I looked into the coherence of DARM with all channels, using 10 minutes of data from 2015-04-02 11.00.00 UTC, which seems a period of high and stable range from the detchar summary page. Here is the result:
https://ldas-jobs.ligo.caltech.edu/~gabriele.vajente/bruco_1112007616/
And as usual, my digest. In very brief summary, nothing really new above 10 Hz, OMC noise below 10 Hz.
- Low frequency noise (below 10 Hz) shows significant coherence with OMC suspension and alignment signals, for example SUS-OMC_M1_NOISEMON_T3, SUS-OM3_M1_LOCK_P_IN1, SUS-OM2_M1_LOCK_P_IN1 etc (see first plot)
- In the 10 to 100 Hz region there is quite some coherence with ITMY longitudinal control signals. Is this due to some subtraction scheme? Even if that's the case, a good subtraction should make DARM incoherent with anything else (see second plot). The same level of coherence is visible with MICH signal (third plot), so my guess is that this is indeed the residual coupling of MICH noise.
- In the 10 to 40 Hz region there is some coherence with BS yaw control (4th plot)
- Coherence with SRCL and PRCL is quite high too in the 20 to 80 Hz region (5th plot).
- The 56.75 Hz line is coherent with ETMY ISI signals, for example ISI-ETMY_ST2_BLND_RZ_GS13_CUR_IN1. Same at 170.5 Hz, 398 Hz, 534.75 Hz
- At 100 Hz there is some coherence with EX magnetometers
- Usual coherence with IMC and PSL periscope signals at the peaks between 180 and 400 Hz
- The band between 414 and 420 Hz is highly coherent with PRCL, same for 971-973 Hz
- At 750 Hz there is high coherence with intensity noise. Same (but slightly lower coherence) at 1980 Hz
- At high frequency intensity noise is still a safe factor below the measured DARM noise
- There is large coherence at 1009.75 Hz with LVEA magnetometer signals: PEM-CS_MAG_LVEA_OUTPUTOPTICS_QUAD_SUM
- In the band 1186-1195 there is coherence with SRCL/PRCL but most interesting with MCL and REFLSERVO signals (plot 6)
- At high frequency there is still the usual broadband low coherence with PRCL and SRCL signals, projecting about a factor ~3 below the DARM noise floor.
We use ITMY as the actuator for MICH and SRCL FF.
The FF is frequency-independent right now, which is probably why there is still MICH/DARM and SRCL/DARM coherence at some frequencies.
In fact, in this period we have been testing the intensity injection at 750Hz to see if the intensity coupling is a function of the MICH error offset or not. That's the reason why there was high coherence between the intensity and darm.