Since the laser power was increased to 10 W, I wanted to see if we can lock the Michelson using REFLAIR_RF45 with PRM misaligned. And yes, the signal to noise ratio was barely large enough to keep the Michelson locked.

Another big motivation of locking the Michelson was that I wanted to calibrate the BS length actuator using the Michelson fringe. I took some data and handed to Yuta who volunteered to do some fitting and so on.

Some observations in the Michelson lock:

One difficulty in the Michelson lock was that, as reported by the LLO commissioners, a bounce mode of the BS suspension at around 17.5 Hz  (see for example aLIGO wiki) limits the control bandwidth to be lower than it. A high UGF maybe able to suppress it, but then a complication is that the control signal saturates the DAC which eventually breaks the loop. This is something we should keep in mind particularly when locking PRMI.

Also because of the low signal to noise ratio at REFLAIR_RF45, the signal above 4 Hz was limited by some electronics noise which was flat. Then this high frequency noise simply saturated the DAC until I newly installed a cutoff filter at 30 Hz (which is now in FM9 of LSC-MICH).

The attached shows in-loop noise spectra calibrated in m/Hz^1/2. The Michelson optical gain was calculated to be 6.79e9 counts/m from the peak-to-peak value of the free swinging waveform which was measured to be 1149 counts. As you can see, the bounce mode was prominent at 17.9 Hz after it got excited by my swept sine measurements.

Also I am attaching the measured OLTF. The UGF was set pretty low at 2 Hz.