I did the same comparison as before using data from today, and the results are a little weird. The gyro noise is the same at low frequencies, and the DC_TRANS spectrum (when multiplied by 10) is also coincident at low frequencies. However, the high-frequency floor of the DC_TRANS spectrum today is higher than it was last night, and there is also a little bit of excess HF noise in the gyro spectrum. I am going to check what the dark noise of the TRANS PD is now.

I also took spectra of the DC outputs of the REFL PDs, and they don't share this low-frequency bump in the spectrum with the TRANS and gyro signals. This is above the ADC noise floor, but it could easily be the electronic noise of the DC output of the PD (which is very noisy for some reason). The point of this measurement was to verify that we saw power fluctuations on the REFL side in coincidence with the TRANS ones. Then, we would see if they were in or out of phase with the TRANS fluctuations to see if they were input power fluctuations or cavity coupling fluctuations. This would essentially give us the information we would get from setting up a pickoff PD near the laser to monitor input power fluctuations, plus a little more.

I will see if this is dark noise and then figure out a way to make a more precise measurement of the REFL fluctuations.