Today I inserted two photodiodes, Thorlabs PDA10CS at (110,44) at dumped end of a PBS before PMC and at (64,28) at dumbed end of INput PBS of Faraday Isolator after PMC. These photodiodes were set at 20dB gain which according to the manufacturer gives us a bandwidth of 1.9 MHz.

Then, I just took Noise Spectrums using AG4395A with the parameter file attached in the data directory, from 0 to 1 MHz at 1 kHz ResBW and 50 averages. The output was divided by the DC level of the photodiode which was measured using TDS 3052B averaged over 10s. This measurement was done with FSS ON or OFF.

Inference

• Whenever FSS is ON, the loop is completed, so noise due to PMC and FSS is present in the measured RIN.
• But when FSS is OFF, before PMC, the noise should be just of free running laser even though PMC is locked downstream.
• The most important observation is that when FSS is OFF, before PMC, the RIN is pretty small with no peaks in 100s kHz, particularly the notorious 433 kHz peak.
• The dome from 1kHz to almost 100kHz peaks at 10kHz is most probably due to FSS as it goes away whenever FSS is OFF.
• The peak at 433 kHz persists even after switching off FSS and is present only after PMC. So this peak is clearly due to PMC and not due to laser as was thought before.
• Conclusion is, that laser is free of any large RIN and most of the RIN introduces is due to FSS or PMC. 
• I've left the PDs in there with termination. I'll change some PMC loop and FSS loop parameters tomorrow to see nature of their contribution to RIN.

North Laser is hereby acquitted.

Data