Due to the DARM coherence with REFL RIN (76766) and measured PRCL/REFL RIN coupling (76805), I tried changing the PRCL offset to see if that would reduce this coupling.
I first began this test very early in the lock, so the coupling to REFL RIN was increasing with time due to thermalization. However, I was able to make a consistent improvement with the coupling by changing the PRCL offset. The improvement held an hour later as well. I found a minimum to the PRCL/REFL RIN coupling. This change also slightly improved PRCL coupling to DARM, and PRCL coupling to PRC2 Y and CHARD Y.
I changed the PRCL1 offset with a 30 second ramp, and started small. I was able to work up to making 10 ct step changes at a time with the long ramp. I also made sure to monitor the POP9 inputs to be sure I wasn't saturating the PD.
A positive PRCL offset made the PRCL/REFL_RIN coupling worse. To confirm I wasn't getting fooled by thermalization, I chopped this a few times. A +10 ct PRCL1 offset made the coupling worse even with the changes from thermalization.
Next, I took negative steps in the PRCL offset. I was able to see the phase flip sign between -60 and -70 ct. I narrowed this down to between -62 and -65 ct. This lines up with Gabriele's prediction from alog 76810.
See final plot of PRCL/RIN and PRCL/DARM here
Plots of PRCL/CHARD Y, PRC2 Y, SRCL and MICH here
I set the offset to -62 ct, and then chopped on and off a few times. PRCL/REFL_RIN coupling reduced by 33 dB. PRCL/PRC2 Y and PRCL/CHARD Y reduced by 7 dB. PRCL/DARM reduced by a few dB below 20 Hz. There was no significant change in the PRCL/MICH or PRCL/SRCL coupling.
I took two sets of quiet time for comparison:
1395862735 -62ct offset ON
1395863349 -62ct offset OFF
1395863966 -62ct offset ON
Gabriele and I do not see any change in DARM with this new offset. However, I think we should check how this has changed CARM at low frequency. Time pending, I'd like to rerun the frequency noise injections and see if there is any change at low frequency.
Other notes: when I first went to run quiet time, I noticed large peaks around 45 Hz in DARM. I checked the PRCL OLG to be sure everything was ok with the new offset. The peaks ended up being HAM3 scattered light from a beam dump Robert had removed. However, I noticed the PRCL UGF was low. Top of the phase bubble is 26 deg phase and around 30 Hz. I increased the PRCL2 gain by 30% to move the UGF from 22.5 Hz to 30 Hz. This is more in line with Evan's measurement, 76605. Plot here
Attached plot shows a comparison of DARM with and without offset. As Elenna said, unfortunately no change.
We also ran two BruCos with PRCL offset and without PRCL offset
Coherence with REFL_RIN is there without the offset, and it's gone with the offset.
Coherence with CHARD_Y is lower with the offset than without the offset. Same is true of coherence with PRLC: there without the offset, lower with the offset
I added this PRCL offset to the guardian. In the set up for "lownoise_length_control", the code sets the PRCL offset to zero, and a tRamp of 10 seconds, then it turns the offset on. Then, in the run state, the PRCL offset is engaged to be -62 in the same step that engages the SRCL offset.
There will be some SDF diffs due to this change.