Q has pointed out that we expect a sign flip in the AS55 signal for DARM as we reduce the CARM offset in the PRFPMI case. Koji also mentioned that the SRC will help broaden the DARM linewidth. I wanted to check and think about these things with my Optickle simulation. Q is working on confirming my results with Mist.
The simulation situation:
* The demod phase for AS55 is set in the MICH-only case so that the MICH signal is maximized in the Q-phase. I do not change the demod phase at all in these simulations.
* Cavity lengths (arms, recycling cavities) are the measured lengths.
* I look at AS55 I and Q as DARM sensors (i.e. I'm doing DARM sweeps) as a function of CARM offset, for both PRFPMI and DRFPMI cases.
Spoiler alert! Conclusions:
* In the PRFPMI case, the DARM signal shows up with approximately equal strength in the I and Q phases, so we suffer only about a factor of 2 if we do not re-optimize the demod angle for AS55.
* In the DRFPMI case, the DARM signal is a factor of 1,000 smaller in the Q-phase than the I-phase, which means that the ideal demod phase angle has moved by about 90 degrees from the MICH-only case. We must either use the I-phase signal or change the demod phase by 90 degrees in order to acquire lock.
* In the PRFPMI case, there is a sign flip for DARM on the AS55 PD around 100pm, so we don't want to use AS55 for DARM until we are well inside 50pm, and aren't going to fluctuate out of that range.
* In the DRFPMI case, there is no such sign flip, at least out to 1nm, so we can use AS55 for DARM as soon as we see a viable signal. This is super awesome. The caveat is that the gain changes significantly as we reduce the CARM offset, so we either need a UGF servo (eventually) or careful watching (for now).
* The AS55 linear(-ish) range is much broader in the dual recycled case, which is yet another reason why getting DARM on AS55 will be easier for DRFPMI.
Why didn't we do it already?
* To put the SRM QPD back, we'd also have to move Steve/EricG's laser. Since I had other things to do, I left the setup for tonight, but I think I will want it for tomorrow night.
* Monday night (and tonight) we can pretty reliably get DARM onto AS55Q for the PRFPMI case, and I don't know what the cause has been for my locklosses, so I thought I'd try to figure that out first.
First up, the current transition we've been trying to handle, PRFPMI DARM to AS55Q. I also plot AS55I, and we see that the signals are roughly the same magnitude (the x axis isn't the same between these plots! sorry), so we aren't screwed if we don't change the demod phase angle. We'll be better off once we can do a re-optimization, but this is assuming we are stuck (at first) with our MICH-only demod phase angle.
Next up, the same plots, but for the DRFPMI case. Note here that there is a factor of about 1000 in the y-axis scales, and also that there is no switch in the sign of the zero-crossing slope for the I-phase.
And here is the same data (DRFPMI case), but zoomed out for the Q-phase, so you can see the craziness of this phase. Again, this is much smaller than the signals in the I-phase, so I'm not too worried.
* Steve leaves the SRM oplev back in its nominal location (we can worry about aligning the mirror, and aligning the beam on the PD, but please put it back approximately where it came from).
* Try DRMI + 2 arm locking, which I don't think we have ever actually done before. Hopefully there won't be any tricks, and we can get to an equivalent place and successfully get DARM to AS55.
* .... Keep going?