Today we started to investigate the PMC loop gain change and its associated bistable behaviour.
Several checks have been made on the PMC loop and also on the slave laser locking loop (since it uses the 14MHz phase modulation as it is done for the PMC).
At the end of th morning we could see again the bistable behaviour. One thing that we remarked was that there was a second PDH signal on the slave laser error signal.
This is probably due to the proximity of a mode hop.
In order to investigate the problem, we have change the master laser temperature from 22.27C down to 19.5C we found again the 2 PDH. Finally we set the temperature to 20.9C and the second PDH disappeared.
We found also more light transmitted by the PMC as it is the case when we were on the right locking point in the past. The explaination could be taht the SL cavity was sometimes locking
on the wrong PDH and we were losing some power on the 14MHz sidebands. The consequence was that we had to change the PMC loop gain to have the PMC loop stable again.
We think that the problem should be solved now and we will keep an eye on the system in the coming days (more details will be posted in a dedicated logentry).
We also investigated the high frequency osccillation we could see in the Power stabilization loop (see attached plot).
Jean-Pierre found that there was no actuation at high frequency.
We found teh current shunt was the culprit but we were not able to fix the electronics in a short time.
Jean-Pierre took it with him and will send us back this electronics and maybe the spare one available at Artemis in the coming days.
We left the power stabilization loop enabled. It improves in any case the low frequency power noise.