Tonight I repeated the PRC length measurement, by locking the CITF on the carrier and the sideband and using the 56MHz phase difference to estimate the PRC de-tuning from perfect sideband resonance.

Locking the CITF was much smoother tonight; the alignment was very stable, there was no ~2Hz oscillation, gains were steady.  The stability was dramatically better than when we made these measurements before (20Feb) or the last time we locked CITF (25 Feb, before the RH was switched off).  The seismic noise was quiet on the 20th, comparable to tonight (not so quiet on the 25th).  Other things that have changed: suspension work, the RH was switched off, and we're using the 6MHz to lock MICH instead of the 119MHz.

Despite the stability, the carrier buildup in the CITF was not as good tonight, the best I could do was about 10mW on B4 DC, about half what we had achieved before.

The phase sensitivity of the B2 56MHz signal to the PRC length is 9x better than the 6MHz, so we expect 7deg of phase shift per millimeter of PRC length detuning.  Tonight I collected four periods of data, two each of sideband and carrier, and as before used the PRCL line injection and the B2 56MHz modulation monitor to estimate the phase shift between the carrier and sideband locks.  We'll make a more quantitative statement tomorrow, but the results are consistent with our previous (noisier) measurement; the PRC length is not perfect, and is mistuned by 3-4mm.

The plot attached shows the duration of the measurements.  Here are the quiet times:

Between the 1st and 2nd carrier locks, I improved the alignment; the first two locks have the same (not great) alignment, and the second two locks should have the same alignment as well.  Outside of these times I also adjusted loop gains and phases, so you need to look at both the DPHI channel in the frames as well as the SIB2_B2_56MHz_phi0 channel.  We saw before that the alignment can change the demod phase, so the measurements to compare are the first pair or the second pair.  These give either (0+0.4) or (0.15+0.25) ~= 0.4 rad as the difference between carrier and sideband locks.  That gives 23deg of phase difference on the 56MHz signal, or about 3.3mm of detuning.  The 6MHz data is also available for analysis.

Notes:

• I have modified the LOCKED_CITF_DF state to use the B1p 6MHz signal for MICH (not normalized), this worked well.  Locking CITF tonight was a breeze.  B1p was quiet dark and often had a clear bulls-eye mode shape.  Does this mean the cold RH state is better, or does the lower buildup mean we're less stable?
• I added a demod phase monitor for the B2 56MHz signal, using the PRCL line
• I added the SIB2 B2 56MHz phi0 channel to the frames