of the output beam and relatively low price of used units, Compass lasers are often used by amateur holographers.

   The laser presented in this article is Compass 315M-100, a nice and shiny-gold module emitting 100mW of 532nm light. I bought the laser head together with a controller and connection cable at an internet auction as a part disassembled from old AGFA CTP machine. The price was suprisingly low, however, the condition of the laser was unknown and there was absolutely no information besides a note saying 'this is a laser' :P I decided to take the risk and buy this stuff, and it was totally worth it! Actually, it would have been worth even if the laser was broken, but it turned out that it works. Below you can see four pictures showing the laser head and the controller.

   Here's a datasheet with detailed specification of the presented laser:

  • Datasheet

       Also, tons of information related to Compass 315M lasers can be found at Sam's Laser FAQ website:

  • Compass 315M at Sam's Laser FAQ website

       The laser head itself is sealed - as you can see in the pictures, top cover is soldered to the casing - there was no easy/safe way to open it. It would be very easy to overheat the head during desoldering process and thus destroy it. Besides that, I didn't really know what exactly to expect inside, so trying to open the lid would most likely mess everything up. Fortunately, I'm not the only one who's interested in checking what's inside this laser head and pretty nice pictures, like the following one, can be found on the Internet - Sam's Laser FAQ of course :) More very interesting photos can be found HERE (scroll down to the bottom of that page).

    Picture from Sam's Laser FAQ

       As it's snown in this picture, there's a lot of goodies inside. Even though the general principle of operation is the same like in case of cheap grean laser pointers, the laser assembly is much more complex. The main reason is, of course, that the compass laser is a good quality laser, and the Chinese laser pointers are cheap lasers. Another reason is that the 315M lasers are quite old - my laser has been manufactured in 1999. These days they use slightly different construction and different materials, however, the assembly complexity is pretty much the same.

       Powering this kind of laser head is not an easy job. Besides diode current stabilization and output beam power monitoring, there are several TECs mounted inside the laser head, not only to improve cooling but to allow the controller to stabilize the optics temperature at certain, optimum level. Different optical elements, like pump diode, Nd:YAG crystal and nonlinear crystal use separate TECs and temperature control circuits.

       Laser controller is not as much sensitive to contamination as the laser head, so its casing is not permanently sealed. I was tempted to see what's inside of course, so here it is:

       There's a bunch of power transistors, chokes, and some other analog and digital electronics stuffed inside quite tihgtly. The controller needs to be powered by 12-28V at maximum power of 150W. Besides the power supply, also a control panel connected to controller's DB15 connector is required. The control panel can be actually a simple analog circuit, so it's not a big deal. That kind of simple controller can be built according to the following schematic:

  • Control panel schematic (from Sam's Laser FAQ)

       Actually, all that is needed to just run the laser are one 10k pot and three switches. All the rest on the schematic is for controling LED indicators. Those indicators are useful, but of course not absolutely necessary.

       In the first of the following pictures you can see even simpler control panel. I built it basing on the schematic from the link above, but I modified it a bit. There's only one switch, which is 'set power', and one green LED that lits up when laser is ready (i.e. after power-on ramp-up). This control panel has auto-start feature made of a transistor and RC circuit. When the power is tuenrd on, that autostart circuit shorts pin 2 od DB9 connector ('Laser diode on') to pin 11 (VCC) after about one second delay. Thermal control (pin 3) always has to be turned on before turning on the diode In my control panel pin 3 is permanently shorted to VCC.

       Luckily, the laser run from the fisrt kick! :D The three pictures next to the control panel picture show the whole laser setup in operation, and a closeup of the laser head.

       Four following pictures show some other 'beam shots' and comparison with cheap Chinese 100mW laser module. The beam emitted by Chinese laser looks more powerful because it's thicker. The difference between beams is actually huge, but they differ in parameters that can't be noticed with a naked eye, like for example M2, longitudal modes etc. In the fourth picture you can see that this laser can easily burn some stuff, even with collimated beam :]

       Warm-up and stabilization of this laser takes about 3 minutes. During this time, output power slowly goes up and stabilizes for several seconds at about 30% and 70% of set power level. Then it finally reaches the set power and since that moment the beam is stable. Power level is very stable, and frequency (single mode) is stabilized as well.

       Power measurements presented in the following four pictures prove that the laser actually keeps 100mW pretty nice. The power meters used to check this are not the best - the Melles-Griot one is a powerful equipment but has not been calibrated for a looong time, and the Spectra-Physics meter is actually dedicated for measuring argon lasers and has not been calibrated either. Thus, the measurements are not very trustworthy, however, the reasults seem to make perfect sense. You can also see IR check by placing filters on the way of the laser beam. First filter is just a piece of red glass, it stops almost all of the 532nm light and transmits most of IR - 808nm, as well as 1064nm. As you can see in the picture, the power meter is not detecting any power on the sensor, which means that there's no infrared. In the next picture the filter comes from a digital camera. It is supposed to stop IR and transmit visible light. Power drop caused by inserting the filter into the beam is low - about 5mW - and can actually be caused bt reflecting some of the green light off by the surface of the filter. Apparently, infrared is very well eliminated bu the optics and filters inside the laser head itself.

       An interesting feature of the Compass laser head, however, common for all serious lasers, is a built-in running time meter. It's a little circuit with microcontroller that simply counts the laser's running time. In the Compass head there's no display or any eunning time indicator, so it's not easy to read the counter. There's only an infrared LED that shoots out some unknown digital signal. After some research I found out that there are some clues on it in the Sam's Laser FAQ, but nobody has fully deciphered the signal. So I had to do that :) I didn't have any digital oscilloscope to record the signal, so I took a quick-and-dirty way to just hook up a photodiode directly to the soundcard's line input and record the signal with an audio editing software :D This approach actually worked out very well. In the first of the following pictures you can see a closeup opf the IR LED. You can also notice a PIC microcontroller wigh 32kHz crystal resonator glued to it and a 24C02 memory. All this is specifically for the running time counting purpose. The second picture shows a photodiode placed just next to the LED.


       The recorded signal is presented in the next picture. The signal is clear enough to recognize logical ones and zeros, so now the 'only' thing is to figure out what those ones and zeros mean :P

       After running the laser for a while and recording new signals I took a piece of paper and a pen and started to try all different combinations and translations, checking if the results make any sense. That took me a while, fut finally I figured it out! There are 9 frames of bits, where first and lase are start and stop frames. 7 frames in the middle are six digits and a point, in XXXXX.X format. Each digit frame consists of 7 bits, where bit first and second are always '1' and the last one is always '0'. Besides that, seems like infrared on is logical '0' and off is logical '1', and the data is coded little endian. Bit 3, 4, 5 and 6 are digit representation in BCD code! Then the running time of my laser is 03972.3 hours! Eureka!
       Some of these facts has been figured out by other people, but noone before figured out completely how to convert that data to time. I sent a more detailed description of what I figured to Sam and he published it on his Laser FAQ, so that information can be found HERE.

       There are not many things a laser hobbyist can do with a laser like this. Since it's very sophisticated and fancy equipment with extremely good parameters, it would be a crime to use it for laser show projector! Some people have done it, though :P The only good use I've seen is holography. For making holograms a very narrow spectral line (single longitudinal mode) and high frequency stability is needed. High power is also appreciated, because it shortens the film exposure time, which makes the whole process less sensitive to vibrations. 532nm wavelength is also very good for exposure of home-made dichromated gelatin holography plates, which are very popular among amateur holographers. All that, and reasonable price for and availability of used Compass laser makes them perfect for amateur holography! I actually sold the laser presented here to a holographer from Netherlands - I hope you like the laser, Thieu! :)

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