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AdV-DET (Commissioning)
mwas, gouaty - 22:42 Wednesday 11 April 2018 (41074) Print this report
OMC calibration and tuning of PZT modulation depth

This afternoon shift was dedicated to new calibration measurements of the OMC with different PZT modulation amplitudes as a  followup to previous results reported in entries 41021 and 41034. We also tested the effect of adding offsets in the OMC1 error signal, and we found that an offset equal to 1.6e-11 m minimizes the coupling of OMC length noise.

 

First, in order to be able to lock the 2 OMC, we reduced the amount of power transmitted by the Faraday Isolator by rotating the Faraday waveplate from its initial position -50900 to the detuned position -27950. This reduced the power measured on B1p and B1s1 by a factor 6.

Since last week results suggested that the amplitude of the PZT modulation was too large on OMC1 we started by reducing it by a factor 3 ( OMC1_PZT_amp(0.1) ). Then we took data while injecting the calibration line at 119 Hz:

- Calibration line injected on OMC1: start: 1207495961 - stop: 1207496021

- Calibration line injected on OMC2: First segment : start: 1207496021 - stop: 1207496082 / Second segment : start: 1207497100 - stop: 1207497160

We then noticed that the demodulation phase of B1_f1 was not properly tuned and we corrected it in order to obtain a bit more than a factor 100 of separation between the I and Q demodulated signals. The loop gain was reduced to avoid loop oscillation. New calibration data were taken after this demodulation phase adjustment:

- Calibration line on OMC1: start: 1207499311 - stop: 1207499371

- Calibration line on OMC2: start: 1207499371 - stop: 1207499431

 

At this point we repeated calibration measurements for different values of offset injected in the error signal of OMC1:

- OMC.OMC1_offset = 1e-5

  • Calibration line on OMC1: start: 1207499751 - stop: 1207499811
  • Calibration line on OMC2: start: 1207499811 - stop: 1207499871

- OMC.OMC1_offset = 1e-4

  • Calibration line on OMC1: start: 1207500007 - stop: 1207500067
  • Calibration line on OMC2: start: 1207500067 - stop: 1207500127

 - OMC.OMC1_offset = 5e-5

  • Calibration line on OMC1:  start: 1207501213 - stop: 1207501273
  • Calibration line on OMC2:  start: 1207501273 - stop: 1207501333

 As explained in entry 41034 , the height of the 1f calibration line at 119Hz in the error signal can be used to estimate the total RMS of the locking accuracy. A quick look at the data indicates that this total RMS is reduced by an order of magnitude when injecting an offset equal to 5e-5 (corresponding to 1.6e-11 m once calibrated). This will be further investigated.

Keeping this offset of 1.6e-11 m in the error signal, new calibration data were taken after reducing further the PZT modulation amplitudes in both OMC (OMC1_PZT_amp(0.03) and OMC2_PZT_amp(0.003).

  • Calibration line on OMC1: start: 1207502261 - stop: 1207502321
  • Calibration line on OMC2: start: 1207502321 - stop: 1207502381

These lower modulation amplitudes were left as permanent setting.

 

After unlocking the OMC2, we performed some temperature scans of OMC2  (with OMC1 still locked) in order to have an independant way of cross-checking the calibration. The following data were taken:

  • Scans around the TEM00, nominal temperature set point:  start: 17:39:00 - stop: 17:44:00 utc
  • Scan of 2 full FSR: start: 17:45:00 - stop:17:57:00 utc
  • Scan around TEM00, TEM01, TEM02: start: 17:57:50 - stop: 18:01:00 utc

After unlocking the OMC1, some scans were also performed:

  • Scan around TEM00: start: 18:05:30 - end: 18:09:00 utc
  • Scan of full FSR: start: 18:09:30 - end: 18:17:00 utc

At the end of the shift, the rotator of the Faraday Waveplate was put back to a position (-47150) maximizing the power on B1p and B1s1.

 

Results of the analysis of these date will be posted later.

Comments to this report:
mwas - 14:17 Thursday 12 April 2018 (41081) Print this report
Looking at the various calibration measurements, the result is the same for all the measurements done on Apr 11, in particular they don't depend on the PZT modulation amplitude (because the error signal is normalized by it)
OMC1_err: 3.3e-7 +/- 0.1e-7 m
OMC2_err: 7.0e-7 +/- 0.1e-7 m

Figure 1, in purple is a time without an offset added to the error signal and in blue is with the offset of 5e-5 in OMC1_err. This offset of 5e-5*3.3e-7 = 1.65e-11 m is consistent with the previously estimated l_RMS of 2e-11m.
Clearly B1_f1_i (from which OMC1_err is derived), goes from a mean of zero to some offset value, as expected.
But with the offset the height of the 119Hz calibration line in B1 PD1 goes down by a factor 10, which would suggest that B1_f1_i doesn't have its zero in the right place.
However at the same time the height of the 119Hz line *increases* in B1s1 (and the offset in B1s1_f1_i increases), so the two error signal (transmitted and reflected), don't agree on what is the right offset to reduce the coupling between OMC length and transmitted power fluctuations.

The value of that offset has not changed when decreasing the two OMCs modulation depth. Something we haven't studied is if the needed offset to minimize the coupling depends on frequency, we should try next time to inject broadband noise and see if offset changing has the same impact on the length to power coupling in the whole 10Hz-1kHz band.

For the last calibration done in the 2 minutes starting from 1207502570:
OMC1
l_RMS = 2.1e-12
l_cal_mod = 2.9e-13
l_err_mod = 8.4e-13
OMC2
l_RMS = 2.5e-12
l_cal_mod = 6.8e-13
l_err_mod = 5.1e-13
So there seems to be some residual offset for both OMC1 and OMC2 that dominate the estimated l_RMS, but the modulation of the calibration line and the error signal are only a factor 2-3 smaller than that.
Figure 2 and 4 shows the calibrated spectrum, where clearly the OMC2 lock is much poorer with a precision of 1.1e-12 m instead of 2e-13 m for OMC1.
Looking at the time series 3 and 5 there is some slow oscillation (period ~ 1minute) in the OMC2 lock.

Figure 6, looking at half an hour of data, this oscillation is present in both OMCs, but happens to be in a larger swing for OMC2 during this calibration measurement (from 17:22:30 to 17:24:30). The two oscillations are quite correlated, but not fully, so it is probably not an issue with the frequency of the input light, but maybe gain peaking at the cross-over between Peltier and PZT actuation. To be further investigated.

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