The attached plot shows the coherence between B1_DC and the input beam jitter as measured by the B2_QD_8MHz signals (first plot).

High coherence is visible up to 10 Hz especially with the vertical jitter.

At higher frequency no measurement is possible since the quadrants are limited by their sensing noise.

A similar result can be found using B4_DQ_6MHz but in this case the coherence is higher (second plot).

I looked at the data taken yesterday between 9h00 and 9h30 when the SRCL offset was changed.

The SRCL Input decreased from about 24 to about 14 (top right plot), so a little less than a factor of 2.

The change in the SRCL offset determined an increase in the CMRF (third plot from the left on the top).

The change in the SRCL offset also triggered a worstening of the COMMp alignment (two top-left plots) which is known to cause an increase of the CMRF (see https://logbook.virgo-gw.eu/virgo/?r=59508).

The interferometer unlocked at the end of this test, so probably, the working point was not very stable due to the change in the DARM TF that a change in SRCL offset induced.

It would be good to repeat this test while keeping COMMp aligned and making sure that DARM loop is still doing its job properly.

The change in the sidebands unbalance mentioned by Ricardo is due to the change in the 56 MHz modulation frequency (see attached plot).

As expected the change of the modulation frequency changes the ITF locking point.

Indeed one can see also changes in SRCL input and in SSFS_Err_Q (an out of loop sensor of a mixture of MICH and PRCL, the mixture depending on the B4_6MHz demodulation phase).

It might be good to check that the FmodErr tunings that are done sometime at the beginning of the lock are not providing us with different locking points.

Very interesting measurements!

There is one thing to keep in mind. The BPC is measuring the beam jitter at the BPC quadrants on EIB. The beam jitter at the IMC input might be larger since the beam is travelling in air from the BPC quadrants to the IB tower input.

Maybe a reshaping of the IMC AA loops filters can reduce the noise at 100Hz.

This said, there is still one thing that I do not understand. The IMC AA WFS is measuring the misalignement between the noisy in air input beam and the silent suspended under vacuum IMC. Three of the four IMC AA WFS signals are correctly used to act on the BPC i.e. in the noisy beam. But the fourth acts on the IMC payload. Wouldn't be possible to use also the fourth to act on the last BPC degree of freedom i.e. BPC_Y? 

Given that the two MC mirror angular degrees of freedom can be controlled using a dithering like system that keeps the beam in the center of the MC mirror, we would then be left with IB_TX. Can this remain under local control?

If not, can a blending of the fourth IMC AA WFS between IB_TX (at the IB suspension resonant frequencies) and BPC_Y (at DC and at high frequency) be the solution?

The fact that the laser noise (frequency and intensity) coupling into DARM changes when the 56 MHz modulation amplitude changes may mean that the ITF working point is changing. This can be due to the fact that there are control loops using the 56 MHz sidebands whose error signals are not kept at zero by the loop because of offsets added on the error points. Two examples are SRCL and DIFFp. When the 56 MHz sideband amplitude is changed the optical gain of the 56 MHz error signals change. As a consequence the ITF working point that null the imposed offsets change.

Indeed the locking point has changed as one can see from the sidebands balance (see attached plot).

It has changed since Tuesday PM.

The sidebands are better balanced on B1p and worst balanced on B4.

It might be good to investigate if there a varying offset on the MICH error signal.

The responsible for the 19:17 UTC jump has been identified: the new filter used for phase noise subtraction was not removing enough the DC component (see entry 59652) leading to offset in the MICH error signal. This filter was changed exactely when the jump was observed (at 19:17 the standard filter was restored, see entry 59644). The new filter was responsible for adding unwanted offsets on MICH and SRCL. These offsets were creating the observed change of working point.

The figure attached shows the beams shape on B4 last week at four different moments. These are shown together with the COMMp WFS i.e. ASC_B4_QD2_6MHz_H and ASC_B4_QD1_6MHz_V during the week.

The four beam images are taken at the following moments.

- The beams shape on B4 before the COMMp change on March 29th at 13h00 UTC. These are the beams as they have been for two weks since the first long lock was achieved.

- The beams shape on B4 after the COMMp change on March the 29t at 15h30 UTC.

- The beam shape on B4 on March 31st at 11h00 UTC when we had the best sensitivity, the best CMRF and the COMMp closer to zero.

- The beams shape on B4 on Apr the 2nd at 14h00 UTC after the ASC shift and before the TCS change.

The bottom line is that the best beams shape was on March 31st at 11h00 UTC when we had the best sensitivity, the best CMRF and the COMMp closer to the optimal alignment.

The goal of the afternoon shift was to scan the OMC in CARM_NULL_1F over the side band modes of order 0, 1, 2 during the thermal transient, to measure the contrast defect and check the impact of the DARM offset on the sensitivity. Unfortunately only the first activity (OMC scan during the thermal transient) could be completed due to the bad weather conditions and the difficulties the lock the interferometer.

Noticing that the B1p beam was slightly off centered vertically on the camera, we used fixed setpoits for SDB1 at 16h37m33 UTC > this allowed us to recenter the B1p beam on the camera, but this did not make any difference on the lock acquisition.

CITF locked in 1F at 17h10 utc. We stay in this state for a few minutes to let the ITF realign itself.

The ITF locked in CARM_NULL_1F at 17h25 utc.

Waiting for B1p to become darker. Opening OMC shutter at 17h30m00 utc.

From a past OMC scan (Fig.1), we estimated that the first side band TEM00 should be located at about 22.54 deg (Fig.2).

We first adjust the OMC temperature to be ~50 mK  below the first 80 MHz SB TEM00, that is to say at 22.49 deg. We start an OMC scan with an amplitude of 0.2 and a frequency of 0.001 Hz.

Scan started at 17h30m52 UTC

Adjusting scan amplitude at 0.17 after the first up-rising scan.

Increasing integration time of camera at 17h47m25 utc.

LSB 84MHz  TEM00: 17h50m20

LSB 56MHz  TEM00: 17h50m40

carrier TEM00 17h51m25

USB 56 MHz TEM00 17h52m05

USB 84 MHz USB TEM00 17h52m22

We note about 35 mK of difference in temperature position of the modes between incerasing and decreasing scan.

Scan stopped at 18h35m10 utc.

The overall scans during the thermal transient are shown on Fig.3. Fig.4 shows a zoom on the first scan with a preliminary identification of the modes (to be confirmed with deeper analysis).

We locked the OMC and then performed the hand-off of DARM to B1_PD3 at 18h47m28 utc (Fig.5). Unfortunately the ITF unlocked at 18h48m02 utc soon after the engagement of DC readout on B1_PD3 (Fig.6). This unlock seems very similar to the one that occurred at the end of the morning shift at 13h38m27utc: https://logbook.virgo-gw.eu/virgo/uploads/59586_1680359343_FIG7_DCreadout_secondUnlock.png

Again we noticed that the DARM_GAIN was increased from 0.070 to 0.077 before the unlock. Thus maybe it would be the case to increase the DARM GAIN in the ITF_LOCK.ini file. To be checked by ISC experts.

After this unlock we did not manage to lock again in CARM_NULL by the end of the shift.

The sideband balance did not came back to the original value.

Moreover the CO2 beam positions have been checked and show no change.

So, the probability that the ON/OFF done on the DAS's on the night between Thu and Fri can be at the origin of the change in the SB balance is reducing.

The other thing that changed in the night between Thu and Fri is the COMMp alignment (see attached plot).

Making the difference between the two is always a little difficult because on one hand a DAS change will affect COMMp but on the other hand COMMp can change by itself without any change of DAS.

Before the shut down we used to control COMMp by closing the loop on the NE using the B4_6MHz_ QD2 Wave Front Sensor.

After the shut down this was changed. For the horizontal degree of freedom we continue to use B4_6MHz_QD2_H while for the vertical we moved to B4_QD1_6MHz_V.

This change was driven by the observation that the response to the vertical COMMp degree of freedom was clearer on the B4_QD1_6MHz_V.

The explanation for this change was the retuning of the B4 telescope done during the shut down.

Last Wednesday the COMMp was aligned again. This was done using the zero of the B4_6MHz_QD2 (59492).

This operation improved the CMRF and the sidebands shape on B4. It did not change in a significative way the SB balance on B1p.

On the following day (Thursday) we observed the ITF going through a minimum of the CMRF.

This minimum was coinciding with balanced SB on the dark fringe (59537)

On Friday and also today the SB were balanced and the CMRF is better than what we use to have.

One thing that is changing again is the aligment of the COMMp.

The natural variation of the COMMp alignmnent (not uder control) pushed the B4_QD2_6MHz_V on the opposite side compared to the position we started with on Wednesday i.e. towards positive values (see attached plot). Please note that I am using the ASC signals not the SPRB ones (there is different demodulation phase in the middle).

At the same time the B4_QD1_6MHz_V is going to zero and there is an interesting correspondance between good CMRF, balanced sidebands on the dark fringe and the zero of this signal (see the attached plot).

The bottom line is that probably the choice that was made on January to control COMMp using B4_QD1_6MHz_V for the TX control and B4_QD2_6MHz_H for the TY was the good one.

I checked the ITF noise after the cable had been disconnected.

The peak height did not change (see attached plot).

This rules out the possibility that the magnet resonance is driven by some current noise in the coil.

This is a consequence of the WI DAS switch OFF of tonight.

As a consequence we herited a different ITF thermal state this morning.

The SB are better balanced and the CMRF fluctuations are smaller (see attached Figure).

As a consequence the BNS range raised to 19 Mpc.

The minimum in the CMRF corresponds to the time the power of the two sidebands on B1p cross i.e. they are equal (see the two attached Figures).

This is the time at which the effect of the thermal lens in the two input mirrors are similar.

This underlines the importance of having balanced sidebands powers.

We used to have better sidebands balance in October/November 2022.

It should be possible to improve this balance acting on the DAS differential power.

I looked at the coherence between the variation in the misalignment of COMMp and the ITF CMRF during the long lock done last week on March the 21st.

The CMRF is evaluated looking at the SSFS line in the DARM signal.

The result is shown in the first Figure attached to this entry.

The coherence is non negligible indeed.

The second plot shows the coherence between the same COMMp misalignement and the power in the north cavity (B7_DC).

Also in this case there is a non negligible coherence.

The DARM noise contains several structures at high frequency (above 1 kHz). The spectrogram collected last week shows that this noise changes considerably in time (see Figure 1).

Figure 2 shows the evolution of the COMMp misalignment in the horizontal direction (bottom plot) together with B1_DC on the top (showing the period the ITF was locked on B1) and the sidebands power on B1p in the middle.

When the B4_QD2_6MHz_H signal increases in absolute value, the noise at high frequency increases.

Figure 3 is a superposition of Figure 1 and Figure 2 to show the coincidence between the two effects.

During the lock of March 18th the horizontal misalignmnent was artificially increased in the positive direction. The noise at high frequency increased during that period.

During the lock of March the 21st there is a progressive increase of COMMp misalignment in the negative direction. This comes with an increase of the noise structures at high frequency.

There is a visible misalignment during the end of the week and this makes the structures in the DARM noise at high frequency quite visible.

A similar effect was visible in the period end of January beginning of February (see Figure 4).

On January the 27th the COMMp was better aligned and the noise at high frequenct was lower. Early February the aligmnent was worst and the noise at high frequency was higher.

The fact that when the COMMp is more misaligned the noise, probably from the input port, reaches more easily the dark port is expected.

Keeping COMMp close to zero should help.

It might be worth noticing that the fact that a tilt of the north arm axis in the vertical direction produces a relatively large change on the B4 WFS in the horizontal direction is not normal.

I see two possible explanations:

1) When we move the beam on the NI vertically the illumination of the point absorbers changes. As a consequence the distribution of the heat source on the mirror surface changes as well. As a consequence the thermal gradient inside the mirror substrate can change also in the horizontal direction and this will be seen by the WFS on B4 as a misalignmnent in the H direction.

2) The COMMp is so misaligned that we have couplings between the H and V degrees of freedom on B4. Indeed we rarely have had the B4 WFS as misaligned as it is now.

The shape of the 56 MHz sidebands is so deformed that I do not know what to choose between 1) and 2). Maybe we have both.

This said, it is worth reminding that the COMMp misalignemnt is not the only possible source of sidebands unbalance (and of SRCL offset).

As an example, in January we had a good COMMp alignmnent but bad sideband unbalance,

A differential lens in the input mirrors can also create sideband unbalance.

Maybe also an offset on MICH can create a sidebands unbalance (the MICH signal originates from the sideband unbalace produced by a MICH motion).

P.S. By the way, the shape of the B4 beam when the COMMp sensor (B4 WFS) has been brought closer to zero (Figure 1) and when it was brought back to the positon in which we are operating the ITF in these days (Figure 2) are shown in the two figures attached to this entry. No comments are needed.

I looked at the data collected one week ago (on Mar 18th) while Romain was changing the COMMp alignment.

The plot shows 1.5 days of data around the test (done in the evening of Mar the 18th).

During the test the COMMp alignment was changed as it is visible in the two bottom plots.

The vertical misalignment as seen by the B4 QD2 WFS decreases while it increases in the horizontal direction.

The interesting plot is the top right one.

This shows the balance of the sidebands on the B1p phase camera.

Usually the sidebands balance is bad and the LSB's are larger then the USB's (this is true for both the 56 MHz SB and the 6 MHz SB).

During the test the SB's balance is inverted.

This means that somewhere in the middle it is possible to balance the two sidebands.

Between Wednesday evening and Thursday evening a diff DAS experiment was made.

As reported in https://logbook.virgo-gw.eu/virgo/?r=59274 the NI DAS was increased by 10% and WI DAS was decreased by 10% on Wed around 19h (the CH were not changed).

Then on Thu around 13h the opposite step was made (https://logbook.virgo-gw.eu/virgo/?r=59288)

The attached figure shows the effect on various ITF signals.

The bottom right plot is the power of the sidebands on B1p.

Initially the 56 MHz sidebands power are different by a factor of three.

At the end it is same (there is no hysteresis).

In the middle they differ by about 20%.

Jumps have been reported when the two sidebands were closer (https://logbook.virgo-gw.eu/virgo/?r=59289)

One possibility is that the differential DAS step came with some common DAS variation and that this change in the common curvature have brought the PRC cavity closer to the bistability.

If so, the solution could be to add a bit of curvature with the PR CHROCC.

I tried to see if the centering of the input beam on the input mirrors changed during this experiment but the cavity locks in single arms were a little short to make a realiable measurement.

The figure shows various interferometer signals in the last half a day.

The 56MHz sidebands unbalance on B1p (bottom-right plot) is smaller now compared to yesterday evening.

Something has changed in the working point.

Indeed the inversion in the SB 56 MHz imbalance on B1p between the lock on March 5th and the lock on March 6th is surprising.

March 5th is the last time we had relatively round SB's.

On that day the 56 MHz USB SB on B1p was larger than the LSB. Since then it is the opposite.

Another difference is the SR alignment. On March 5th SR was under local control. On March 6th it was driven by the maximization of the DARM line.

It might be good to check that the SR alignmnent control with the maximization of the DARM line is still working fine.

Since the maximization of the DARM line uses the DARM signals built from the 56 MHz SB's, the SR alignmment loop might suffer from the 56 MHz SB imbalance.

This evening we locked the OMC in dark fringe.

ITF locked in CARM_NULL_1F at ~20h05 utc.

Enable AATuning servo at 20h20m53 UTC.

SDB1 high band width control enabled at 20h21m56 UTC.

Putting DARM offset 0.20 at 20h31m20 utc.

OMC scan at 20h34 utc:

Getting 20 uW on order 4, 13 uW on order 3, 8 uW on order 2, 10 uW On order 1, 5 uW on TEM00.

DARM offset increased at 0.3 at 20h43m25 utc > we found 3 uW on TEM00.

DARM offset at 0.5 at 20h49m40 utc > less than 1 uW on TEM00.

DARM offset at 0.0 at 20h54 utc > 22 uW on TEM00.

We keep a DARM offset of 0.0 to lock the OMC. We update the parameter darm_offset_rf = 0.0 in the ITF_LOCK.ini file.

OMC locked at 21h18 utc (see Fig.1). ITF unlocked at 21h18m29 utc (see Fig.2). This seemed to be caused by an oscillation of the DARM loop, presumably because the DARM gain was left too high in the ITF_LOCK.ini file. Fabio and Camilla reduced it from 0.07 to 0.04 (parameter darm_gain).

ITF locked again in CARM_NULL at 21h55 utc.

Enable AATuning servo at  21h56m32 UTC.

Enable high bandwidth control of SDB1_LC at 22h04m40 UTC

OMC locked at 22h10 utc (Fig.3). ITF unlocked at 22h10m37 utc. Again the same kind of unlock with an oscillation in LSC_DARM at about 20 Hz (Fig.4 and Fig.5).

Conclusions:

• We can lock the OMC in dark fringe with a DARM offset of 0.0.
• Despite the fact that the hand-off from RF to DC readout was commented, we systematically unlock while reaching the state LOCKED_OMC_DARM_B1_PD3. Having adjusted the DARM gain did not fix the problem.

The two first plots attached to this elog entry show the sideband shape as seen from the phase camera on Sunday afternoon (Figure 1) before the CO2 beam centering on the NI done on Monday morning (https://logbook.virgo-gw.eu/virgo/?r=59151) and on Monday afternoon (Figure 2) after these changes were made.

The sidebands are now more aberrated both on B4 and on B1p.

The third and fourth plots show the same but in CITF.

In this case some difference is visible only on the 56 MHz B1p images (also in this case it is better on Sunday).

The main variations are coincident with the activties on the PSL bench.

The WE SAT problem is back (see figure).

It would be good to have a monitoring that tells us when the problem is there.