By pushing on the table to force the MZ output through full swings, I have determined the amplitude of the beat and therefore the calibration. The peak-to-peak amplitude is ~3400 cts, so the calibration is:

(1 / 1700 cts) * (1064 x10^-9 m) / (2 * pi) = 9.96 x 10^-11 m/ct

Here is the calibrated spectrum:

Comparing it with the spectrum I linked to in the previous post shows that the noise below 1 Hz has gone down significantly (a factor of ~8 at 100 mHz), while the high frequency noise has suspiciously gone up. For the resonances, this could be due to Q enhancement from going into vacuum, but it is also noteworthy that there was a possible calibration issue in the old spectrum, while this time I have used the full swing of the beat in counts to do the calibration without having to multiply by other conversion factors. I will always do it this way in the future so that there is no ambiguity. No matter how you look at it, though, the low-frequency noise is better.

It is also worth noting that this MZ-type noise is expected only to couple in as phase noise at the gyro output beat signal. As such, its influence will be rotated away by a factor of f at low frequencies, where we will instead be dominated by the "FSR modulation noise". Hopefully, the low frequency improvement we see here will have a counterpart in that, as well.