I asked Gabriele why it looked like for the PRCL sweep REFL 55 I&Q were zero at zero, but for the MICH sweep only REFL55 I was zero. He took a look at his code, and found that he was not at the correct locking point. Here is his email back to me:
I found the reason for the not zero value. Indeed, if you could zoom into the PRCL sweep, you would see that the error signals does not cross zero exactly at PRCL=0, but instead some 50 pm away from zero. This is enough to change a lot the PRCL signal when sweeping MICH. If I put PRCL to the correct zero point, and I sweep MICH, I now get everything at zero. I'm sending again the plots.
The fact that such a small detuning is enough to change PRCL signal when sweeping MICH is due, I believe, to the fact that MICH optical gain is much smaller than PRCL one.
Here are the redone plots:
Phase not tuned:
POP22 resonance for MICH and PRCL:
POP110 resonance for MICH and PRCL: