[Kenta, Komori, M. Honjo, Michimura]

To estimate losses in the IFO, we did X arm and Y arm cavity scan to measure the modematching and modultion index.
Modematching is 97(1)% for Xarm and 94(1)% for Yarm for single arm case (numbers include misalignment).
Modulation index is 0.09(2) for f1 and 0.11(4) for f2, measured at arms, which is a factor of 2 smaller than the previous measurement in 2019.

What we did:
 - Locked Xarm and Yarm each with PRM misaligned.
 - Did a cavity scan using ALS, with the same method in klog #31112 and klog #30825 (scan was done by adding 27000 counts in K1:ALS-SUM_OFS_SLOWOUT_CALI_OFFSET and turning on/off FM1 that has -1 gain with a ramp time of 300 sec to do a linear scan)
 - During the cavity scan, LO frequency was 5.624 369 113 MHz.
 - GPS times of the scan are as follows.

startTime=1428131200  # Yarm scan April 8, 2025, misaligned, slow scan                                                                                               
durTime=350                                                                                                                                                            

startTime=1428130200  # Yarm scan April 8, 2025, aligned, slow scan                                                                                                     
durTime=350

startTime=1428139080  # Xarm scan April 8, 2025, misaligned, slow scan                                                                                                 
durTime=350                                                                                                                                                            

startTime=1428141500  # Xarm scan April 8, 2025, aligned, slow scan                                                                                                    
durTime=350

Result:
 - Attched are time series data during the scan, when the arms are aligned.
 - From (carrier TEM00 power)/(carrier TEM00 power + carrier TEM1 power + carrier TEM2 power), modematching was calculated.
 - Pure modematching calculated with (carrier TEM00 power + carrier TEM1 power)/(carrier TEM00 power + carrier TEM1 power + carrier TEM2 power) is 98.9(1)% for Xarm and 98.4(2)% for Yarm.
 - From 2*sqrt(upper or lower sideband TEM00 power / carrier TEM00 power), modulation index was calculated.
 - Uncertainties presented here is just a rough estimate.

Discussion:
  - As for modulation index, it was measured to be the following in 12 July 2019 (klog #9505), but it is somehow half now. What happened??
Modulation index of f1 @MIF = 0.22(2)
Modulation index of f2 @MIF = 0.23(2)
 - Resolving f1 from TEM1 was still very hard. They are too close (klog #30825).
 - It has been said that there is a 15% loss from BS to OMC DC PD during O4a from DARM shot noise. For this estimate, modematching between IMC output beam and PRFPMI was estimated using the modulation index of 0.2. We might want to re-evaluate using the updated modulation index.

Next:
 - Derive FSR, TMS (g-factor), Finesse from these measurements
 - Measure modematching between IMC output beam and PRFPMI using REFL power PRFPMI locked/unlocked (REFL PDA3 can be used, as this is not affected by HWP rotation during the lock)

I also analyzed the data of the shaker injection at the PR3 beam-duct.

Similar to the ground shaking, the strain spectrum around 80Hz got noisy when the shaker vibrated around 190Hz (and around 173Hz).

At 89 Hz, a liner-like response was found.

I analyzed the data of the shaker injection at PR3 ground.

At the time of the around 192Hz injection, the strain spectrum was noisy.

The projection, by assuming these noises were caused by the injection, was much smaller than the current sensitivity.

Abstract

I ran OLTF measurements before leaving Mozumi yesterday.
The front-end calibration of of MICH and PRCL was updated based on these measurements.
Maybe this is the first update after increasing 10W(?).
Update of parameters was done in 11:34:57 JST.
 

Changes

MICH optical gain (OLTF shows on Fig.1-2)
old: 176.915dB [Oct. 9th]
new: 173.024dB (-36%)

PRCL optical gain (OLTF shows on Fig.3-4)
old: 188.240dB [Oct. 9th]
new: 186.923dB (-14%)

No other changes in Sensing and Actuation functions which has frequency dependent responses.

Note

A template for MICH OLTF (/users/Commissioning/templates/diaggui/MICH/MICH_PRFPMIOBS_OLG.xml) was update as that lower limit of frequency was decreased from 10Hz to 3Hz because frequency range was not so proper for current UGF as shown in Fig.1.

A template for PRCL OLTF (/users/Commissioning/templates/diaggui/PRCL/PRCL_PRFPMIOBS_OLG.xml) was updated as that excitation amplitude was reduced as a factor of 0.1 (0.3 -> 0.03) because SNR of swept sine is close to ~10000 and too loud non-linear noise enhancement can be seen during the measurements. And also, measured TF has strange bias from the OLTF model with this too loud amplitude as shown in Fig.5-6. Some people measured PRCL OLTF several times in recent one month. Was there no same problem? Or did they reduce amplitude manually? BTW, Fig. 3-4 was measured with an amplitude as 0.01. It seemed to be suitable around 40-100Hz. On the other hand, we have poor SNR below 20Hz. So we may need to set frequency dependent amuplitude instead of constant amplitude.

Oscillation of OMC DCPD over 5~10 seconds

• 2025-04-06 02:53:10 UTC
• 2025-04-06 03:09:56 UTC

For these two lockloss, actually Yokozawa-san was changing the filter, as chatted with Yokozawa-san. As a result, the lockloss occurred. So, they are not new phenomena.
The LSC_LOCK guardian state was at WORKING_WITH_OBSERVATION_WITHOUT_LINES (this state was set properly). I missed this information.

I checked the additional 5 lockloss occurred at 4/7. The previous lockloss investigation was posted in klog33288. The plots are summarized in wiki.
One good news is that the lockloss due to the OMC saturation with 40 Hz has occurred much less frequently, thanks to the commissioners' parient work.

1Hz seismic motion made the IMC lockloss

For 2 lockloss, we saw the excess of the seismic motion with 1~10 Hz, which made the oscillation of the IMC length control and made the IMC lockloss.

• 2025-04-07 10:21:44.437500 UTC (765 s) (Fig 1)
• 2025-04-07 15:40:13.562500 UTC (1790 s) (Fig 2)

PR2 and PRM oscillation at 0.6~0.8 Hz

For 1 lockloss (2025-04-07 19:58:49.562500 UTC), there was an oscillation in the PR2 pitch and PRM pitch with 0.6~0.8 Hz (Fig 3 and 4). It started at -30 s of the lockloss. The trigger seems to be the excess of the seismic motion. (Fig5)

Others

For 1 lockloss(2025-04-07 11:53:44.437500 UTC (4084 s)), there was less hint. The OMMT2 TRANS was stable (Fig6). I checked the BPC controls, because this lock was over one hour. The ITMX yaw and ETMX yaw seem to be drifting (Fig7) so that the error signal is away from zero. But,  I'm not confident that this is the lockloss cause.
Just after this lockloss (<1s), the excess of the seismic motion was observed in 1~30Hz frequency bands. The lockloss occurred before this excess (Fig8).
At -18 min of the lockloss, there was the excess at 10~20 Hz (Fig9).

For 1 lockloss at WORKING_WITH_OBSERVATION_WITHOUT_LINES, the excitation was underway. I will let it be.

The REF BRT HEAD temperature started increasing again, while the BRT4/3 temperatures kept decreasing. Finally, the BRT3 temperature is now below the critical temperature (157K).

Additional notes on the DARM calibration measurement on April 7, 2025

• Before the first measurement, we waited for the alignment to settle at PRFPMI_RF_LOCKED and INCREASING_LAS_POWER, monitoring the ARM BPC signals.
• We performed two rounds of measurements on this day. The first set (filename prefix: 1350_*) was taken without performing OMC alignment.
• Due to a lock loss during the first set and the lack of proper OMC alignment (klog 33214), we decided to repeat the full measurement again as the second set.
• The second set (filename prefix: 1454_*) was taken after OMC alignment.

I changed the setpoint of the heater from 27°C to 25°C at 8:45 JST.

By the way, did you add and turn on several circuits in the EXV area? If so, the temp increase by ~ 0.6 °C can be explained. Anyway, Takahashi-san performed the off load for F0 and F1 because it drifted a lot. 

At present, no "small" cooler in the EXV area for the temp compensation.

[Kenta, Yuzurihara]

For the lockloss (2025-04-06 20:26:15 UTC), the lock duration was longer than other recent locks (50335 s = 13.98 hours)!! So, we checked the stability of the lock.
There are two isses: (1)​​ t​​​he BPC for EY pitch was drifting away from 0. (2) the unknown excess in 500~2000 Hz started in the middle of the lock

Details

Figure 1 shows the BPC error and feedback signals during the lock. We found that the BPC for EY pitch had drifted so that the error signal (K1:BPC-PIT_ETMY_OUTF_INMON) was away from 0. See the y-corsor located at y=0. (In my understanding, there was no BPC control for IY pitch.)

Figure 2 shows the normalized the spectrogram of the strain signal. From -5 hours before the lockloss (~15:00 UTC), the excess in 500~2000Hz had appear. The BPC frequency for EY pitch and yaw was 860 Hz and 855 Hz, respectively . So, this excess could disturbed the BPC control.
Note that, during this lock. the laser power and AS17 were stable (Figure 3).

Figure 4 shows the non-Gaussianity monitor (Gauch) of the corresponding time. I can't find any coincident change (also in other time and channels).It's possible that the excess was not far from Gaussian noise or that the time resolution should be much more large to catch this excess.

Figure 5 and Figure 6 show the behavior around 860 Hz and 855 Hz of the strain channel. I picked up them from line monitor (developed by Takahiro S Yamamoto-san). The amplitude of the injected line for BPC was changing in time, as seen Figure 1.
In addition,the magnitude of the spectrum was different before and after 15:00 UTC. This is corresponding with the finding in Figure 2.

Figure 7 shows the channels related to the PMC. Once the PMC temperature control was saturated, it recovered from the saturation.

[Kenta, Yuzurihara]
We​​​​​​ performed the lockloss investigation for the recent lockloss from the OBSERVATION state of the LSC_LOCK guardian, between 2025-04-05 03:54:16 UTC and 2025-04-07 05:54:54 UTC. The previous lockloss investigation was posted in klog33153.
During this period, there were 10 lockloss. The plots are summarized in wiki.

PMC HV Saturation

The 1 lockloss (2025-04-05 03:54:16 UTC) was occured because the PMC HV was saturated. Figure 1 shows the screenshot that K1:PSL-PMC_PZT_HV_MON_OUT_DQ touched the limit value (300).
During this period (4/4~4/7), the most of lockloss was occured under the the saturation of K1:PSL-PMC_HEATER_INMON (15000). Except for one lockloss, the direct lockloss cause was not related to the PMC saturation.

1Hz seismic motion made the IMC lockloss

For 2 lockloss, we saw the excess of the seismic motion with 1~10 Hz, which made the oscillation of the IMC length control and made the IMC lockloss.

• 2025-04-06 06:02:46 UTC (Fig)
• 2025-04-07 00:53:24 UTC (Fig)

Oscillation of OMC DC PD

This phenomena was observed many times in the lockloss in March. In this period, this phenomena was observed in 1 lockloss. (2025-04-06 20:26:15 UTC), as seen in Fig 4. The cause of the oscillation was unclear.

Oscillation of OMC DCPD over 5~10 seconds

• 2025-04-06 02:53:10 UTC
• 2025-04-06 03:09:56 UTC

This new phenomena were observed in 2 lockloss. It took 5~10 secods for the OMC DCPD to oscillate and saturate, as seen Fig5 and Fig6. The oscillation frequency was 44 and 43 Hz, respectively.
As shown the figures, the OMMT2 TRANS was not increased just before the lockloss. It indicates that the laser power arriving the AS port was not increased. But, the laser power of OMC DCPD increased with 5 seconds... We need more samples....
When the OMC DCPD started to oscilalte, the power of IR trans dropped coincidently.
There was no strange behavior on AS17 (=DC OFFSET value).

Others

For 1 lockloss (2025-04-07 05:54:54 UTC), there was no hint, except for the saturation of K1:PSL-PMC_HEATER_INMON. The OMMT2 TRANS was stable.

For 3 lockloss, the excitation was underway. I will let these lockloss be.

Abstract

The front-end calibration was updated with today's measurement results.
There is no large change in the actuator efficiency of TM from previous ones which was measured on Feb. 19th klog#32745.
Because we had large changes in the mirror temperature in Mar. it was not an enough evidence that actuator efficiency had been stable during Mar.
On the other hand, an assumption in recent OLTF measurement on Apr. 2nd (klog#33198) and updated on Apr. 4th (klog#33260 that 1.5dB change in OLTF came from optical gain (klog#33213) seemed to be almost correct. (Mirror temperature was ~80K in Feb. and Apr., but it changed as 80K -> 40K -> 80K in Mar.)
A change in the actuator efficiency of IM is slightly large but it's not so serious for sensitivity evaluation now. So it will be checked later.

Update of the front-end calibration was done at 18:58:26 JST.
So it's valid from an IFO lock at 19:08:59 JST.
 

Changes

Actuator efficiency of TM (Fig.1-2 shows measured TF to DARM)
old: 3.7791e-14 * (10Hz/f)^2
new: 3.7947e-14 * (10Hz/f)^2 (+0.4%)

Actuator efficiency of IM (Fig.3-4 shows measured TF to DARM)
old: 1.39319e-14 * (10Hz/f)^4
new: 1.62888e-14 * (10Hz/f)^4 (+16.9%)

Optical gain (Fig.5-6 shows measured OLTF)
old: 248.094dB
new: 247.993dB (-1.2%)

MN_LOCK_L (Fig.7 shows total filter shape of all FMs)
added 40Hz band stop filter at FM6

IM_LOCK_L (Fig.8 shows total filter shape of all FMs)
added 40Hz band stop filter at FM1

LSC_OMC_DC
removed +2dB and -2.08dB gains at FM2 and FM3, respectively.

The most direct way to investigate the mystery of the power discrepancy would be to measure the power right after the secondary lens, where the IR beam radius would be sufficiently shrinked, when the relevant chambers will be opened.

Abstract:

I tried to lock DARM with newly designed hierarchical actuators but failed.
Though the reason is not clear, DARM loop oscilated at 2.1Hz.
According to the phase behaviour around 2.1Hz, there seems to be negative zero around there.
So, decoupling of the actuator might solve the problem.

Detail:

First, I measured transfer function from ISC_INF_EXC to DARM1 with only turning on one of MN, IM, and TM actuator switches with ALS_CARM_LOCKED state (fig1:right column).
Pink, cyan, and black lines show the TF when only TM, I, and MN actuators were opened, respectively.
I slightly tuned the notch filter frequency for MN filters but basically the designed filter seemed to be as expected.

Then, TF from ISC_INF_EXC to DARM1 with turning on all actuator switches with ALS_CARM_LOCKED state (fig2: left column).
Red and blue lines show the TFs with nominal filters and newly designed filters, respectively.
With the initial design, hollow around 1.9 Hz is too deep, so I tuned the notch filter of TM _LOCK_L to mitigate it.

After that, I tried to lock the ALD_DARM with new hierarchical actuators but started to oscillate at 2.1Hz (fig2).
2.1Hz gain should be enough high after engaging ALS_DARM, so it doesn't seem gain peaking.
According to the TF in fig2, the phase was rotated by 360 degrees between 1.7Hz to 2.5Hz, which implies that there was a negative zero around that frequency.
If there is a negative zero around 2Hz, actuator sign at 2.15 Hz resonance would be flipped, so oscillation might be explained.
I'm not so sure why there was a negative zero but one possibility is actuator coupling from the other DoFs, so actuator decoupling should be performed.

[M. Honjo, Michimura]

We have reduced the PD gains for TMSX and TMSY polarization monitor PDs to avoid saturation in 10 W PRFPMI operation.
Power at TMS PDs seems to have half the power than estimation for some reason (as was the case in 2018 and 2022).

Summary of PD whitening situations:
  X_IR_PDA1: 0 dB, no whitening gain, no whitening filters
  X_IRSPOL_PDA1: 0 dB (was 10 dB), no whitening gain, 2-stage whitening filter
  X_IRPPOL_PDA1: 10 dB (was 30 dB), no whitening gain, 2-stage whitening filter
  HWP angle: 20 deg
  Y_IR_PDA1: 0 dB, no whitening gain, no whitening filters
  Y_IRSPOL_PDA1: 0 dB (was 10 dB), no whitening gain, 2-stage whitening filter
  Y_IRPPOL_PDA1: 20 dB (was 30 dB), no whitening gain, 2-stage whitening filter
  HWP angle: 80 deg
  See klog #30902 for previous settings. Note that X and Y have different settings now.

Calibrated polarization spectra:
  - We calibrated measured spectra into polarization rotation angle using the factor described in klog #30885. But with following changes

if ARM=='X':
    PDp2Rad=1/(4*(300.7-16.3)*np.sin(4*np.deg2rad(20-10.1)))*10**(20./10)  # Calibration from klog #30113, but calibration was done in 30 dB, but now in 10 dB
    PDs2Rad=1/(4*(297.0-19.7)*np.sin(4*np.deg2rad(20-54.7)))*10**(30./20)  # Calibration from klog #30113, but calibration was done in 30 dB, but now in 0 dB
    PDp2Rad/=130*10.8/1.1/2  # 130 for PRFPMI instead of single arm, 10.8/1.1 for input power difference, 2 for installation of additional BS in TMS (klog #32183)
    PDs2Rad/=130*10.8/1.1/2
elif ARM=='Y':
    PDp2Rad=1/(4*(358.0-24.1)*np.sin(4*np.deg2rad(80-72.5)))*10**(10./20)  # Calibration from klog #30827, but calibration was done in 30 dB, but now in 20 dB
    PDs2Rad=1/(4*(352.0-24.2)*np.sin(4*np.deg2rad(80-117.4)))*10**(30./20) # Calibration from klog #30827, but calibration was done in 30 dB, but now in 0 dB
    PDp2Rad/=130*10.8/1.1/2  # Same with X (klog #32181)
    PDs2Rad/=130*10.8/1.1/2 

See attached for the original data in counts and the calibrated data for X and Y.

TMS power budget:
  - Using the following, power transmitted from ETMX and ETMY would be 420 mW and 395 mW, respectively. Since we have 4 BSs to IR_PDA1 now (JGW-T1808962), power at IR_PDA1 would be 26.2 mW and 24.7 mW, respectively.

PIMC=10.8
PRG=13.0
BS=0.5
T_ITMX=0.444/100 # PhysRevApplied.14.014021
T_ETMX=6.8e-6  # PhysRevApplied.14.014021
T_ITMY=0.479/100  # PhysRevApplied.14.014021
T_ETMY=6.92e-6  # PhysRevApplied.14.014021
RTLX=50e-6  # Round-trip loss klog #30823
RTLY=60e-6

def cavitypowertransmission(T1,T2,L):
    L=L-T2   # Remove ETM transmission from round-trip loss
    t1=sqrt(T1)
    t2=sqrt(T2)
    r1=sqrt(1-T1)
    r2=sqrt(1-T2)
    rloss=sqrt(1-L)
    return (t1*t2)**2/(1-r1*r2*rloss)**2

T_XARM=cavitypowertransmission(T_ITMX,T_ETMX,RTLX)
T_YARM=cavitypowertransmission(T_ITMY,T_ETMY,RTLY)
P_TMSX=PIMC*PRG*BS*T_XARM
P_TMSY=PIMC*PRG*BS*T_YARM

 - During 10.8 W PRFPMI lock
K1:TMS-X_IR_PDA1_INMON = 10200 cnts
K1:TMS-Y_IR_PDA1_INMON = 9530 cnts
  These corresponds to 10.6 mW for X and 9.83 mW for Y, using 40/2**16 V/cnts, 1.5e3 Ohm transimpedance gain, and 0.39 A/W for PDA100A2. These are roughly a factor of 2.5 smaler than the estimated power above.

Past TMS power measurements and discussions:
  - According to measurements in October 2022 (klog #22456, klog #22339), power around RLNS2 was 529 uW for X and 480 for Y with 2.9 W input, single arm. For 2.9 W single arm, using T_PRM=0.135 (JGW-L1605744), they should be 1170 uW for X and 1100 uW for Y. Here, measured values are a factor of 2.2 smaller. Note that these measurements were done with a power meter. So, the discrepancy is consistent between power measurement methods.
  - This was also the case in December 2018 (klog #7415, klog #7419).
  - Note that all the calculations above assume 100% modematching of IMC transmission to the main IFO. This should be a good approximation due to DARM shot noise estimates and past modematching measurements.
  - We are consistently loosing half the power since 2018. Something inside BRT???

Next:
  - Redo polarization calibration under 10 W PRFPMI configuration
  - Set HWP angle to reasonable values and make PD gains same for X and Y

[Kimura and Yasui]
　To improve the convenience of cryo-cooler maintenance,
the CRY Group has installed special tools for the cryo-coolers in the X-end and Y-end machine rooms.
 The special tools are stored in the KTC tool box shown in the photos.

I offloaded the F0 and F1 GAS filters with the FRs.

Yokozawa-san turned off the FFU on the pre-booth of the PR3 at 15:02 today.

I compared the accelerometers' ASDs before and after this work. There are no significant changes.

I analyzed the magnetic injection data on April 3rd, based on the coupling function model.

• Fig.1: ASDs of the strain and the magnetometer (at BS), for the reference and each injection time
• Fig.2: Coupling Function upper limit,  (Bx^2 + By^2 + Bz^2)^0.5 -> strain
• Fig.3: Projection upper limit