[Enomoto, Yuhang, Eleonora]

Yesteday, just after openening the gate valve between the pipe and the end chamber we were able to lock the filter cavity again (after more than 2 months!) 

The alignment was recovered in the past days and we were able to see some dim flashes even with the closed gate valve.

The lock acqusition was smooth with the 1/f filter shape but we got an oscillation at 155 kHz when swhiching to the 1/f^4.

The gain setting was:  PIEZO GAIN = 5.   INPUT ATTENUATION = 9.5 (almost maximum)

The oscillation is likely to be caused by a piezo resonance. (See main laser piezo resonance characterization in entry #859). We couldn't get rid of the oscillation by changing the piezo gain but we noticed that for some gain values, the oscillation is at lower frequancy, about 75 kHz (probably another piezo resonance.)

In the end we found a good setting of the gains (we had to change the input amplification also): PIEZO GAIN = 4.   INPUT ATTENUATION = 1.6 (much lower than before)

In this configuration the UGF is about 16 kHz with phase margin 62 deg (see pic 1). By eye, the spectrum of the error signal seems even lower than before but we need to check again the calibration to confirm this.

Note that:

- the locking photodiode is now the qubig one without DC (as the one with DC is used for OPO) but the gain of the RF channel shoud be the same.

- the change of the PIEZO GAIN affects the piezo dynamics, as reported by Pierre in entry #747.