After the recent recovery of FC alignment, FC GR lock is very unstable. I turned off the END picomotor driver, but it didn't change the situation.

I checked the FC GR correction signal and laser PZT signal. The FC GR correction signal is PZT mon signal amplified by 50 with SR560. For laser PZT signal, I divided the signal sent to laser into two. One is sent to laser and one is for monitor. The relation between FC GR correction signal and PZT signal is as follows.

FC GR correction signal = PZT mon*50 = (1/100*PZT)*50 = 0.5*PZT

Fig. 1 and 2 show the FC GR correction signal (CH1) and PZT signal (CH2). As you can see, CH1 with 500mV range and CH2 with 1V range are completely overlapping, which means the FC GR correction signal is exactly half of the PZT signal as expected.

Fig. 1 shows that when the FC GR correction signal reaches +/- 1.1V (PZT signal reaches +/- 2.2V), FC unlocks (the signal becomes 0). According to a manual of laser, the PZT can accept +/- 65V (PZT tuning coefficient is 1MHz/V and PZT tuning range is +/- 65MHz). So the PZT signal is well within the PZT range.

Fig. 2 shows that FC also unlocks even without saturation.

We will investigate the reason of this unlock.