Yesterday we found that we could only get about 1 dB squeezing and ~4dB antisqueezing at 25 mW injection. Inspecting the homodyne showed that some bright leakage from OPO was interfering with the measurement. Initially I thought this was CC related, and it goes away when I block CC.

While checking alignments to OPO, I decided when sending BAB to also check green alignment - when sending green to locked OPO, the transmitted BAB beam will be amplified and deamplified according to the nonlinear gain. I found that green was heavily misaligned to OPO in yaw. The nonlinear gain before adjustment was less than half of the optimal amount (actually quite less, maybe 1/3 to 1/4). I reoptimized the green alignment to OPO by maximizing the nonlinear gain of transmitted BAB. I also checked PLL frequency and temperature. The optimal temperature did not change (7.117 kOhm). The optimal ppol frequency was found to be 39*5 = 195 MHz, although it has been seen on several occasions that this causes a large amount of noise in the CC2 error signal, so we usually slightly offset to 205 MHz. The optimal temperature/ppol without green was found to be 7.117 kOhm/240 MHz, the same as written on the tape label next to the ppol monitor (2024-01-09). A rough reading of the power meter in BAB transmission gives NLG 980 uW/310 uW ~ 3.2, which is consistent with previous measurements although a bit lower.

Anyway, this isn't the first time the green path randomly received large yaw misalignment. We may need to watch out for this in the future. A suggestion from Yuhang was that the green phase shifter gets kicked a lot and has some hysterisis.

I decided to check squeezing with 195 MHz ppol. However, I completely lost CC PLL output. Both CC fiber PDs are receiving about 1.1 mW power. Both are outputting some signal, although I am not sure what is supposed to be correct in this case. On the battery operated fiber PD I see about 115 mV and on the power supplied PD I see about 12 V. Like I said, I don't know if it's correct, but it's something. Somehow I cannot see anything on CC PLL MON while adjusting CC temperature. Also, the CC laser power reaching the fiber coupler is only 500 uW, but should be 3.3 mW from the wiki. We should probably buy a spare power supply for the battery operated fiber PD (after the Thorlabs ban ends...)