Aritomi, Eleonora and Yuhang

Today we monitored BAB PDH signal for more than one hour, which is taken by collecting filter cavity BAB reflection on an RF PD. During this time, filter cavity got unlock several times. 

The orange color line is normalized PDH signal.

The cyan color line is PDH signal before normalization. 

The dark blue color line is filter cavity control loop correction signal.

• We could see basically the PDH signal doesn't drift and it means the BAB is always on resonance. And there is fluctuation, by taking the pk-pk value of this thick line(let's say 8count pk-pk) and take calibration factor of 2Hz/count. We should have RMS locking accuracy of 11Hz. And this is roughly consistent with the measurement of locking accuracy spectrum which give result of 6Hz.
• We could see that in the middle, there is a time filter cavity got unlock, and normalized PDH has an offset from zero.
• There are many times filter cavity got unlock and everytime when it got lock again, the PDH went back zero again.

But we observed at least twice a strange behavior. The PDH becomes smaller and BAB transmission becomes also smaller while GR transmission is kept. After we tune AOM frequency, the BAB transmission was recovered and the detuning was changed a bit.