A couple of things I notice:
1) If you scan the cavity and look at the relative sizes of carrier and sidebands in transmission, then as you move away from having a zero error signal in the region far from resonance, you can increase the relative size of the sidebands. In other words, the point where we have zero error siganl far from resonance, is also a place where we are not maximizing the modulation depth through the EOM. This is another way of saying we have some offset.
2) If you close the shutter on the laser there is still a 19MHz signal coming from the PD. There is some sort of coupling happnening, and this will give us some offset. I measure roughtly 4mV offset in the error signal with the laser shutter closed.
3) Rotating the QWP also changes the power going into this loop of the gyro since there is a PBS after it. This is a total pain because when you put a PD in and try to reduce the RFAM you are also measuring the reduction in power caused by the PBS. The only way to measure this is to rotate the QWP, measure the power, adjust the power to the same level, then measure the RFAM, and then to iterate.
Attached is a plot of measuring RFAM as a function of polarizer position. Although we moved the EOM, it seems that the position of minimum RFAM is still 100 degrees (actually just a fraction before 100 degrees) where is was possible to get the signal down to approx 450uV (blocking the cavity and using our REFL PD to make the measurement).
Again I saw that this minimum value started to vary a bit, so I set it to the minimum and locked the actuator on the wave plate. Then I recorded the RFAM for a while to see how far it would change. The graph is attached. It varied up to just below 5mV and then started coming back down again.