After injecting some squeezing in the IFO and saw nothing we thought it's a good idea to go back to the HD and measure squeezing with PSL LO. Sadly due to another 30% pump transmission drop (alog46290) the max non linear gain we could get is 2. The phase noise measured from LO error signal (in loop) suggested 3 mrad. Fringe visibility was 98.5% (I used 98% for the model below).
The measured loss in the path can be found in Haocun's github here. The calculated total efficiency is 0.8147. This measurement suggested we have extra loss up to ~20% that was unaccounted for.
The first measurement Haocun took after the vent can be found here (we locked LO with Mephisto LO at the time). We still had intensity noise issue at the time so the data points were all over the place because the nlg was unstable. I'm not sure if the extra loss has always been there or degraded over time after the vent.
We could not have measured ~4.5 dB of squeezing in 45586 with losses as high as in this recent measurement. We can be confident that we understood our losses in the homodyne path in November, regardless of the fluctuating green power and nonlinear gain.
This afternoon Nutsinee went to the table to look at the seed power transmitted, we can compare this to earlier measurements of the seed power to check for changes in the losses inside the chamber that is independent of the homodyne path losses.