I have looked at the CARM and DARM RF loops, assuming the loop shapes that we've been using, and it pretty much looks like a miracle that we were ever able to make the transition. The CARM and DARM loops are very marginal.
The ALS CARM loop was already pretty close to marginal, but we lose an extra 12 degrees of phase with the REFL loop:
However, if our cavity pole compensator's zero frequency is too low, we get all of that phase back, at the sacrifice of 2dB of gain margin at both ends of the phase bubble.
I looked at an Optical simulation to check what the cavity pole frequencies are expected to be, with the losses that we've measured. In both cases, I assume the Xarm has about 150ppm of loss. The DARM cavity pole is about 4.5kHz no matter what the Yarm loss is. The CARM cavity pole is about 172 Hz if the Yarm has 500ppm of loss, or 120 Hz if the Yarm has 200ppm of loss.
In the plots below, I use a CARM cavity pole frequency of 150 Hz, to roughly split the difference.
Edit, 13Mar2015, JCD: Rana points out to me that I was using from Foton the analog design strings, without including the fact that these are actually digital filters. This means that I am missing some phase lag. Eeek.
The ALS loop includes:
The REFL loop includes:
The first plot is the case of perfectly matched cavity pole and compensating zero (150Hz, with compensator having 3kHz pole):
This next version is the case where the compensating zero is a little too low, which is the case I think we have now:
The last plot is a DARM loop. Everything is the same, except that the RF plant has a 4.5kHz pole, and no compensation: