This morning between 8:00 and 10:25 UTC I slightly changed the position of the central crop in B1p camera image, in order to better catch the central spot. See previous crop in first attachment (in LN3), current one in second attachment (in arms locked)

We started at 14:00 UTC and to not interfere with the ITF and risk to unlock, we manually injected the SQZ. 

• System initial status: ITF LOW NOISE 3; SQZ_MAIN @SQZ_Locked_NO_FC

• 14:28:26 AA loop engaged

We performed a first CC phase scan (Fig. 1-2): 

• 14:39 CC scan started DCP = 195 Hz

• 15:02:21 CC scan ended (fast shutter closed), ITF in shot

• 15:26:15 fast shutter opened

• 15:36:22 ITF in SQZ phi0 = 0.65 rad

• 15:39:19 SQB1 TX angle set to 300 (initially at 310)

• 16:42:58 SQB1 X position set to -3200 (original position)

• 16:47:58 ITF in SQZ

After the first CC scan we realized the magnitude was similar to the last shift (entry #64094) even though there should be a higher level of injected SQZ. We decided to try to improve the magnitude by acting on SQB1_TX and SQB1_X DOFs. We worked on this from 15:39:19 to 16:42:58 UTC. Finally we left SQB1_X as it was and we changed the set point of SQB1_TX from 310 to 330 urad. The fluctuations on the magnitude are reduced (Fig. 3).

We then performed another phase scan (Fig. 4-5)

• 17:05 CC scan started DCP = 195 Hz

• 17:28 CC scan ended (fast shutter closed), ITF in shot

After the second CC scan we realized the SQZ angle is actually around 1.05 rad, so we decided to repeat the SQZ mode acquisition.

• 17:39:53 fast shutter opened, ITF in SQZ phi0 = 1.05 rad

• 17:49:08 CC loop engaged, gain=75000 (glitch between 17:54:50 and 17:56:14)

• 17:58:54 ITF in ASQZ phi0 = 2.75 rad

We were able to get another CC phase scan with DCP setpoint at 225 Hz (Fig. 6-7). Fig. 8 reports also the shot SQZ and ASQZ acquisitions made after the scan.

• 18:11 DCP value set to 225 Hz

• 18:44:11 ITF in shot

• 18:46: CC scan started DCP = 225 Hz

• 19:09 CC scan ended

• 19:15:07 ITF in ASQZ phi0 = 2.7 rad

• 19:22:48 ITF in SQZ phi0 = 0.95 rad

• 19:28 DCP value set back to 195 Hz

This morning I slightly tweaked the alignment of the optics showed in entry 56246 inside the AEI sqzueezer board, in order to improve the MZI contrast and the pump beam alignment on the OPA. 

I worked on MZI between 7:43 and 7:47 UTC, and on OPA between 7:52 and 8:19 UTC.

The MZI at the beginning was already quite high, and I could only improve it marginally  - from 68% to 73%, see first attached plot.

After tweaking the pump beam alignment, the OPA peak trasmission improved by about 5% (see second attached plot) and the main HOM in OPA scan was reduced by a comparable amount (see third attached plot).

SHIFT GOAL:

The goal was to perform several CC scan at different SR angle. Nevertheless, due the not optimum condition of the MZ and OPA (waiting for the retuning) we decid to perform a couple of scan without changing the SR angle to check the SQZ alignment-

SHIFT activity:

We started at  16:00 UTC and to not interfere with the ITF and risk to unlock, we manually injected the SQZ. 

• System initial status: ITF LOW NOISE 3; SQZ_MAIN @SQZ_Locked_NO_FC

• 16:10:39 EQB1  fast shutter opened (process: SQZ_ctrl/shutter ‘open’)

After that the CC_loop was engaged and then  the AA (process: SQZ_PLL_AA/ Matrix and Filter ‘AA dither enable’).

• started a CC 4MHz phase scan @ 16:52. 

  • time per point = 6 s

  • - initial phase  = 20 degree

  • - final phase = 230 degree

  • - phase steps = 1 degree

  • - random phase = False

  • - Closed shutter sleep time = 120 s

  • - AA closed on dither = False

  • - cc loop gain   = 75000.0

N.B: No glitch during the scan, it was just before and after (see plot)

In meantime Henning checked the MZ and changed lowered again the offset at 0.035 V due to the humidity  misalignment that spoiled the contrast and induced the set point near the actuator saturation. After the Henning action we redid the scan. 

We stopped at 19.25 after that some data have been collected at the CC phase value corresponding to the SQZ and ASQZ, or better to the minimum and maximum values in the BRMS range @210 MHz and in other ranges.

Further data  and plot in the following comments.

Today we recovered SQB2 and FLT.

Sunday Morning we realized that SQB2_MotSwitch was not recheable anymore. This morning Fiodor switched off and on the motor crate. Then I restarted the VPM process and this restored the communication

Today Matteo recoverd both the vertical and the horizontal loops of SQB2

After that also the filter cavity has been recovered

Today, at 15.06 UTC, the WI chiller set point has been decreased from 19.00 C to 18.98 C. The BCK chiller set point has been changed accordingly. 
The effect was significat: the total power emitted by the laser increased  but it altered the polarization. In agreement with Fiodor, to avoid some instability problems related to the power change, the step has been reverted. Thus, at 15.28 UTC, the WI chiller set point (and that of the BCK one) has been increased back to 19.00 C. Then,  Francesco alerted me of the TCS DMS flags which were red. So, with ITF in down, I checked all CO2 powers restoring the nominal ones.  This activity has been completed at 16.47 UTC. Now, the laser WP came back to the pre-step condition (see attached figure).

The oscillations in range are partly due to changes in the optical gain, see first plot. But not only: oscillations in phase with Etalon temperature are also visible in the BRMS of B1 power around 100 Hz; the second attached plot compares the BRMS around 100 Hz in strain units and in power units: oscillations in Hrec BRMS around 100 Hz are only a factor ~2 higher than oscillations on B1 BRMS. Here the BRMS in power units is computed as the BRMS in strain units times L*OG/sqrt[1+(f/DCP)^2], where L is the lenth of arms, OG is the optical gain, and DCP is the double cavity pole frequency.

Such oscillations become clearly visible after tuning the DCP frequency to ~170 Hz. The third attached plot shows the BNS range vs DCP frequency. 170 Hz DCP might not optimise the BNS range, and it might correspond to larger fluctuations.

Yesterday morning we noticed a strange behavior of the Etalon loop, both on NI (fig. 1) and WI (fig. 2) tower, probably due to the restart of some ACL servers that generate the signal for the control loop while the loop was still ON.  

In particular, the loop seemed to have an inverted sign as it was heating the tower when the RH probe temperature was above the setting point. A stop/start of both loops (NI: 15h53m-UTC; WI: 17h57m-UTC) seemed to solve the problem (see fig. 3). However, as observed by Michal, it is possible that this huge and fast change of temperature of the two towers (fig. 4) had an effect on the lock stability and on the sensitivity.

Due to the slow control of the loop, the effect of the issue were visible with a huge delay; we then decided to postpone the start of the etalon scan, expected for the beginning of the ER. From the data of this morning, I would suggest not to start the scan before the end of the day in order to start with a stable initial state.

Beside, it is worth to notice also a strange behavior of the NI RH temperature probe. In figure 5 we can observe that after the lost of data due to the server restart of tuesday morning (between 7:10 and 8:30 UTC), the temperature read on the NI RH did a jump (and the same did the NI etalon err signal), while on the WI its behavior was as expected.  

The BNS range has been slowly oscillating on daily time scale in recent days. The attached plot shows BNS range and Etalon temperature over time from March 3rd to this morning. BNS range seems to nicely follow the Etalon temperatures, in particular when NI and WI temperature oscillate in phase.

The SRCL set control was disabled in LN3 since last Wednesday, and enabled again yesterday, FIg1. With SRCL set control disabled, the SR mirror TY angle drifts much more to keep the DCP frequency, Fig2.

With SRCL set disabled the DARM response is likely less accurate, see drift in low frequency Hrec TF phase during the night between 7/3 and 8/3.

After enabling again the SRCL set control the Hrec noise is much more correlated with SR mirror angle, see Fig3. Comparing data with SRCL set enabled yesterday and before Wednesday, Fig3 and Fig1, it seems that the correlation of sensitivity and SR angle fluctuations are now much higher. This might be due to a worse SNR in the SCRL set control.

A new noise bump between 60 and 70 Hz popped up recently and is now limiting the BNS range, see first attached plot.

The fluctuations in SSFS coupling and optical gain are larger since yesterday afternoon, see second attached picture. Is this due to the resumed control on the optical spring?

The first NE RH step yesterday improved the fringe contrast (power on B1p decreased), the second step instead worsened the fringe contrast  (power on B1p increased). See again second attached picture.

The attached plot compares the Hrec sensitivity with (purple) and without (blue) squeezing injected, the main difference is below 250 Hz, as expected from the extra losses above the DCP frequency due to SR misalignment.

The CC scan confirms that the phase of injected squeezing was correct, i.e. the drif in Hrec sensitivity over night was due to something else.

A line at 40.04 Hz popped up in the sensitivity curve since after the Tuesday maintenance on 27/2.

During the afternoon, the ITF unlocked sistematically trying to lock the OMC. The fringe appeared too bright [0.22-0.28 mW].

At 17.16 UTC, the SR RH power has been settled at 23.13 V (P=7 W) form 6.6 W.

During the SR RH thermal transient, we tried to decrease B1p by acting on the DAS. 

The effects on the fringe were vey poor, thus at the end of the shift, we reset the DAS powers at the nominal ones.

This afternoon, we checked the effect of the SR RH power decrease (63456).

The ITF locked in LN3 at 15.50 UTC and unlocked at 17.06 UTC.

The ITF relocked at LN2 at 17.40 UTC but it has been manually unlocked at 18.11 UTC because of an earthquake compromising the lock.

The ITF relocked at LN2 at 19.47 UTC. We left the ITF in this state for approximately 30 minutes to observe the behavior of the DCP before proceeding to LN3.
During this lock, I also tried to investigate some changes of NI DAS powers to improve the CD but the results were confusing and, despite this, we never reached the BNS range of this night lock.

So, discussing also with Fiodor, at 22.13 UTC, in LN3, we settled 6 W (21.41 V) on the SR RH, which is halfway between 5 W and 7 W.

The main ITF signals since yesterday morning are shown in the attached figure.

Comparing the Hrec specta for the two periods, the change in sensitivity is due to a small (3÷4%) change in the level of broadband noise, corresponding to the ~3.5% change in optical gain; plus the enhancement of some spectral lines, e.g. at 111 Hz, 120 Hz, 122.5 Hz, 133 Hz etc. See first picture.

Indeed the Hrec BRMS in 110÷133 Hz changed by about 10%, and the DARM BRMS in 110÷120 Hz changed by about 8% between 16:10 UTC and 16:30 UTC, see first and last plots in the second picture. But the Hrec and DARM BRMS  around 220 Hz, where no spectral lines were enhanced, did not change significantly, see traces in red.

The table mentioned in the text was missing...

Though not explicitly mentioned, the computation for the optical gain included the effect of double cavity pole frequency: In particular the Hrec BRMS noise was rescaled in Fig2 and FIg7÷11 according to OG/(1-(f/DCP)^2), where OG is Hrec_ORgain_meancavities and DCP is Hrec_ORpole_meancavities.

In attachment the BRMS noise in Hrec and rescaled to B1 power for two different frequency intervals (110÷133 Hz and 110÷120 Hz). The rescaled noise levels in B1 power are similar, though the noise levels in Hrec are not, the difference being due to the slightly different DARM response between 100 Hz and 200 Hz. 

The attached table contains the data shown in the various plots; the columns are respectively: 

1) GPS time

2) contrast defect (B1p power)

3) Hrec BRMS in 110÷133 Hz

4) DCP frequency

5) low-frequency optical gain

6) SSFS coupling to DARM

7) BRMS noise on B1 power in 110÷133 Hz

8) same as 7) but with contribution from SSFS coupling subtracted

About the impact of SR misalignment on SSFS coupling, this is evident in FIg3. With SR aligned the SSFS coupling is usually larger. However, rescaling Hrec noise for the optical gain does not remove the contribution of SSFS noise; and subtracting the (usually small) residual contribution from SSFS coupling after rescaling seems correct: there are data with high optical gain and high SSFS coupling (e.g. during the Etalon tuning on 29/12), and data with low optical gain and low SSFS coupling, see again Fig3.

This is clear in the attached plots FIg1a÷FIg11a, which are the same as in the previous entry, with two different colours to show data with DCP frequency above 300 Hz (red) and below 300 Hz (blue). 

The Pearson correlation coefficient r from data in the plots is:

r=-0.47 for Hrec BRMS noise and optical gain

r=0.53 for Hrec BRMS noise and contrast defect

r=0.56 for SSFS coupling and contrast defect

r=0.28 for B1 power BRMS noise and contrast defect

r=0.07 for B1 power BRMS noise and contrast defect, after subtracting SSFS coupling contribution from B1 power BRMS

The recent changes in input laser power to the ITF allow to better estimate the power-dependent losses in the arms.

Fig1: measured power in the arms (average of LSC_B7_DC and LSC_B8_DC, rescaled with 5.2 ppm from optchar calibration for NE transmission) and input power to the interferometer (INJ_ITF_input); data since January 2023, each point is an average over 1000 s. The input power was changed from ~31.5 W to ~24 W on February 2023, then reduced to ~11 W in October 2023, increased to ~14 W in January 2024, and to ~16.5 W last week. In general the circulating power in the arms followed the trend in input power, but there was a significant increase after the replacement of the NE mirror in June 2023.

Fig2: carrier recycling gain from the data of Fig1; the operations in vacuum chambers for magnet fixing on WI and NE did not produce significant changes in arm cavity losses, despite the appearance of a new point absorber on the WI mirror.

Fig3: mean arm cavity round-trip losses vs time, from the data of Fig1.

Fig4: arm cavity round-trip losses vs. input power; data with old and new NE mirror are shown with differen colors, with linear fits shown in solid lines; the accuracy on the power-dependent part of the round-trip losses, i.e. the linear slope, is limited by the maximum input power with new NE mirror, and by the small range of input powers with the old NE mirror. Moreover, the circulating power is also affected by ETM curvature thermal tuning (see Fig1÷Fig3). The accuracy could be improved with a precise independent estimate of the cold RTL from arms locking. The slope with new NE mirror (0.73 +/- 0.05 ppm/W) is about 25% lower than with old NE mirror (1.00 +/- 0.08 ppm/W).

Fig5: power in the arms vs input power. Again data with old and new NE mirror are shown with differen colors. Solid lines are from linear fits of round-trip losses (see Fig4); the black line represents an ideal case with no power-dependent losses, and 75 ppm cold RTL.

Fig6: same as Fig5 with larger span, showing the consequence of power-dependent losses on the extrapolated circulating power for input powers up to 80 W.

In summary:

- thermal effects in the arms will not allow to reach 250 W circulating power with 80 W input power.

- Replacing a single test mass out of 4 reduced the thermal losses by 1/4.

- With the set of O2 test masses, the circulating power with 80 W input power would not have reached 200 kW; replacing NE mirror, the maximum circulating power with 80 W input power would be less than 250 W.

- Point absorbers by contamination, e.g. appeared on WI mirror after the intervention on May 2023, have much lower impact on intra-cavity losses.

Summary:

1) Hrec noise in the bucket scales inversely with optical gain; the noise in W/sqrt(Hz) is independent on optical gain;

2) the coupling of frequency noise in DARM is correlated with contrast defect;

3) when subtracting the (usually small) contribution from SSFS, Hrec noise in the bucket is basically independent on contrast defect.  
 

The attached plots contain all data from December and January at 12 W input power; each point is an average over 100 s.

- Hrec noise is the BRMS of Hrec in the band 110÷133 Hz

- optical gain is Hrec_ORgain_meancavities

- noise on B1 is the noise in power units, i.e. Hrec BRMS multiplied by the optical gain in W/m and by the arms lenght in m.

- SSFS coupling (or CMRF) is the magnitude of the 227 Hz line in DARM

- contrast defect is LSC_B1p_DC

Fig1: Hrec noise vs optical gain clearly shows inverse scaling;

Fig2: noise on B1 vs optical gain is basically constant, although it sligtly increases for small values of the optical gain;

Fig3: CMRF vs optical gain also shows some inverse scaling;

Fig4: contrast defect vs optical gain clearly shows inverse scaling;

Fig5: contrast defect vs CMRF shows clear linear correlation: poor contrast defect usually implies poor CMRF; this might be partly due to the BS CMRF loop not working in LN2;

Fig6:  Hrec noise vs contrast defect; a linear correlation is clearly visible. 

Fig7: noise on B1 vs contrast defect: some linear correlation is also visible, though smaller than on Hrec.

Fig8: noise on B1 after subtracting the contribution of SSFS coupling, as computed from Fig7

Fig9: noise on B1 vs contrast defect shows some linear correlation

Fig10: after subtracting the contribution from SSFS coupling, noise on B1 is independend on contrast defect

Fig11: after subtracting the contribution from SSFS coupling, the smal residyual dependence of noise on B1 from optical gain (Fig2) disappears too

In conclusion, HOMs might affect the CMRF for frequency noise, but have no effect on the mystery noise. Frequency noise is not contributing significantly with good CMRF, but CMRF fluctuations produce not negligible contribution.

At 18:12 UTC an automatic squeezing injection started, as the SQZ node was still in FIS injection after the last weekend. This however was not triggered by the LN2 transition, which occurred about 90 minutes before, see plot.

The slow shutter on SQB1 got stuck during the ER. This morning the VAC team vented the tower, Francesco opened the controls for bench and suspension, and Fiodor had a direct inspection. After gently loosing the blocking screw of the micrometric actuator, the shutter could be moved both manually and by the motor across the full range, see pictures corresponding to roughly open and closed position in attachment. The current position is closed, TBC in the presence of a beam.

On Fiodor's request, we conducted some cross-check simulations to understand the behavior of the optical gain of DARM as a function of the angle of the SRM.

We begin with a brief description of our interpretation of the effect of SRM tilt on the transfer function "DARM motion -> DARM sensor (B1)," conveniently referred to as DARM TF. In Fig.1, the DARM TF is shown for three different optical configurations:
1) DRMI - Dual Recycled Michelson Interferometer, the currently used scheme.
2) PRMI - Power Recycled Michelson Interferometer. In this case, the SRM has been removed from the model.
3) SRM fully misaligned - This configuration is a functional equivalent of when the SRM is placed in the "parking" position, i.e., it is widely misaligned. In this configuration, the light reflected by the SRM is lost. Therefore, this configuration was emulated by setting the mirror losses to the mirror reflectivity value (L=0.6), reflectivity to zero (R=0), and leaving transmissivity unchanged (T=0.4).
The optical gains are all normalized to the DC gain (at 10 Hz) of the DRMI curve. 

When the SRM is operated in Resonant Sidebands Extraction conditions, the SRM-Arm optics can be treated as a single cavity with lower finesse than that of the individual arm cavities. Lower finesse corresponds to a broader bandwidth, and for this reason, in Fig.1, the optical pole frequency of the DARM TF is higher for the DRMI curve compared to the PRMI curve. Regarding the optical gain, this is inversely proportional to bandwidth. Therefore, a wider bandwidth corresponds to reduced optical gain. For this reason, the optical gain is higher in the case of PRMI compared to DRMI.

It is interesting to note what happens in the "SRM fully misaligned" case. In this case, the DARM TF is identical to that of PRMI, with the optical gain reduced by a factor R. For clarity:
G2 = R  G1
where G1 is the optical gain in the PRMI case, and G2 is the optical gain in the "SRM fully misaligned" case. This is expected as the signal 'reflected' from the SRM is lost in this configuration. Anyway, the important point here is that the DARM TF for the case of complete misalignment is analogous to that of a PRMI.

For smaller angles than the "parking" one, our interpretation is that the system is in an intermediate configuration between "SRM fully misaligned"  and DRMI. Tilting the SRM decreases the equivalent reflectivity of the SRM and increases equivalent losses. Therefore, as the SRM angle increases, the "signal recycling" effect is increasingly lost, and the DARM TF tends towards that of the "SRM fully misaligned” case (which is identical to a PRMI-like response).

This interpretation seems to be compatible with experimental observations and the results of our simulations. We calculated the DC optical gain of the DARM TF with Finesse varying the SRM angle; the result is shown in Fig2, where we display the DC gain as a function of the SRM angle. The DC gain is calculated at a frequency of 10Hz. The result appears to be compatible with experimental data.


The purpose of the simulation was to investigate the behavior of the DC gain of the DARM TF as a function of the misalignment of the SRM. In the past, the behavior of the DCP in relation to misalignment had been studied. The results can be found here: https://logbook.virgo-gw.eu/virgo/?r=55862

- The Finesse model used is the common Kat file for Virgo, which is available at: https://git.ligo.org/finesse/finesse-virgo/-/blob/main/src/finesse_virgo/katscript/00_virgo_common_file.kat
- Experimental data is referenced to GPS time: 1386088818

After the lock around 13 UTC, an excess broadband noise appeared below 200 Hz, which was then found to be due to the green beam still present at NE because the flip mirror failed to move after lock acquisition.

After flipping the mirror to remove green beam, the excess noise disappeared, see plot.