[Koji, Jenne, Diego]
Summary: We really don't have any MICH signal in REFL 165. Why is still a mystery.
We made several transfer function measurements while PRMI was locked on REFL33 with the arms held off resonance, and compared those to the case where the ETMs are misaligned. We fine-tuned the REFL165 demod phase looking at the transfer function between 10-300 Hz (using bandpassed white noise injected in the MICH FF filter bank and looking at REFL165Q), rather than just a single line. We did that at CARM offset of 3 counts (ALS locked), and then saw that as we reduced the CARM offset, the coherence between MICH injection and REFL165Q just goes down. Any signal that is there seems to be dominated by PRCL.
So, we're not sure why having the arms eats the MICH 165 signal, but it does. Everyone should dream tonight about how this could happen.
Koji suggested that if the signal is just lost in the noise, perhaps we could increase our modulation depth for 55MHz (currently at 0.26, a pretty beefy number already). Alternatively, if instead the problem is that the MICH signal has rotated to be in line with the PRCL signal, there may be no hope (also, why would this happen?).
Anyhow, we'd like to understand why we don't have any MICH signal in REFL165 when the arm cavities are involved, but until we come up with a solution we'll stick with REFL33 and see how far that gets us.
The only really worthwhile plot that I've got saved is the difference in these transfer functions when PRMI-only locked and PRMI+arms locked. Green is PRMI-only, with the demod phase optimized by actuating on PRM and minimizing the peak in the Q signal. Blue is PRMI with the arms held off resonance using the ALS signals, with the demod phase set again, in the same way. We were expecting (at least, hoping) that the blue transfer function would have the same shape as the green, but clearly it doesn't. The dip that is around 45 Hz can be moved by rotating the demod phase, which changes how much PRCL couples into the Q phase. Weird. At ~3nm we had somewhat reasonable coherence to RELF165Q, and were able to pick -102deg as the demod phase where the dip just disappears. However, as the CARM offset is reduced, we lost coherence in the transfer functions.