Gabriele and I talked for a while on Wednesday afternoon about ideas for transitioning to IR control, from ALS.
I think one of the baseline ideas was to use the sqrt(transmission) as an error signal. Gabriele pointed out to me that to have a linear signal, really what we need is sqrt( [max transmission] - [current transmission] ), and this requires good knowledge of the maximum transmission that we expect. However, we can't really measure this max transmission, since we aren't yet able to hold the arms that close to resonance. If we get this number wrong, the error signal close to the resonance won't be very good.
Gabriele suggested maybe using just the raw transmission signal. When we're near the half-resonance point, the transmission gives us an approximately linear signal, although it becomes totally non-linear as we get close to resonance. Using this technique, however, requires lowering the finesse of PRCL by putting in a medium-large MICH offset, so that the PRC is lossy. This lowering of the PRC finesse prevents the coupled-cavity linewidth of the arm to get too tiny. Apparently this trick was very handy for Virgo when locking the PRFPMI, but it's not so clear that it will work for the DRFPMI, because the signal recycling cavity complicates things.
I need to look at, and meditate over, some Optickle simulations before I say much else about this stuff.