I took spectrum of Out-of-loop (OOL) photodiode and In-loop (IL) photodiodes with transmitted light from the cavities when ISS is on in both paths at gain value of 2x10000.

Measurement:

• In-loop photodiodes of ISS have been disconnected from Acromag cards as this was injecting noise into the loops.
• So, without witnessing the DC level, we can increase the ISS loops' gain to 2x1000 with HF and LF both set at 300 Hz with 6 dB/octave roll off.
• A major problem in this measurement is the slow drift of DC power level during the time of measurement.
• I was measuring DC value of photodiode averaged over 10 s with an oscilloscope just before the measurement was taken.
• In fact, one can see that between two measurements of the different spans, the DC value has changed enough to show mismatch in South In-Loop RIN.
• The out-of-loop photodiode was measured with sequentially blocking either the north laser or the south laser.
• The same issue with knowing the exact DC level persists here as well.

Inference:

• As expected the out-of-loop noise is higher than the in-loop noise, but I did not expect a factor of ~5 difference.
• However, this method of measuring RIN isn't very faithful.
• Overall for the experiment, it is clear that increasing the gain of ISS further did not lower the beatnote spectrum much.
• This suggests that we have damped down photothermal noise in the experiment below other fundamental noise sources.
• My future efforts would be focused solely to get rid of the peaks in the range of 200 Hz to 1 kHz.
• I see that the sharp peak at 480 Hz has changed into a tri-peak around the same frequency without any changes in the experiment other than ISS.
• This atleast indicates that this peak was not undefeatable, and so must be the remaining ones.

Data