[Yuhang, Eleonora]

Filter cavity lock was recovered easily.

Then we checked green alignment into OPO: we sent BAB and maximized its alignement into OPO by moving the lens in pic 1. We scanned OPO and measure the BAB transmitted TEM00 with and without green pump. The green phase was modulated at 10 Hz. We optimized the alignment of the green beam into OPO by maximizing the BAB transmission. The ratio between BAB transmission with green (pic2) and without green (pic 3) gives the parametric gain which is ~ 25.4 for 50mW of pump. It seems fine. Note that recently we observed a reduced parametric gain but this measurement is in agreemen with the early value (see entry #1131).

We changed OPO temperature from 7.185 to 7.215 kOhm to maximize the non-linear gain, (with the PLL offset without green set to 180 MHz.)

We check the p-pol PLL offset when green is injected into OPO and optimized it by maximizing CC1 error signal. Now the good values for PLL offset are:

We found that p-pol was not well aligned into OPO, and the alignment was very unsatable and difficult to optimize. Eventually we discovered the p-pol laser temperature was 2 degree away from the optimal temperature. So the issue was likely due to mode-hop. We put it back at the nominal value and we confirmed that all the lasers are at the optimal temperature, namely:

Then we aligned p-pol and CC into OPO. Spectrum is shown in Pic 4 (blue is p-pol, yellow is CC).

Finally, we measured again squeezing with 50mW green pump. The measured squeezing level is 5.9dB which is consistent with the measurement done before. Besides, the spectrum is as flat as before.