The Pcal reciever-side photodiode at EX is showing unwanted spikes at various frequencies. Jeff helpfully pointed out that these changes started after we performed an end station calibration during maintenance yesterday. Looking at the spectra of the transmitter-side photodiode and reciever-side photodiode before and after maintenance (pcalx_spec.pdf), we can see that there is indeed a significant difference, and the fact that we see it in both photodiodes tells us that these changes are happening inside the transmitter module.

Looking average time series of both photodiodes before and after maintenance (pcalx_trends.png) shows that the voltage output of these sensors is much less stable after we perform the end station calibration.

Given that the voltage on the photodiodes (and hence the laser power) appears to to be unstable and larger than before, I would hazard a guess that the issue is with the Optical Follower Servo and not with the laser dying. However, I'm not sure how our work yesterday would have caused this. We will continue to investigate.