Measurement

• Everything is almost the same as the last measurement.
• This time, if you notice from orange to yellow curve, I had to manually lower down the gain on the south common path by 12 dB instead of 3 dB to ensure the south path doesn't go into oscillations.
• From there onwards, the gains were reduced normally by 3 dB in each step/

Inference

• The beatnote spectrum indeed drops down to lowest when the gains are 3 dB below the maximum gains where I keep the loops.
• Maybe keeping a large gain margin is important. But I need help to understand this result.
• Also, it is kind of clear that the laser noise starts showing up from the yellow curve onwards only. So in current gain settings, the noise floor is probably due to something else.
• Another weird fact I saw was that the RMS of error signal taken at "Mixer" port which is the TP5 point in LIGO-D040105-C_CTN_North increases significantly only after the gain values of the loop is decreased by more than 15 dB (to -1 dB each on FAST and COM).
• The same thing can't be said for the south path where I see a gradual increase in this error signal RMS value as the gains are reduced.
• If someone has a better idea of understanding these results or has some suggestions on further tests or a combination of parameter changes I should do, please let me know.

Data