The goal of these measurements is to check if one can find some correlation between the CFP lock losses observed during a run and some CEM noise in the bunker.
the main issue is coming from a 20Hz noise apearing/disapearing in the laser cavity PZT signal.
is it a real cavity length noise that must be compensated or is it a pickup noise in the error signal which produce some unwanted compensation ?
the second issue (apearing much less often) is a higher frequency noise visible mainly in the transmission signal of the CFP which becomes wider and noisy during 1-2 seconds.

This morning, I installed a simple wire loop all around the table (the machine is OFF).
this wire is connected to the wire of a BNC connector at one end and to the ground of the same BNC connector at the other end to form a loop.
this BNC connector is connected to a 1kHz low pass filter to remove high frequency CEM noises and connected to a 10-1000 variable gain amplifier, plugged to the CH3 of the CFP scope (instead of the PDH/PZT-CAV signal).

I locked the CFP above 80kW after tuning the CEP.

I took several pictures to illustrate what I observed :
- on the right, the scope signal in time domain (blue = TRANS / green = PZT laser / cyan = REFLECT / pink = LOOP noise)
- on the left, the spectral analysis in frequency domain (red = PZT laser / white = LOOP noise)

in these 3 measurements, I don't do anything to the CFP, I just wait for the signals.

1) normal condition : see picture "Without Noise @ 20Hz"
the PZT signal in time domain is relatively flat and the 20Hz noise is barely visible in frequency domain.
we don't see any correlation with the LOOP noise for which one can see clear 50Hz and 100Hz peaks.

2) 20Hz noise condition : see picture "With Noise @ 20Hz"
the PZT signal in time and frequency domain exhibit a clear 20Hz oscillation.
the REFLECT and TRANS signals are also correlated to this 20Hz oscillation.
but still no correlation with the LOOP noise in time or frequency domain.

3) 20Hz noise lock loss condition : see picture "Delock @ 20Hz"
because of the lock loss the spectral analysis of the PZT signal is meaningless,
but in time domain, one can clearly see the growth of the 20Hz signal, inducing large noise on TRANS and REFLECT signal but still no correlation with the LOOP noise.

Conclusion : the 20Hz noise seems not related to any CEM noisy signal but more probably to a real mechanical noise in the CFP (not in the laser cavity because it would be seen in the phase noise measurements).

in this measurement, I tried to produce a tone around 400Hz with my voice, close to the FP cavity.
it is difficult to right volume as the cavity can easily lose the lock.

4) Voice noise condition : see picture "Voice noise @ 400Hz"
here, I changed the spectral span to 500Hz.
one can see a small bump in frequency domain around 400Hz due to the accoustic noise in the PZT signal.
the general shape of the PZT spectral signal is maybe related to the PID parameters : one can see more signal at lower frequencies.
in time domain, one can see TRANS and REFLECT signals are more noisy than before and these signals shapes seems identical to what has been observed when one has lock loss due to high frequency noise.
maybe we could put a mic in the bunker, connected to one scope to check if sometime one doesn't have much more accoustic noise...