Keita and I have been thinking about the 100 Hz noise in DARM that appeared when we had low CARM gain. Tonight I reproduced the 100 Hz DARM noise in NOMINAL_LOW_NOISE. I did this by lowering LSC-REFL_SERVO_IN1GAIN by -12 dB. The second plot is DARM before and after I reduced the CARM gain by -12 dB. We can see the return of the ~ kHz noise, as well as an increase in 100 Hz and below. The first is DARM and CARM coherence before and after. This is particularly interesting: DARM vs CARM coherence is completely unchanged, even at higher frequencies. The pdf is the CARM OLG I was taking when I lost lock. CARM UGF was ~ 4.5 kHz. Unclear what could be causing this nonlinear coupling between CARM and DARM. But it is good that we can turn a knob and control this noise. It is extremely suspicious that the 100 Hz and lower noise is not coherent with anything. Further investigations when we lock again. EDIT2: I was able to relock and get another spectrum with -6 dB of CARM gain (plot 2). Unfortunately I lost lock when I reduced the IMC gain by 6 dB to try and get a DARM spectrum there. The IFO doesn't like when you take away its laser frequency control...
This plot shows IMC-F, MICH, PRCL, and SRCL coherence with DARM at a time of high CARM gain (green), and low CARM gain (red). It seems that PRCL and SRCL become a lot more coherent with DARM at high frequencies when we have low CARM gain. But there are no broad features at or below 100 Hz. EDIT: Added the -6 dB LSC signals. Green is normal CARM gain, blue is -6 dB, purple is -12 dB CARM gain.
"DARM vs CARM coherence is completely unchanged, even at higher frequencies"
IMC board output is not CARM (Common ARM) in that it's not what the IFO sees. It's totally dominated by the frequency noise of PSL. No change in the coherence (which is mostly f>6.5kHz) is because the residual frequency noise coming from PSL is much considerably larger than the shot noise at that frequency.
If you look at REFL CM board output you'll see that the coherence changes with CM gain.
I and Daniel went to the floor on Friday while CM was at the nominal gain and the power was 20-something W to observe REFL_A_RF9 RF monitor.
On the scope it was about 10mV pp, the monitor has 23dB attenuation so the PD output should have been about 140mVpp. The largest component was 9MHz, closely followed by 72MHz, everything else was small.
RF power monitor in the demod board sometimes shows -5dBm or so but mostly -10dBm-ish.
From INMON channels it's clear most of the remaining RF was in Q phase (note the analog whitening gain of 12dB).
Nothing stood out.
