[Tanaka, Ushiba]

Abstract:

We measured OLTF of MICH and PRCL when PRMI_3F_LOCKED state but overall gain was reduced.
After several investigation, we realized laser power on REFL PDA1 was smaller than before even though HWP angle was same.
Since it seemed happened when REFL table realignment was done, we need to check what happened on REFL table (accidentially rotate HWP by hands, clipping, PD break, etc).

Detail:

Since PRFPMI handover was not stable now (it can be passed but probablity of the success of handover was lower than before), we investigated what happened.
So, we measured OLTF of MICH and PRCL (not attached here but hopefully will be attached tomorrow), and found that overall gain was reduced a lot from the reference (July, 2024).
First, we suspected the phasing but it doesn't seem to change overall gain drastically.

After several investigation, we found laser power on REFL PDA1 was reduced by a factor of 4-5 even though HWP angle was same as before.
This was happened on December 12, 2024 (fig1), which was the day we performed REFL beam realignment (klog31986).
According to the ime stamp of the photos taken during the REFL work, the timing of the power reduction is working around HWP, so it would be caused by accidential rotation of HWP.
Another possibility is clipping somewhere though the beam path doesn't seem to change so much because REFL camera image is almost same as before.
Also, it would be better to check the PD itself because even we increased the laser power close to the previous values, OLTF could not be measured well.