THG Stability trouble on Coherent 355-1500

[icampbell2011]

We recently purchased a machining tool with an older Coherent 355-1500 laser. There are some problems:

Cold start: there is a Fault 30 SHG Battery error, that shouldn't be to hard to fix. Clear that error and SHG and THG temps rise and stabilize just fine. Turn on the diodes (90%) PRF=10-15Khz. Turn on Pulses and the THG temp will rise from 58.06 to 58.88, then start to fall. The problem is it doesn't stop, it just keeps dropping until Fault 7 shuts the laser down. To correct the trouble I turn off and on the power supply, then THG temp will rise and stabilize normally, until you start pulses again. This cycle takes about 5 minutes. The previous owners had the same issue back in 2007. They were trying to figure it out when the system was moth balled. This is an old laser, no longer supported by Coherent. Any ideas? I need to change the desiccant in the laser head, but i don't believe that has anything to do with it. I have tried varying PRF and Diode power. I can't keep the laser on long enough to optimize any temperatures. TT=2120. Diodes have about 6,000 hours. crystal has 4 used positions. I tried different positions 5 and 22 and back to 10.

Ian

[krazer]

If the crystal is damaged it can heat excessively when you hit it with power (which might explain the brief rise in temp) but the fact that is drops shortly thereafter is really weird. The heater is clearly working, because you were able to get it to warm up in the first place, my best guess is that the controller is freaking out either due to the initial temp rise or the low battery.

The battery would be my first suspect, my understanding is that it us there to allow for a controlled cool down of the crystal in the event of a power loss (to avoid cracking it), so it is possible the crystal controller is initiating an emergency shutdown of some sort and not telling the rest of the laser about it.

If you need a crystal heater I have one (and the entire stage assembly, and a few crystals) out of various coherent THG yags. There are 2 heater coils in the heater assembly, so I suppose it is possible one of them burned out...

Do you get good power before the temperature starts to drop?

[icampbell2011]

Thanks krazer.

I guess get decent power out 1.6 watts, around 100uJ at 15Khz. A damaged crystal doesn't sound good. I have the maintenance log book form the previous owners with emails to Coherent and things they have tried, it all points to something in the head. I've worked with Lambda-Pys KrF lasers, but I haven't taken apart one of these before. I'll get working on the battery, and at least get that fault gone.

Thanks,
Ian

[krazer]

I have a few pictures of the insides of a larger Avia laser; yours should have the pump diodes inside the power supply and just have fibers headed into the laser head but the SHG and THG optics should be pretty similar. http://www.krazerlasers.com/lasers/avia/

Good luck on the repair