SudarshanS, TravisS, JoshuaF
Summary
After some investigation we did yesterday, it looks like the X-end Pcal laser is clipping on the way out (after hitting the test mass) inside the chamber.
Details
We were aware about the slow drift in Pcal laser as observed by RxPD (receiver side photodetector) at X-end sometime before ER7. We let the pcal run without fixing it because we were sure it was clipping on the way out. This was confirmed by looking at the data from ER7 which shows the pcal induced displacement on DARM corroborates with the calibrated TxPD that is on the transmitter side of the Pcal enclosure.
Yesterday we wanted to investigate if the clipping was on the aperture of the RxPD and/or steering mirror in RxPD enclosure, which could have been an easy fix. However, its not the case. One of the beam(out of two) is clipping inside the chamber on the way out. We did measure the power of each beam individually before it enters the chamber and after it exits as follows.
| Inner Beam | Outer Beam | |
| Before it enters the chamber | 0.735 W | 0.715 W |
| After it exits the chamber | 0.726 W | 0.077 W |
This still doesnot effect the operation of the Pcal because the displacement can be calibrated using TxPD. However, looking at the trend plot we know that this changed happen sometime around May 20 and we should be able to revert back into the old configuration after some more investigation of what changed during that time.
