Koji was right, and I was using much too large of a CARM offset. Tonight, I set either my CARM or DARM offset to 3 counts, and was able to easily acquire PRMI lock using REFL33.
For either CARM or DARM offset reduction (the other one was kept at zero offset), I was able to get to about 0.5 counts, but I lose lock when I try to go to 0.4 or 0.3 counts. One time, I tried "jumping over" the resonance, by going from minus 1 to plus 1 in CARM offset. Plots of this below.
ALS locked with "Xarm" servo as proxy for DARM, and "Yarm" servo as proxy for DARM. Pushing only on ETMs today, not the MC.
MICH / PRCL:
Input matrix: 1's in REFL 33 I&Q, if not using power normalization. 200's in REFL 33 I&Q if power normalization used (either POPDC or POP22). 200 used because that's about the average value of POPDC or POP22 when PRMI sideband-only resonant.
Trigger: POP22, up 100, down 10.
Power normalization: 1's for both MICH and PRCL in POP22I for one trial. 1's for both MICH and PRCL in POPDC for another trial. Both seemed to work equally well, although that may change when I'm actually getting IR resonance in the cavity.
FM triggers: MICH = FM2. PRCL = FMs 2, 3, 6, 9. Trigger up = 35, down 10. PRCL delayed by 0.5 sec, MICH delayed by 5 seconds.
Servo gains: MICH = 0.4, PRCL = -0.01
When I approach the situation of both arms resonating, it pretty consistently looks like the PRM is getting pushed in pitch (and not in yaw). I don't know why this could be, but it seems like this is the big symptom before lockloss - if the POP spot starts moving (and the PRM suspit signal starts moving), PRMI lock is going to be lost.
I don't know if it's imperfect alignment, imperfect mode matching, or something else, but I see lots of high-order higher order modes on both the POP and AS cameras when the CARM or DARM offset is less than 1 count. I tried to take a video, but the brightness and contrast aren't set as high as on monitors 3 and 5, so it's hard to see the dim stuff. Youtube. At the midpoint of the video, you see a lockloss.
Even though I have overridden the transmission triggers so that I only use the QPDs for the transmission signals, I'm only seeing arm transmission values up to about 50 from each arm. If we had ideal PRC gain, we expect something like 650 or 700.
A few plots
All of the raw data for these plots, and several other channels, is in /users/jenne/PRFPMI/PRMI_2arms_8Apr2014/m1_to_p1_carmOffset_1081065069. As mentioned above, "XARM" is CARM, and "YARM" is DARM. So, the XARM_IN1 tells us about the CARM offset that I was applying. The start time is 1081065069, and the plots are all 8 seconds long.
First, the transmitted power and the CARM offset.
The REFL_I error signals and the CARM offset.
The RF signals that we will eventually chose from for CARM and DARM control. Note that I'm not sure about the AS55 phase, so I plot both I and Q.
The PRM suspit and sus yaw angular signals and the CARM offset. I don't see a huge change in the suspit signal, but it does seem to change character once we approach arm resonances.