* confirmed that there is an offset in the OMC1 lock point, it seems stable compared to last week
* unclear whether the slow oscillation is a cross-over issue or some real signal
As the dark fringe couldn't be reach stably, the work was done on a single bounce beam from NI.
Continued work on understanding the OMC lock RMS started last week
Turned Faraday rotator to send most of the beam toward the squeezing bench: Channel 1 axis 2, -46150 lowered to -10150
Figure 1, OMC1 offset at 5e-5 is still a good way to lower the 119Hz line coupling.
Figure 2, this offset is also lowers the coupling of a broad-band noise injection (1e-4 V/rtHz from 0 to 5kHz)
17:54:30 (1min) injecting 3e-5 V/rtHz up to 50kHz with 0 offset
17:56 (1min) injecting 3e-5 V/rtHz up to 50kHz with 5e-5 offset
Figure 3, the lowered coupling is up to ~10kHz, above that there isn't much difference
Exploring further an offset of 5.5e-5 is even better. Figure 4, a factor 100 better than without offset as shown on figure 1.
For OMC2 I have tried to optimize the offset, but kept going in circles, it seems the slow oscillation is dominant.
Unlocked OMC2, and look at the offset to minimize the coupling of the 119Hz line, it changed to 4e-5. The slow oscillation of +/- 0.2 V in the PZT correction is still present (and it corresponds to an oscillation in 119Hz line coupling factor, with the coupling large when the PZT correction changes quickly). So the oscillation is real, and is not due to some OMC1/OMC2 cross-talk.
18:48-19:09 injection into OMC1 PZT, shaped (amplitude 3.0, pole at 10mHz, zero at 100mHz, butterworth at 0.5Hz)
changed PZT feedback filter replaced Q=1 pole-zero pair at 0.1-0.3Hz, with simple pole-zero at same frequency
19:23-19:33 repeating same injection into OMC1 PZT, the cross over moved from 15mHz to 250mHz (see figure 5)
relocked OMC2 in this asymmetric configuration (with OMC1 cross-over much higher than OMC2)
19:40-19:46 stable data
changed OMC2 PZT feedback to filter the same way as OMC1
19:47-19:52 stable data in this configuration, it doesn't seem like anything changed in the lock (figure 6).
Could try to boost the control filter and suppress this signal (but first I need to learn how to do that without large transients)
putting back usual PZT filter and unlocking OMC2
19:55 injection into OMC1_err (length loop)
20:01-20:11 increased amplitude from 1e-5 to 3e-5
20:15-20:24 injection with additional shaping to increase low frequency
more analysis on the loop measurements and steady state data will follow
Put Faraday rotator back to maximum throughput, seems to be a very different place -37650, maybe went down too fast and some commands where not actually executed by the rotator?
