Yuheng, Michael

We tried to look at squeezing again to see if our issues went away.

We initially tried replacing the ND filter covering the IRMC reflection photodiode. Currently, we have for screw on ND filters, ND 0.3 (too weak), ND 1.0 (too strong), ND2.0. But no ND 0.5 lying around. I purchased a variable ND filter a while ago so we tried putting it between IRMC reflection and the error signal PD. I could adjust it so that the base level of the reflection spectrum is close to saturation (~ 8V) and the PDH error signal is the usual shape with 1.25 Vpk (3 times what it was before). The attentuation was 1.06/2.81 = 0.377 which is ND 0.42. The threshold was adjusted to the good point and the IRMC was locked. The gain had to be turned down from ~ 10 to about 4 to prevent oscillation. However, the homodyne spectrum did not look so good. At first I guessed this was due to the influence of the variable ND filter, so I removed it and put ND1 back (which is what we were using before). But then it turned out that the homodyne was quite misaligned. I rebalanced the homodyne and the spectrum became ok. Anyway, I should purchase some more screw on ND filters since that variable one is not AR coated for 1064.

I could see the CC error signal that indicates that nonlinear gain is present (roughly, nonlinear gain*sin(green phase mismatch)). It seems stable and not causing problems. So the squeezing output seems fine.

I wanted to demonstrate LO/sqz overlap using AMC, but then the IRMC lock became unstable again. I am not entirely sure what is causing the IRMC lock issue. Staring at it for a while, the PDH error signal sometimes becomes much smaller. It seems to go away after we leave and then turn off the high voltage driver and come back some time later. It could be that the high voltage driver is faulty. Switching for an optimal ND filter might be a temporary fix, but it seems we need a new HVD...

Another issue was that the GRMC kept unlocking. It seems the phase of the PDH error signal is randomly changing in a seemingly not continuous manner. Sometimes it is in perfect I-phase and sometimes it goes to almost Q-phase. I don't know what causes it right now but it could have something to do with the 88.3 MHz lock scheme.