We measured the REFL signals at 2W with the ISS second loop engaged and off (REF traces). The RF signals measure residual RF modulation (RAM), whereas LF measured the RIN. The REFL PD sees about a factor of 10 more light than the ISS second loop PDs when unlocked.
There seems to be a wide scatter peak at ~16.5 Hz. The 9 MHz demodulated signal shows some strange smooth excess noise below 100 Hz, whereas the 45 MHz seems to see more of the jitter peaks. The 9 MHz signal has been scaled to give the same digital and electronics gain as the 45 MHz. The transimpedance gains are very similar to start with.
The 16.5 Hz peak appears to be the same one reported about two years ago in 15223. That peak is caused by scatter of the REFL beam that is transmitted by IM4, goes out the viewport and onto IOT2R. It might be worth checking if the beam dumping on IOT2R is as good as it can be. Longer term, this beam should be dumped in vacuum, on HAM2 (this was done at LLO in the post-O1 vent).
Some notes on scaling REFL_LF for RIN measurements
see also https://doi.org/10.1364/AO.48.005423
We measured 12.9 mW with 2.0 W input, when the PRM is aligned, but the interferometer is not locked. This scales to Punlock ~ 160 mW at 25 W.
When locked and after the modulation index reduction for the 9 MHz RF, we have about Plock ~ 2.2 mW of light, which is carrier dominated.
To calibrate the REFL_LF in RIN we need to divide the signal by √ Punlock x Plock ~ 18.8 mW.
The shot noise in units of RIN is then √ 2 h ν Plock / √ Punlock x Plock = √ 2 h ν / Plock .
When we are locked at 25 W, the second loop ISS diodes are exposed to 15 mW and 16.5 mW for inner and outer, respectively.
This makes REFL_LF a better sensor for relative intensity noise measurements by about a factor of 3.
Here is a measurement of the REFL signals when locked at 25 W. The reference traces are measured with 2 W and unlocked (no second loop ISS).
The red horizontal cursor corresponds to the ISS second loop shot noise. It is well reproduced by REFL_LF between 200 Hz and 3 kHz. One striking feature is the 260 Hz periscope peak that is suppressed by the ISS near shot noise level, but shows up again much stronger in REFL_LF. This peak is most likely regenerated in the interferometer from the jitter and an rms common misalignment. The situation is a little different for the set of peaks around 4 kHz. It looks like they appear weaker in REFL_LF, as if they get regenerated but with a negative sign, so that their amplitudes subtract.