Sheila, Nutsinee
Following up alog47184 we investigated on how the other two peaks (one at 177.7Hz and the other at 262Hz) behave with different CLF power and UGF. The peaks only responded to decreasing/increasing loop gain but not to increasing power. We didn't do other tests (eg. noise eater on/off) since the IFO lost lock. We also tried decreasing LO loop gain but that didn't do anything. The mystery continues.
Note that the high CLF gain with 0.02mW going in to the coupler is what we are normally operating.
Same CLF gain label of the third plot means same UGF as the first plot.
I think we left the CLF power at 0.08mW into the coupler. For optimized IFO squeezing tonight (if needed) I recommend putting it back to 0.02 mW.
Interesting. While I was debugging at LLO, I noticed that turning down the CLF gain by a lot showed noise peaks in DARM. My belief is that any phase noise is visible due to backscatter light being modulated by the parametric amplification. I believe this is also what causes the 2f noise seen when driving ZM's 43807, and the 1f peaks are from direct modulation of backscatter. We haven't tried driving the CLF/LO loops and looking in DARM but that might be a good test since you can calibrate that drive level to phase RMS. If you have done tests for the backscatter level, then you might find a correspondence as we did roughly did in 43881.