Summary:
The Pcal Y laser beam is likely clipping somewhere in the beam path. This will need to be addressed ASAP. In the future we need to keep a close eye on the Pcal summary spectra on the DetChar web pages.

Details:
Jeff K. and I noticed that the spectrum for the Y-end Pcal seemed particularly noisy. I plotted some TX and RX PD channels at different times since Oct. 11. Several days since Oct. 11, the Pcal team has been to EY to perform some Pcal maintenance. One of those times (I think Oct. 18, but we don't have an aLOG reference for this), we realigned the beams on the test mass. Potentially, this change caused some clipping.

Attached are the spectra for TX and RX. Notice that there are no dramatic changes in the TX spectra. In the RX spectra, there is structure becoming more apparent with time in the 15-30 Hz region and 90-140 Hz. Also, various other peaks are growing

Also attached is a minute trend of the TX and RX PD mean values. On Oct 18, after realignment (the step down), the RX PD starts to drift downward while the TX PD power holds steady. The decrease in RX PD is nearly 10% from the start of the realignment. 

The Pcal team should address this ASAP, hopefully during tomorrow's maintenance time.