[Yuta, Rana, Evan]

When Stefan left Friday evening, PRMI wouldn't lock. We poked around at MEDM screens for a while before deciding that a more systematic diagnosis was in order. We decided to attack just the Michelson first.

We parked PRM and misaligned ETMX. We then adjusted the LSC MICH filter bank to duplicate was was done for Kiwamu's and Yuta's previous Michelson lock characterization (elog 9698, 31 Jan 2014). Even with a 1:0 integrator engaged, we found that the Michelson would not lock for more than 30 s, and the error signal drifted by about a third of its peak-to-peak.

We were able to measure the OLTF, and found that it had a UGF of 3 Hz with no phase margin. Rana suggested we notch out the bounce mode of the BS suspension with filters from LLO. We got the filter, adjusted the frequency to the LHO BS (17.8 Hz, as measured from the REFLAIR_A_RF45_Q_ERR spectrum), and then added it to FM6 on LSC_MICH. After doing this, we found that the Michelson lock is much more stable --- it appears to lock indefinitely.

In order to calibrate REFLAIR_A_RF45_Q_ERR in terms of mirror motion, we let the Michelson swing freely and recorded the fringing. We know that the fringing amplitude (in counts) as a function of asymmetry l is A sin(4 pi l / lambda), so the linear portion has a slope of A * 4 pi / lambda, in counts per m. I took the swinging data, trended the minimum, median, and maximum, and then took the median of the trended minimum and maximum values. A histogram of these values is attached. From this I find A = 643 counts; this gives the conversion factor as 7.6 counts per nanometer.

We used this value to get a calibrated spectrum of the dark noise of REFLAIR_A_RF45_Q_ERR, which we measured with the modecleaner unlocked. A trended 10-minute time series is attached; we see that the drift is on the order of a few nanometers over this time period. Also attached is a spectrum of the dark noise, along with Yuta's estimate of the control signal (LSC_MICH_OUT) the Michelson, given in terms of length. The estimated length noise was 1.1 um RMS.

An OLTF of the improved Michelson loop is attached. The UGF is now 7.5 Hz, with a phase margin of 20 degrees. Also attached is Yuta's model of the expected OLTF; the agreeement is excellent around the UGF, except for the flat gain. This model uses already existing an already of the triple suspension of the BS (/ligo/svncommon/SusSVN/sus/trunk/Common/MatlabTools/TripleModel_Production).

We assumed that the suspension model gives BS actuation efficiency from H1:SUS-BS_M2_LOCK_L_OUTPUT to the actual M3 motion in m/counts. However, there is a missing factor of 1.7e-3 in this actuation efficiency to fit to the measured OLTF.