Virgo Logbook

AdV-COM (AdV commissioning (1st part) )

bersanetti, sorrentino, vardaro, mwas - 20:27 Friday 20 March 2026 (68921)

Lock acquisition with SRC detuned and SQZ measurements

Today we aimed to inject again squeezing in the interferometer, while working in the SRC-detuned configuration.

At first we tested the lock acquisition with the SRC already detuned from the beginning: usually we get to CARM Null with a non-zero optical spring, then the SRCL_SET servo brings it and keeps it to a setpoint, which is 0.5 (roughly in Hz). Today we immediately put a setpoint of 10, meaning that the servo's job was to just keep the optical spring in place.

This happened at 12h53m34 UTC, then we unlocked at 13h07m54 UTC as we were testing this in anticipation during other activities.

We repeated the procedure with the lock in CARM Null starting at 14h11m23 UTC, with the servo setpoint at 10 starting at 14h12m48 UTC.

Later, while still in CARM Null, we made a noise injection on DARM (to be analyzed later) in order to understand the DARM plant and check the optical spring: DARM_noise shape, amplitude = 20, 180 seconds starting from 14h39m00 UTC.

Then we continued the lock acquisition while keeping the detuned configuration, and that worked with no issues.

We reached LOW_NOISE_3_ALIGNED at 14h59m44 UTC. At 15h04m05 UTC we lowered the SRCL_SET setpoint down from 10 to 9.

Then we made another noise injection on DARM with the same parameters of the standard injection for Science mode: DARM noise shape, amplitude = 0.2, 180 seconds starting from 15h11m30 UTC.

At around ~15:20 UTC we started the squeezing injection using the SQZ_MAIN Metatron node, going to the SQZ_INJECTED_NO_FC state.

At 15:35 UTC we changed the squeezing phase from 3.8 to 4.1;. 4.1 was the value used yesterday.
At 15:37 UTC we changed the squeezing phase to 1.1
At 15:40 UTC we started a 1000s long scan over -7 radians of the squeezing phase, with a manual ramp. We then understood that the automatic alignment is not supposed to be enabled all the time, but only when anti-squeezing is injected.
At 15:56 UTC we set the phase to 3.1 rad for anti-squeezing, and we let the automatic alignment run for 10 minutes.

We went back to the SQZ_LOCKED_NO_FC state and run the /virgoDev/AEI_SQZ/coh_scan.py script, to make the scan in the usual way. Initial GPS time: 1458058066.74 (16h07m28 UTC).

Then we increased the detuning of the SRC by progressively increasing the SRCL_SET setpoint, targeting a pole of ~200 Hz (read by Hrec) as yesterday:

16h37m35 UTC SRCL_SET_SET = 11;
16h50m34 UTC SRCL_SET_SET = 12;
16h56m11 UTC SRCL_SET_SET = 14;
17h06m20 UTC SRCL_SET_SET = 13.

The small rollback was due to the pole reading starting to drop to 170 Hz, so we got back to around 200 Hz.

At 17h33m09 UTC we started another scan of the squeezing phase. Initial GPS time: 1458063207.03.

None of the two scans reached the same increase in sensitivity as yesterday's first one, but additionally the second one showed a lower effect overall. Additionally, something else was changing in the interferometer at the time, as the frequency noise and the PSTAB couplings we higher. More analysis on that will follow.

WIth the same conditions as the second scan, we performed a calibration measurement using the CALI node, state CALIBRATED_DF_DAILY; the procedure started at 18h08m11 UTC.

We decided to keep the detuned configuration for the current lock and for the weekend, using the loop and the automation: we set the SRCL_SET_SET to 10 in CARM Null (line 274 of ITF_LOCK.ini) and to 12 in LOW_NOISE_2 (line 522 of ITF_LOCK.ini), which are the only places where it is set in the automation. I added the reset to zero of the setpoint in the DOWN state of the DRMI_LOCK node.

The SRCL_SET setpoint was also put manually to 12 at 19h13m46 UTC. In the figure a snapshot of the figures of merit of the interferometer.

Images attached to this report

Comments to this report:

mwas - 22:03 Friday 20 March 2026 (68922)

The interferometer has unlocked with SRCL INPUT running away from -20 down past -30, and also with SR and SDB1 alignment moving away. I have reduced the optical spring set point from 12 to 11, so that is further away from a potential instability.

Images attached to this comment

68922_1774040485_Screenshot from 2026-03-20 21-59-11.png

mwas - 7:10 Saturday 21 March 2026 (68923)

Figure 1. In the detuned SRCL configuration last night the alignment of SR and SDB1 was quite unstable, with SR moving by +/-0.3urad on time scales shorter than one hour, and SDB1 moving by +/-3 urad.

Figure 2. in the tuned SRCL configuration on March 18 the fluctuation of SR alignment compared to the ground were a bit smaller at +/-0.2 urad, and especially for SDB1 they were a lot smaller at +/-1 urad.

We had seen during the shift this week that SR and SDB1 alignment changes during the detuning. And we would make sense for the SR alignment signal to be affected as it looks at the demodulation of the phase of the 491.3Hz line at frequencies of the SR alignment dithers.

Figure 3, 4 and 5 show analytical models of the optical response for a detuning of respectively 1nm, 10nm and 20nm. For a small detuning the phase at 491.3Hz decreases with increased losses, and the SR alignment signal tries to maximize the phase, so reduce the losses due to misalignment. But for a detuning of 10nm the phase at 491.3Hz stop depeding on losses, and for 20nm detuning it reverses direction, higher losses increase the phase. That would explain the SR behavior, the current alignment signal looses SNR with larger detuning, and eventually for higher detuning it will flip signs.

This would also explain why the squeezing phase scans last night had a lower effect for larger detuning. If SR alignment becomes erratic, there will be more losses in the SRC, and squeezing will become less efficient.

This should be tested and if confirmed there can be several solution. Use a line at DARM line at lower frequencies for the SR alignment, or use a line at higher frequencies with an opposite sign once the detuning is large enough, or think about possible other error signals for SR alignment in the detuned conifguration.

/users/mwas/detchar/toySensitivity_20260225/toySR_TF.m

Images attached to this comment

68923_1774072776_Screenshot from 2026-03-21 06-54-19.png

68923_1774072877_Screenshot from 2026-03-21 06-58-30.png

68923_1774073053_Screenshot from 2026-03-21 06-58-45.png

68923_1774073065_Screenshot from 2026-03-21 06-58-57.png

68923_1774073070_Screenshot from 2026-03-21 06-59-02.png

vardaro - 11:43 Saturday 21 March 2026 (68924)

I checked the effect of the phase scan in different range of Hrec. The following plot are composed by three subplots:

First line: Hrec BRMS in different bandwidth normalized in dB, 0dB means shutter closed of the SQZ
Second line: Blue plot B1_4MHz_mag i.e. the magnitude of the beat note between 4MHz CC beam and B1 Beam on B1_PD
Third line: BNS Rang

I did the plots with three different GPS:

Figure1: 19 March - 16:00, SR tuned Best alignment of the squeezing performed during the week
Figure2: 20 March - 16:10, SR detuned First phase scan
Figure3: 20 March - 17:33, SR Detuned Second phase scan

Not clear if the SQZ injection improved the range with SR Detuned, interesting that:

with SR tuned the maximum of the 4MHz mag correspond to ASQZ in high frequency and that the phase dispersion between frequencies range in the bucket and at higher frequency is quite negligible
with SR detuned the maximum of the 4MHz mag correspond to SQZ in high frequency and that the phase dispersion between frequencies range in the bucket and at higher frequency is big, they have quite opposite behaviour

Unfortunately the plot of Fig1: the long stretch of SQZ was performed with too high gain in the CC loop this is why that part is more noisy

Images attached to this comment