Locking refinement

After last time locking, we did a lot of things. This caused the unlocking of Filter Cavity.

I didn't know the drift problem before. I took the picture at laser injection window of PR chamber. You can see from Fig.1 the drift after around 12 hours. I remember that it will not happen when the local control is off.

After fixing them, we still cannot fix the problem. We discussed this during the meeting and Eleonora suggested to decrease the gain. So I decreased it from 100 to 20. This is really effective. We can easily lock the cavity then.

But the problem is the locking of TEM00 is not very good. For transmitted signal, we can see from the oscilloscope that the peak is around 1.2V. But after locking, the transmitted signal can be only 0.4V in the best case. Besides, we can lock with higher-order modes and they have almost the same level with TEM00. This should come from the bad alignment. But I don't know how to make alignment better. It's really difficult to find a good way to improve this. We can see almost no change on screen the modes, on oscilloscope by changing the local control offset of mirrors.

We also measure the noise spectrum of transmitted signal and error signal with different gains. Please refer to Fig.2 and Fig.3