First, we suspected that the AOM changes laser power when the ISS is not locked, but Ch6 shows that the AC signal is not going to the AOM when the ISS servo is turned off.
There are several possibilities for this instability,
1. the PMC servo works something wrong.
2. the feedback signals of IMC control signal change the laser power
3. the voltage of the AOM from the oscillator is malfunctioning.
However,
1. if the power fluctuation comes from PMC, there must be something very strange things on the error signal of PMC. PMC itself is a quite simple servo, which is not very likely.
2. feedback to EOM and PZT basically does not change power. Thermal may change power, but we do not see such a large fluctuation at the output of the main FB laser(Ch5).
3. it remains a possibility of malfunction on AOM driver or something, but it does not seem to be very likely.
After some consideration, we noticed that it may be a mode hop of the main laser. If you see Ch5 carefully, you can see a little fluctuation at the same position as Ch3 has a big jump. If it is a mode hop, everything looks quite natural. PMC only locks to a certain frequency, the power is reduced by the amount of power that has gone to a different mode.
If it is a mode hop, it is understandable that ISS can't work. We should change the temperature of the laser first.
For your information: https://klog.icrr.u-tokyo.ac.jp/osl/?r=16697
The behavior of change of the PMC trans DC power was quite different when ISS locked loss(Fig.1., Fig.2.) and naturally PMC power became lower(Fig.3.).
For Fig.3., as already described by Akutsu-san, we sometimes detected the sudden worse alignment of PMC, we also need the investigation
For Fig.1. and Fig.2., the fluctuation of PMC trans DC power was much faster than Fig.3. it will be occurred in other reason, I think.
