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AdV-ISC (Commissioning up to first full interferometer lock)
mantovani, pinto, bersanetti - 21:08 Wednesday 14 October 2026 (69924) Print this report
Detuned configuration

The morning shift started by relocking the interferometer, which had no issue on that front for the whole shift.

The idea of the shift was to start studying again the SRC longitudinal detuning. Before starting that, and also because we are now working decentering, a very basic set of angular noise injections was done (DIFFp, PR). The log file is in /virgoData/NoiseInjections/ASC/ASC_injection-1476000002.txt

After that, the tests on the SR detuning started; the first few tests were done naively by changing the setpoint of the SRCL_SET loop, which in the past did not give very good results, as the signal was not carrying enough information to drive it properly in the intended way (higher frequency for the optical spring). This happened in LOW_NOISE_3_ALIGNED.

Here the recap of the tests done in this way:

08:14:10 UTC : SRCL_SET_SET = 0.5 -> 5.0 in 300 s, then 480 s of clean data;
08:27:30 UTC : SRCL_SET_SET = 5.0 -> 10.0 in 300 s, then 600 s of clean data.

Around 8:31 UTC we observed some bursts of noise in the MICH loop (and, fainter, in the SRCL one) at around 3.6 Hz. Given that in that state the UGF servo loops are not operational anymore (with the exception of the DARM one), the gains of both of them were raised around 8:35:30 UTC. We unlocked at 8:38:52 UTC.

In the following lock, that started at 09:16:53 UTC:
09:27:42 UTC: SRCL_SET_SET = 0.5 -> 10.0 in 600 s
09:46:39 UTC: unlock

There were a couple of unattended tests during the lunch break, which look very similar and probably do not carry more information.

Later, we verifired offline from the tests that the OS signal moved back to a more manual approach: LOW_NOISE_2 state, SRCL_SET loop technically close but in reality frozen in place (LSC_SRCL_SET_CAL = 0), then LSC_SRCL_SET moved manually as a static offset.

Test #1:
12:29:00 UTC: start of LOW_NOISE_2
12:33:00 UTC: SRCL_SET loop frozen (value of _SET_TOT at the time ~ -8.3)
12:33:52 UTC: SRCL_SET = 0 -> -5 in 300 s
12:41:00 UTC: SRCL_SET = -5 -> -10 in 300 s
Noise injection on DARM (and clean data): logfile in /virgoData/NoiseInjections/LSC/LSC_injection-1476017575.txt
13:01 UTC: SRCL_SET = -10 -> -15 in 300 s
13:20 Noise injection on DARM (and clean data): logfile in /virgoData/NoiseInjections/LSC/LSC_injection-1476019008.txt
13:29:21 UTC: unlock
We continued to change the SRCL set at step LN2 (in order to have all the servo for the gains active) and we took :

- DARM OLTF measurements to fit the OS for different SRCL set

- Angular noise coupling (DIFFp) to see if the coupling could change with the SRCL set

the gps are:

SRCL set @ -10 gpsDARM=utc2gps(2026,10,14,14,32,45); dur DARM=120;

SRCL set @ -20 gpsDARM2=utc2gps(2026,10,14,14,47,50); dur DARM2=120;

            gpsDptx=utc2gps(2026,10,14,14,57,00); dur Dptx=120;

            gpsDpty=utc2gps(2026,10,14,15,04,10); dur Dpty=120;

It is visible that the -20 configuration shows a lower coupling of DIFFp TX (Figure 1)

After that we tried a SRCL set up to -30 but it failed the LN3 aligned state transition (a 3.5 Hz oscillation on MICH - most probably low gain, which is surprising since we let the servos to adjust the gain in the SRCL set increase and to stabilize).

We have then reached the LN3 aligned configuration and performed a SRCL detune to check the sensitivity (to be able to compare with the one that will be achieved with the diaphragm)

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Comments to this report:
mwas - 12:00 Monday 19 October 2026 (69949) Print this report

Laser frequency noise has been a major limitations during these measurements, making any conclusions on the effect on the BNS range in general, or on the HOM noise (mystery noise) difficult to make. The higher laser frequency noise is due to something that happened on Oct 7 around 19:00 UTC and above 300Hz due to the more noisy LNFS: https://logbook.virgo-gw.eu/virgo/?r=69929.

Figure 1 shows a noise budget in the starting tuned SR configuration. The fitted HOM noise level is comparable to the case of January, when the first stage of SR diaphragm was already installed, for the times when the etalon effect was making the B1p higher. It is unclear in what position the etalon effect was on Oct 14. The estimated frequency noise contribution is comparable to the HOM noise, so the fitted level of HOM noise depends strongly on whether that frequency noise estimation is correct or not.

Figure 2 shows the SR detuned configuration. The frequency noise estimation decreases, because the coupling was lower by a factor ~2 in detuned SR. The estimated HOM noise also decreases by about 20%, which is less of decrease than what was measured in March with the SR diaphragm. It is hard to draw a conclusion from that, as it may be due to many reasons. A tentative conclusion is that the SR detuning is not dramatically better at reducing the HOM noise without the SR diaphragm. At low frequency the sensitivity is much worse, because this data is in LN2 (with many dither lines on), unlike the figure 1 in LN3. 

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