Using the PDA520 detector on the AS port I tried to get some better estimates for the round-trip loss in both arms. While setting up the measurement I noticed some strange output on the scope I'm using to measure the amount of reflected light.
The interferometer was aligned using the dither scripts for both arms. Then, ITMY was majorly misaligned in pitch AND yaw such that the PD reading did not change anymore. Thus, only light reflected from the XARM was incident of the AS PD. The scope was showing strange oscillations (Channel 2 is the AS PD signal):
For the measurement we compare the DC level of the reflection with the ETM aligned (and the arm locked) vs a misaligned ETM (only ITM reflection). This ringing could be observed in both states, and was qualitatively reproducible with the other arm. It did not show up in the MC or ARM transmission. I found that changing the pitch of the 'active' ITM (=of the arm under investigation) either way by just a couple of ticks made it go away and settle roughly at the lower bound of the oscillation:
In this configuration the PD output follows the mode cleaner transmission (Channel 3 in the screen caps) quite well, but we can't take the differential measurement like this, because it is impossible to align and lock the arm but them misalign the ITM. Moving the respective other ITM for potential secondary beams did not seem to have an obvious effect, although I do suspect a ghost/secondary beam to be the culprit for this. I moved the PDA520 on the optical table but didn't see a change in the ringing amplitude. I do need to check the PD reflection though.
Obviously it will be hard to determine the arm loss this way, but for now I used the averaging function of the scope to get rid of the ringing. What this gave me was:
(16 +/- 9) ppm losses in the x-arm and (-18+/-8) ppm losses in the y-arm
The negative loss obviously makes little sense, and even the x-arm number seems a little too low to be true. I strongly suspect the ringing is responsible and wanted to investigate this further today, but a problem with c1psl came up that shut down all work on this until it is fixed:
I found the PMC unlocked this morning and c1psl (amongst other slow machines) was unresponsive, so I power-cycled them. All except c1psl came back to normal operation. The PMC transmission, as recorded by c1psl, shows that it has been down for several days:
Repeated attempts to reset and/or power-cycle it by Gautam and myself could not bring it back. The fail indicator LED of a single daughter card (the DOUT XVME-212) turns off after reboot, all others stay lit. The sysfail LED on the crate is also on, but according to elog 10015 this is 'normal'. I'm following up that post's elog tree to monitor the startup of c1psl through its system console via a serial connection to find out what is wrong.