[Louis, Gabriele]
The DHARD_Y to DARM coupling always showed two regimes: a steep coupling below 20-30 Hz, and a flatter coupling above 20-30 Hz. We've been able to change the flatter coupling above 20-30 Hz by changing the ITMT Y2L coefficient.
Today we confirmed a suspicion: the steep low frequency coupling is due to length to angle coupling at the AS WFS. We changed the beam position on the WFS by adding an offset to the WFS centering (H1:ASC-AS_A_DC_YAW_OFFSET) and saw a change in the DHARD_Y to DARM coupling.
A value close to -0.14 gives the minimum coupling below 20 Hz. we now have two independent knobs to minimize the DHARD_Y to DARM coupling at all frequencies.
Incidentally, the higher frequency couping is now lower than yesterday, with the same ITMY Y2L coefficient of -1.65
We did a scan of the AS_A_WFS Y centering from -0.2 to -0.1 in steps of 0.01, an analysis will follow tomorrow:
-0.200: 1394510627 - 1394510727
-0.190: 1394510777 - 1394510877
-0.180: 1394510927 - 1394511027
-0.170: 1394511077 - 1394511177
-0.160: 1394511227 - 1394511327
-0.150: 1394511377 - 1394511477
-0.140: 1394511527 - 1394511627
-0.130: 1394511677 - 1394511777
-0.120: 1394511827 - 1394511928
-0.110: 1394511978 - 1394512078
-0.100: 1394512128 - 1394512228
0.000: 1394512278 - 1394512378
We are leaving a value of -0.14 in the WFS offset
Attached is a comparison of the DARM sensing function with no AS A centering offset vs an offset of -0.14. With an AS A centering offset of -0.14, which we found to be the value that results in the minimum amount of coupling to DARM below 20Hz, the sensing function clearly shows optical spring-like characteristics. This brings to mind a few thoughts: 1. This supports the idea that coupling from the DHARD loop into DARM has a noticeable effect on the structure seen in the sensing function at low frequencies. We've been wondering about this for some time, so it's nice to finally have a direction to point in. 2. We tend to adjust the src detuning by constantly measuring the sensing function and trying to find an SRC offset that results in a flat sensing function at low frequencies. The fact that DHARD also couples with DARM in such a way that it can affect the shape of the sensing function at low frequencies begs the question: could we be in fact further detuning the src while intending to do the opposite due to confusion caused by the dhard coupling effects? 3. I recall being told that sometimes squeezing gets better with some level of detuning. If our only measure of SRC detuning is from measuring and inspecting the sensing function then this measurement hasn't been clean due to the DHARD coupling. lots to think about..
Here's a more detailed analysis of the AS WFC centering steps.
The first plot shows the steps in ASC-AS_A_DC_YAW_OFFSET compared with a DARM spectrogram, during a DHARD_Y injection. The spectrogram shows that there is minimum in the coupling of DHARD_Y to DARM around -0.15 / =0.16.
The second plot shows the transfer function from DHARD_Y to DARM for all values of the offset. A value of -0.15 gives the lowest coherence and the lowest coupling, so that seems to be the optimal value. One can notice how the transfer function phase flips sign as expected when one goes through the minimum coupling.
Changing the AS_A centering offset also moved SR2, SRM and BS.
I tried stepping the REFl WFS A and B DC offsets in yaw similarly to see if the CHARD Y coupling to DARM would change. In summary, I stepped between -0.2 and 0.2 for both WFS and saw no change.
Method: I set a 30 second ramp on the offsets because the DC centering loops are slow. I stepped first in steps of 0.01, and then 0.02. I injected a broadband CHARD Y injection and measured the transfer function to darm between 10-30 Hz. I saw no change in the coupling while I made these steps.
Before checking on the calibration change in DARM and DHARD, I check on the thermalisation effect with the coupling.
I chose long duration locking time (Mar. 16, 05:30:00 UTC ~ 15:30:00 UTC) without centering offset, and selected start, middle (10:30:00 UTC) and end time within the time window.
Three plots are; 1) DARM, 2) DHARD PIT, 3) DHARD YAW.
In addition, I included screenshot of ndscope to confirm the time window.
As the 'end' time data in all plots show different trend compare to the other times, it seems that the thermalisation affects DARM and DHARD.
Checked on the calibration lines in DARM and DHARD with centering offset on/off conditions.
To minimize the thermalisation effect, time for the comparison were chosen within short time window.
Figures are; 1) Comparison altogether, 2) DARM comparison, 3) DHARD PIT, 4) DHARD YAW, 5) Screenshot of the ndscope around comparison time.
It can be confirmed that the peaks of calibration lines were same in DARM with and without the centering offset. However, for DHARD, only YAW showed calibration lines, and with different peak magnitude (lower in without offset).
