PMC mode overlap was found to be 68.77%. I realigned this to 91.99%. Here, the percentage is the percentage of power that went through the PMC.
Then I found that the alignment into Faraday isolator in the south path was also very poor with only about 75% of light going through. I aligned this also to 87.11%, but unfortunately, due to this exercise, the south cavity is misaligned now.
However, this will help in keeping the required power low at the PMC stages. So for some reason, the PDH error signals of PMC locks on both paths saturate to some value and do not increase further. This is right after the mixer so it is not the fault of the Servo Cards. As I tried a pristine different RFPD too, it is not the fault of the RFPDs either. I checked the modulation depth, and it is good as well. So only two things can happen here:
This problem seems unsolvable, so I basically have to keep power low enough at the PMC stage. My goal is to send 3 mW power on the cavities. Then the FSS RFPDs will be shot noise limited.
p.s. I know this seems too elaborate exercise at this point when I know a much larger issue, the 50 Hz noise source. But to investigate that, I need rest of the table working and since I'm sort of rebooting everything, I want to optimize it as good as I can and I want to keep the record of all problems and efficiencies as well.