It seems that the ALS laser at end X is occasionally having a sudden drop in green power, starting about 6 hours ago and getting worse. I looked through 200 days of trends of the green power and don't see anything like this. This is the same laser which has its diode current increasing, see alog 36417
I have not been able to lock ALS for a while, most of the locklosses aren't at times when this power drops, but I am wondering if the laser or SHG is the cause of the problems.
The second screenshot shows some end laser powers over an hour, you can see that at some times the IR power drops at the same time as the green power, and that the TMS QPDs also see the power drops, so I think this is a real problem not just a problem with the monitor diode.
It could be that the laser is mode hopping and we just need to change the temperature to find a mode hop free region, or maybe we need to get out a spare laser.
before I left last night, I drove to End X and power cycled the laser, at 4:59 UTC on May 29th. In the attached screenshot you can identify this by the time that the LASERDIODEPOWERMONITOR goes to 0. ALthought the power out of the laser decreased by about 3% for IR ( but only 1% for green, which should depend on the IR power^2) the sudden power drops didn't happen for 10 hours after I power cycled it.
I was able to lock ALS for about 6 minutes, but am going to head to the end station now to check for mode hopping in the ALS laser.
I was able to move the laser temperature so that the beat note moved by +-500MHz, without seeing any mode hopping.
Attached are two screenshots of two different kinds of power drops, one that only appears in green channels and one that only appears in IR channels.
I also adjusted the temperature of the doubling crystal, which was at 33.51 when I started, to get about 3% more green power out. I don't think this will fix the problem.
To prepare for swapping the laser, I checked the green irises on the input beam right before the periscope, the beam is well centered on them. I also added two irises in the IR path right after the laser before the steering mirror, using a card not a viewer. I attempted to add an irises to the green path but did not have the right base plates.
I also added and aligned 4 irises right after the laser, in the IR path the two irises are in the longish beam line before any steering mirrors, for green they are in the path after two steering mirrors before the EOM. I also put 4 dog clamps marking the location of the laser base plate. We can use the dog clamps and the irises in the IR path for placing the laser, and use the irises in the green path for the two steering mirrors right after the laser, to hopefully get us to roughly the right alignment.
