I'm working on some more modelling investigations of this whole situation. The main thing I wanted to do was to look at the complex field amplitudes / IFO reflectivity to see how the PDH signal is affected by different field components. 

I still have plenty more to do, but I got a result which I though I should share. In addition to Jenne's simulation, I also see that between our "nominal" and "canceled" states as defined in Kojis ELOG 11036, there is a factor of ~20 difference in the PRCL signal in REFL33. 

The plots below are kind of like "PDH Signal Budgets" of the two states. 

Specifically, the reason our gain gets reduced is that, in the "canceled" state, the 44*11 and 55*22 products conspire to weaken the signal by having a slope opposite to the -11*22 type products. In contrast, in our "nominal" case, all of the products slope together. 

However, this also predicts that the nominal REFL33 is more sensitive to Carrier*33 than to the signal we desire, -11*22. The only reason it ever worked seems to be the biggest contriubutor, the unexpected 44*11! 

The "residual" trace is the difference of REFL33 and the sum of the field products shown, to justify that all relevant products had been included. 

The simulation that produced this was set up to create 4 orders of modulation at each EOM, with 3 orders of sidebands on sidebands. The demodulation phase was taken by lining up a PRM excitation entirely along I, as we would do on the actual instrument. 

MIST Simulation files attached!