I took beatnote spectrums in the current modulation index (was set to around 0.3 earlier). Then I took spectrum after attenuating the modulation signal power to both EOMs on North and South Path by 3 dB and 6 dB. This should reduce the modulation depth by  and 1/2. After every change, a small displacement happens in the thermal control of the cavities, so I had to wait for some time to let it settle. The gains of the FSS loops were kept constant to make sure only the modulation depth is the parameter that is changing. Gain values were 24 dB and 16 dB for South COM and FAST gains, and 11 dB and 10 dB for North COM and FAST gain.

Inference:

• There is almost no difference in noise upto1 kHz.
• There is a new bump I'm seeing between 100 and 200 Hz though which was there on Dec 12th but not on Dec 13th measurements.
• This bump might have been reduced due to a decrease in laser power or something might have changed between the two days. I'll check with increasing power what happens.
• Above 1 kHz, we see that as modulation depth is deceased, we see a reduction in noise. The same goes for power.
• Overall, this does show that the effect of these changes is more or less as expected and the effect of non-linearity, if any is not much.
• It is still valuable to quantify properly how much the non-linearity might be affecting the final beatnote spectrum though. Working on it.

Data