Vicky and I went to SQZT7 while calibration work was happening, to follow up on some of our observations from 72525 (and Vicky's comment). 

Polarization issue:

​​​​​​​With the seed dither locked, we placed a PBS before the half wave plate in the homodyne sqz path and measured 67.2uW transmitted (vertical pol) and 750uW reflected (horizontal) (817uW total, 8% in the wrong polarization, 16.5 degrees polarization rotation).  After the half wave plate we measured 5.48uW transmission through the PBS (vertical) and 802uW reflected (horizontal) ( 807uW total, 0.7% in the wrong polarization, polarization less than 5 degrees away from horizontal).  We also placed the PBS right at the bottom of the periscope, and there measured 70uW transmitted and 820uW before the PBS was inserted (8.5% in the wrong polarization, 17 degrees polarization rotation away from horizontal).  This would not limit the squeezing measured on the homodyne since we are able to correct it with the HWP, but measuring the same polarization rotation at the bottom of the periscope suggests that the beam could be coming out of HAM7 with this polarization error, which would look like an 8% loss to the squeezing level in the IFO. 

In Sept 2022, during the vent for the OM2 swap, we measured the throughput of the seed beam from HAM7 to HAM6 65110, which agreed well with the only loss between HAM7 and HAM6 being the 65.6% reflectivity of SRM, and suggests that there was not an 8% loss in the OFI at that time. 

Loss on SQZT7 (not bad):

Comparing the total power measurements here, we have 820uW at the bottom of the periscope, and 807uW measured right before the homodyne, so we have something like 1.6% loss on SQZT7 optics (small compared to the type of loss we need to explain our squeezing level).  

Seed transmitted power over reflected power ratio has dropped:

We also measured the seed power reflected from the OPO, so that we could compare the ratio of transmitted to reflected seed measured at the time of the squeezer installation in HAM7 in Feb 2022: 61904 (3.9% trans/refl). Today we saw 0.82mW seed transmitted, and 27mW of reflected seed at the bottom of the periscopes (3.03% trans/refl).  This is 78% of the ratio measured at installation.  Because this seems like a large drop, we repeated the measurement twice more, and got 3% each time.  We also checked that the dither lock is locking at the maximum seed transmission.  

Homodyne PD QE check (QE of PDB might be low):

We used an Ophir which was calibrated in 2018 to measure the LO power onto the homodyne PDs, the filter and head are SN 889882 and the controller is SN 889428.  For PDA we saw 0.6mW, for PDB we saw 0.63mW.

Both the PDs are calibrated into mA in the front end, which includes anti-gain of gain(0.25)gain(0.22027), transimpedance of 0.001 (1kOhm), two anti-whitening filters (and cnts2V and mA factors).  For PDA there is a fudge factor in the filter gain, if we divide this out, the readback is that the PDA photocurrent was 0.512mA, and 0.5126mA for PDB (with a drift of 0.5% over the measurement time).  This gives a responsivity of 0.855A/W for PDA and 0.813A/W for PDB.  For QE of 1, the responsivity would be e lambda/(h c) = 0.8582 A/W, so our measurement is 99.6% QE of PDA, and 95% QE for PDB.  (See Vicky measured higher reflection off PDB than PDA in 63893 and Haocuns' measurement in 43452).