J. Kissel (with information from S. Dwyer)

I have been blaming the rotation stage for the 5 [W] range of IFO input power from lock stretch to lock stretch, but it looks like I've been wrong this whole time. I attach a 25 day trend of the the power into the IMC, compared to 
 - the power transmitted from the PMC, 
 - the power reflected off the input of the PMC. 
 - the relative power diffracted by the AOM (servoed by 1st loop)
 - the 2nd loop Reference Voltage (servoed by 2nd Loop)

One can see 
 - the range of power into the IMC is 5 [W], with an average of 30 [W], which means a relative power range of 16%. 
 - the range of transmitted power from the PMC shows a similar range: 10 [W] or out an average of 60 [W]; a relative range of 16%.
 - the power changes are anti-correlated with the PSL diffracted power, as expected, but the channel appears to be poorly calibrated since it ranges between 2 and 6%. This is likely the cause of the power variations, and they are under reported.
 - the power reflected by the PMC shows some long-term trend over the 25 days, but no correlation with the lock-to-lock difference.
 - the second loop reference voltage seems to track the long term trend of the PMC REFL power.
 - though there may be some correlated in isolated events, the human-adjusted (and NOT every lock stretch) 1st Loop voltage reference is not correllated with the lock-to-lock stretch changes in power.

The fundamental problem -- the DC voltage reference for the ISS' 1st loop is not stable. 

Currently, we have some alarms on the diffracted power, but they're not very tight, and only designed to prevent saturation of the servo. The power fluctuations *could* be remedied with some hacky slow digital servo, or a tightening of the threshold on the diffracted power, but it seems like we should be able to get a more stable voltage reference.