koji.arai@LIGO.ORG - posted 00:28, Tuesday 14 March 2023 - last comment - 17:20, Tuesday 14 March 2023(67968)
OMC ASC: QPD offset sweep
[Vicky, Koji]
Summary
At the current Nominal Low Noise state, OMC QPD A and B are used for OMC ASC. The OMC is made such that the beam spots are NOT on the center of the QPDs.
We wanted to check the dependence of the optical gain, the squeezer beam mode matching, and the other noise couplings (sqeezed level, jitter, low freq noises, and so on) with the OMC alignment.
We set the new offset values to be
(NEW)
H1:OMC-ASC_QPD_A_PIT_OFFSET -0.350
H1:OMC-ASC_QPD_A_YAW_OFFSET -0.144
H1:OMC-ASC_QPD_B_PIT_OFFSET -0.150
H1:OMC-ASC_QPD_B_YAW_OFFSET -0.050Method
- The OMC ASC offsets for the QPDs are added at the filter modules
- The OMC ASC offsets for the QPDs are added at the filter modules
H1OMC-ASC_QPD_X_YYY (X=A or B, YYY=PIT or YAW) and the conventional values are (OLD) H1:OMC-ASC_QPD_A_PIT_OFFSET -0.368
H1:OMC-ASC_QPD_A_YAW_OFFSET -0.219
H1:OMC-ASC_QPD_B_PIT_OFFSET -0.150
H1:OMC-ASC_QPD_B_YAW_OFFSET 0.000- We want to sweep these offsets one by one and check the following quantities
1) DARM optical gain:
-> Monitor 10s avg of H1:CAL-CS_TDEP_KAPPA_C_OUT or H1:CAL-CS_TDEP_KAPPA_C_OUTPUT
The optical gain (aka the IFO beam mode match) change is partially compensated by the increase of the darm offset to keep the DCPD DC value.
Therefore the IFO beam mode match is supposed to be proportional to the square of this value assuming the IFO optical gain itself is stationary. (cf LHO ALOG 67963 by Craig)
Therefore the IFO beam mode match is supposed to be proportional to the square of this value assuming the IFO optical gain itself is stationary. (cf LHO ALOG 67963 by Craig)
2) Squeezer mode matching
-> Monitor the integrated RMS of the calibrated DARM (H1:CAL-DELTAL_EXTERNAL_DQ) around 725Hz. In fact
,H1:SQZ-DCPD_RATIO_3_MON corresponds to this value. We measure both.-> We also injected 1300Hz line from the squeezer. This should tell us how much the squeezer field is transmitted (we need to think about it a bit).
3) Jitter noise coupling
-> Monitor the integrated RMS of the calibrated DARM around (say) 100~200Hz
4) Other noise coupling
-> Monitor the integrated RMS of the calibrated DARM around (say) 20~80Hz
or Monitor the integrated RMS of the ratio with between the DARM and the reference DARM spectrum taken at the nominal offset values.
5) Just as a reference, turn on the OMC ASC dither lines and monitor the dither error signals (10s avg value):
H1:OMC-ASC_P1_I_OUT
H1:OMC-ASC_Y1_I_OUT
H1:OMC-ASC_P2_I_OUT
H1:OMC-ASC_Y2_I_OUT- Sweep an offset up and down from the current nominal value, until we see (at least) one of the above quantities gets degraded. Determine the best point from the data points (at least 5 pts, ideally 7 pts or more) for each index.
- If possible repeat the sweep at around the new best offset, as the iteration.
How it went
- We made a script to measure the above values and save H1:CAL-DELTAL_EXTERNAL_DQ spectra into files.
- The offsets were changed with the order of P1, P2, Y1, and Y2.
- We monitored OMC SUS DAC outputs such that they do not exceed the 50% F.S. (we believe the FS is 2^17 count = 131k)
- We used the shot noise level (above 2.) as our reference to optimize the offsets. But the level was drifting so it was a bit confusing.
- P1 and P2 didn't make much change around the conventional offset values.
- Y1 wanted to go positive. Y2 wanted to go negative.
- The new offset shown at the beginning was thus determined.
- If possible repeat the sweep at around the new best offset, as the iteration.
How it went
- We made a script to measure the above values and save H1:CAL-DELTAL_EXTERNAL_DQ spectra into files.
- The offsets were changed with the order of P1, P2, Y1, and Y2.
- We monitored OMC SUS DAC outputs such that they do not exceed the 50% F.S. (we believe the FS is 2^17 count = 131k)
- We used the shot noise level (above 2.) as our reference to optimize the offsets. But the level was drifting so it was a bit confusing.
- P1 and P2 didn't make much change around the conventional offset values.
- Y1 wanted to go positive. Y2 wanted to go negative.
- The new offset shown at the beginning was thus determined.
Discussions
- The BNS range during the measurement and adjustment was ~120Mpc and it was not at the best state.
- The detailed analysis of each index will come later.
- The new offsets were saved in SDF.
- The new offsets were saved in SDF.
Measurement analysis can be found at LHO ALOG 67994