[Trent, Georgia]

We were curious if the coherence between DARM and the PSL periscope accelerometer was improved with Craig's Wednesday night input alignment. Though there wasn't much time without glitches or excitations during this lock, we got a 700 averages with 75% overlap bw 0.1Hz. In the attached plot, top is DARM ASD, bottom is coherence with the PSL periscope accelerometer (witness sensor for jitter), and right plot is zoomed in on some of the jitter peaks. It looks pretty convincing that for the resonances above 100Hz, the coherence was lower with the new input (and full IFO) alignment on Wendesday.

According to some of the sensors in HAM2 and HAM3, we might have walked the input pointing further in PIT than the O4A alignment, so maybe there is an IM1 and IM3 alignment somewhere in between the two times in the attached plot that has even lower input jitter coherence.

Looking at Camilla's Bruco from December, the PSL periscope coherence was not this bad in O4a

Attachment shows DARM × PSL acceleration coherence on 13 Dec (red), 11 Jan (turquoise), and 16 Mar (black). There is no significant difference between the latter two.

OM2 was hot on 13 Dec and cold on the latter two.

New fit done, loaded into FM2, not tested yet.

We tested the new FF, and it didn't perform as well as expected. However

• the old FF filter now performs worse than right after it was tuned
• the new FF filter performs better than the old one now

We therefore used the measurement to fit another MICH FF, we'll test it soon

Gabriele created two new filters in FM1 and FM2 to try out in DARM. Currently, we are using FM3. I ran an injection using the current filter, and then for each of Gabriele's new filters. Overall, if we want the most suppression 10-20 Hz, we should go with the current, FM3. If we want to do better 20-40 Hz, we can choose one of the new filters. FM2 is worse from 40-70 Hz, but I'm not sure how much that matters. FM1 will inject a few dB more noise than the others from 7-10 Hz, again, not sure how much that matters. I am staying with FM3 pending some evaluation.

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).

At high CLF power we have about 12 uW of light for CLF and RLF after the OPO. Each homodyne PD has 0.5 mW of power, or 1 mW total. This yields a CLF/RLF to LO ratio of 0.012. Using the equation Gamma^2/2 to get the modulation index, we obtain an estimate of 150 mrad for the maximum phase modulation. In reality, it will be somewhat smaller since some of the power will be in amplitude modulation. This will limit the maximum amount of achievable squeezing on the homodyne. But, this has no bearing on the DCPDs, since the CLF/RLF to LO ratio there is approximetaly 5000 times smaller.

A bit more details on this.

The power ratio between SB and CR of each sideband is approximated to be ~ (gamma/2)^2 where gamma is modulation depth in radian. Add the two sidebands together you get (gamma^2)/2.

A total power transmits through the OPO during high CLF case was 12uW. A total LO power hitting the HD was 1mW. So the phase noise contribution to HD sqz was

(gamma^2)/2 = 12uW/1mW 

High CLF phase noise (gamma) at the homodyne = 154mrad (max)

A total power transmits through the OPO during low CLF case was 0.6 uW. A total LO power hitting the HD was 1mW. So the phase noise contribution to HD sqz was

Low CLF phase noise (gamma) at the homodyne = sqrt(2*0.6uW/1mW) = 35mrad (max)

These number include amplitude modulation. It's the worse case that could possibly happen. Using 45 mrad of phase noise and *16dB of asqz fits the high CLF squeezing of 6.5 dB in the homodyne. 15 mrad and 16 dB of squeezing fits the low CLF squeezing of 8 dB in the HD.

For the IFO case we've only injected sqz using low CLF so far. All the sideband power gets attenuated by the OMC (a factor of 5000). The LO (IFO carrier) is ~40mW. Phase noise contribution from CLF/RLF sidebands is 0.08 mrad, which is negligible. Even for high CLF power (RF6 = 6 dBm) this phase noise would be 0.3 mrad. That number is still negligible so there's no reason why we shouldn't be able to see good squeezing with high CLF. Using **15dB of aSQZ and a loss of ***30% we have 5 dB of squeezing as observed on Friday. 

*ASQZ trace overlapped with 16 dB aSQZ reference in HD https://alog.ligo-wa.caltech.edu/aLOG/uploads/73562_20231018171710_8dB_hd_sqz_2023Oct18.png

**15 dB of aSQZ in DARM https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=76426

*** If I'm reading this noise budget correctly the inferred loss in the IFO is 30%.