Attachment #1 - In loop error spectra, measured as Koji posted end of last week.

• Main difference is that the line noise seems much lower.
• For the "dark" measurement, I set the IN1 gain of the servo board to the value of +4 dB, which is what it is in lock.
• As Koji mentioned, this isn't an apple-to-apple comparison as the IMC loop will squish the plotted orange trace.
• Nevertheless, the fact that the blue trace is above orange everywhere gives confidence that we are in fact measuring frequency noise.
• For the higher frequency measurement, I used the AG4395 analyzer, which has 50 ohm input impedance. So to get the measurements with the SR785 to line up, I multiplied these by x2.
• For the frequency axis calibration, I used the value of 13 kHz/V for the PDH discriminant, which was what I measured it to be last year (but I didn't check again today).
• Note that the IMC locking loop OLTF has not been undone, so this isn't the actual laser frequency noise on the transmitted beam. In order to measure the latter, we'd have to use (for example) an arm cavity as an analyzer.

Attachment #2 - OLTF of the IMC loop.

• Measurement was made using the IN1/IN2 method, injection was done at the "A EXC" front panel BNC input.
• For comparison, I've overlaid a measurement from the 2017 IMC loop investigations. Doesn't seem to be significantly different.
• UGF and phase margin are in the ballpark of what they were reported to be in the past.

Attachment #3 - Photo courtesy Koji showing the bank of BNC connectors used for these measurements.

Clearly, these measurements were taken in a time when the IMC was "well behaved". How to characterize what's happening when this isn't the case?