[Alastair, Koji, Zach]
As we discussed doing, we removed the PBS that was used to split the power between beams and put a 50/50 power splitter in its place:
The thinking was that doing this would minimize the AM coupling from polarization rotation in the EOM. After realigning the entire experiment, we observed no improvement in the low-frequency noise spectrum (compare with this post):
It does seem like there is a reduction from 10 mHz to 100 mHz, but it is tough to tell whether or not this is meaningful given the FFT bandwidth. There remains the possibility of true AM from the EOM itself, though it seems unlikely that it would be stronger than that from the rotation. I suppose we should measure it either way.
Another not-extremely-likely case is that the phase noise from the two oscillators is the same below 1 Hz. This means that their absolute noise levels (V/rHz) would have to be different by a factor of two to accommodate the fact that the AOM oscillator noise couples in twice as strongly from the double-pass. Since the phase noise tends to go up with carrier frequency, and since the AOM carrier is half that of the PLL, this isn't out of the question. We are planning to beat our two Marconis together to see if the low-frequency noise is enough to explain the gyro noise. Frank and Tara's data suggest that it stays fairly flat at lower frequencies, but they don't have a measurement at the low frequencies we're concerned with.