Darkhan, Yutaro, Kiwamu,
Conducting some more power measurements in order to understand the optical loss in PRMI (9300), we started realizing that the polarization in the central interferometer may not be healthy.
The interferometer beam is supposed to be Spol, but the reflected beam from the ITMs have Ppolarization component at 510% level.
Our wild guess is that this Ppol has to do with the optical loss that we saw in the PRMI.
[Power measurement at POP and REFL ports]
Suspecting that the PR2 transmissivity is not correct, we checked the optical powers at the POP. The results suggest that the PR2 transmissivity is OK.
 POP, Forward propagation beam = 628 mV using PDA100A with 20 dB setting

POP, Backward propagation bam = 307 mV using PDA100A with 20 dB setting
 Note that the reflection from ITMX and ITMY are spatially separated so that the intensity should be half of that in the forward propagation beam due to the BS.
 Therefore the total loss as the beam bounces off of the ITMs is estimated to be 1  307*2/628 = 2.2% (including ITM transmission)
 REFL, PRM direct reflection = 1455 +/2 cnts using REFLPDA1
 REFL, ITMX single bounce = 3.82 +/ 0.02 cnts using REFLPDA1
 REFL, ITMY single bounce = 3.72 +/ 0.01 using REFLPDA1
 Therefore, loss in ITMX = 12% (including ITM trans)
 Therefore, loss in ITMY = 15% (including ITM trans)
From these measurements, we conclude that REFL sees much higher optical loss than POP. This discrepancy can be explained if the polarization is messed up as the beam bounces off of the ITMs.
[Polarization measurements]
Then we moved onto the polarization measurements

ITMX single bounce, POP backward propagation
 S: 135 mV using PDA100A with 20 dB setting
 P: 14 mV using PDA100A with 20 db setting
 This corresponds to loss of 9.4% for Spol

ITMY single bounce, POP backward propagation
 S: 124 mV using PDA100A with 20 dB setting
 P: 6 mV using PDA100A with 20 dB setting
 This corresponds to loss of 4.6% for Spol.
The below are the results from our measurements for the POS beam. However, the beam size was as large as or perhaps larger than the aperture of PDA100A. So the values may not be accurate.
 POS, S: 2.09 V using PDA100A with 40 dB setting
 POS, P: 0.25 V using PDA100A with 40 dB setting