[Ushiba, Tanaka, Fujimoto, Saito]

Following klog:36974, we attempted to achieve lock by adding an integrator to the low-pass filter, but locking was not successful. We also tried changing the feedback sign, changing the frequency relationship between the main laser and the sub-laser, and using the low-pass filter of SR560 instead of Moku:Lab. However, none of these approaches resulted in lock acquisition. In addition, the measured beat signal amplitude was approximately 0.2 mVpp, whereas the value predicted from the laser powers was approximately 0.1575 V, representing a large discrepancy. Therefore, in the next experiment, we plan to investigate the alignment, mode matching, and polarization conditions to determine whether the beat signal amplitude can be increased.
 

• First, a 10 kHz low-pass filter was applied, and the gain was gradually increased. Oscillation occurred once the gain exceeded a certain level. We then inserted an integrator with a corner frequency of 1 Hz while keeping the gain just below the oscillation threshold and tested whether lock could be achieved. However, no behavior indicating that the beat frequency was being pulled toward the LO frequency was observed. In addition, when the integrator was enabled, the beat frequency was observed to move further away from the LO frequency. We also changed the feedback sign and tested both cases in which the sub-laser frequency was higher than and lower than the main laser frequency, but lock could not be achieved in any configuration. Furthermore, we attempted to use the low-pass filter of an SR560 instead of Moku:Lab and varied the gain, but lock acquisition was still unsuccessful.
   

• In addition, when the gain was increased with the 10 kHz low-pass filter in place, the frequency of the error signal at the onset of oscillation was approximately 900 kHz. When the beat signal was observed on the oscilloscope, its amplitude exhibited temporal fluctuations, and the measured frequency was also approximately 900 kHz. This amplitude-modulated signal disappeared when the sub-laser beam was blocked, but remained unchanged when the main laser beam was blocked. The same 900 kHz signal was also observed on the spectrum analyzer. Its level was approximately -43 dB with the noise eater turned off and decreased to approximately -80 dB when the noise eater was turned on. However, changing the state of the noise eater did not enable lock acquisition.

• The beat signal amplitude, converted from the spectrum analyzer reading in dBm, was approximately 0.2 mVpp. At present, the sub-laser power incident on the RFPD is approximately 1.78 mW, while the main laser power is estimated to be approximately 17 μW. Since the responsivity of the RFPD (1611FS-AC) is approximately 525 V/W, the expected beat signal amplitude is (525V/W)×2√(1.78×10^−3×17×10^−6) which is approximately 0.1575 V. This predicted value is about 788 times larger than the measured amplitude. Therefore, in the next experiment, we plan to investigate the alignment, mode matching, and polarization conditions to determine whether the beat signal amplitude can be increased.

[Kimura and Yasui]
 On the afternoon of June 1, we filled two helium compressors (EYC P-53 and EYC P-55) with G-1 grade helium gas to a pressure of 15 bar.
As a result, maintenance work on the EYC cryocooler for the radiation shield has been completed.

[Kimura, Yasui, Tanaka (Edwards) and Okada (Edwards)]
 On the morning of June 1, two Edwards technicians conducted an inspection of the Gamma ion pump power supplies used on the X and Y arms.
This inspection involved connecting a high-resistance dummy pump to the power supply’s high-voltage terminals and verifying that the voltage (~6 kV) and current (~60 μA) were being output stably. (Photos 1~3)
As a result, it was confirmed that the five Gamma ion pump power supplies used on the X and Y arms were operating without issues.
 Subsequently, the Edwards technicians inspected the Gamma ion pump power supply at the X-end that had generated an error. (k-log 36915)
(Photos 4~5)
No loose connections were found in the wiring forming the interlock.
Therefore, it was decided to monitor the situation for the time being.

YamaT-san, Nakagaki-san, Ikeda

> The standalone stepper motor driver for the F3 GAS didn't work. 
We migrated from k1script1 to k1script0 (K-Log #36747), but due to the same issue described in K-Log#36803, we are unable to establish a connection.
As a temporary workaround, the Standalone (IX) stepper motor has been moved to k1script1, and the configuration has been modified so that it can operate there.

Details

Just before starting the work, an earthquake occurred. There were also some aftershocks during the work. The earthquake was likely around seismic intensity 3 near Niigata. In addition, the ground motion was relatively large today.

Preparation

As a preparation, we misaligned SRM and PRM to avoid unwanted resonances. We also confirmed that all Type-A suspensions could go to the LOCK_ACQUISITION state.

The GRX and GRY shutters were opened. The GR transmission was about 0.7 for X-arm and about 1 for Y-arm.

An aftershock occurred at around 06:09.

X-arm alignment

Before starting the guardian-based initial alignment, we slightly moved PR3 to increase the GRX transmission.

The PR3 opticalign values were adjusted to increase the GRX transmission.

After that, we requested IRX_LOCKED. Then we requested GRX_LOCKED_WITH_IRX, but the lock was lost. The IMC did not recover automatically, so we worked on the IO alignment.

We requested PROVIDING_STABLE_LIGHT in the IO guardian and adjusted the PZTs. For example, we changed the PZT1 pitch offset. The original value was 75, and we tried 102 because the corresponding input value was about 27 when the alignment was good. We looked for sudden changes and tried to bring the corresponding degrees of freedom back to the previous good values.

We adjusted by using PZT{1,2}_{P,Y}. After recovering the IMC lock, we waited for the ASC to engage. Finally, the PZT value was returned to 75.

After the IO was recovered, we continued the X-arm alignment using the initial alignment guardian. In the ALIGNING_XARM state, we manually moved ITMX and ETMX while checking the transmission. And completed the Xarm alignment.

Y-arm alignment

We then started the Y-arm alignment. During the Y-arm alignment, we slightly moved BS to improve the alignment.

OMC alignment

For the OMC alignment, we first turned off the GR beams and requested IRY_LOCKED_FOR_OMC. Then we requested ENGAGE_OMMT2_TRANS_CENTERING, followed by ALIGNING_TO_OMC.

During the alignment, K1:OMC_TRANS_DC_SUM_OUT16 increased to about 33 and became stable. The alignment became stable, although the change was not very large.

After that, we requested RECORD_GOOD_VALUES_OMC.

SRY alignment

Finally, we tried the SRY alignment. We requested ALIGNING_SEY and adjusted SRM, referring to the procedure in klog 36967.

Since the alignment did not look good, we set the guardian to DOWN and manually moved SRM. We adjusted SRM while watching K1:LSC-AS_PDA1_DC_OUT_DQ and the POP camera.

We then requested ALIGNING_SRY again and adjusted the SRM opticalign values so that the oplev values became close to the previous good oplev values. We tried ALIGNING_SRY once more, but SRY could not be locked.

The failure to complete the SRY alignment might have been due to the relatively large ground motion today, including the earthquake and aftershocks.

Summary

The X-arm, Y-arm, and OMC alignments were completed. We also tried the SRY alignment, but it could not be completed.

[Yokozawa, Chen, Hirose]

We performed initial alignment procedures.

The X-arm, Y-arm, and OMC alignments were completed. We also tried to align SRY, but it could not be completed.

There was an earthquake just before starting the work, followed by some aftershocks. The ground motion was relatively large today, and this likely made the SRY lock difficult.

Details will be reported later.

SRMI was also locked with 3f signals (REFL 51I for SRCL, REFL 51Q for MICH) under the same conditions. It should be noted that the demodulation phase of REFL51 was optimised not for sensing SRCL & MICH signals, but for avoiding glitches when IR flashes at REFL table. This may cause weird phase rotation of the measured open loop transfer functions, as attached (Fig. 1 for MICH, Fig. 2 for SRCL). For stable locking of SRMI and DRMI, the phasing work seems necessary.

[Fujimoto, Takano]

Summary

We noticed that the previous locking point of SRMI was not as intended, somewhere very weird. Finally, we found the good locking point, where MICH is at the dark fringe and the f1 sidebands are on resonance in the SRC.

Detail

Once we thought that we could lock SRMI (see here), but after further discussion, we concluded that this locking point was not as expected, because the AS34 buildup was -0.04, whereas we observed it can reach ~ 0.9. After several trials and errors, we found out the optimal conditions for locking it at the point we wanted: MICH is at the dark fringe and the f1 sidebands are on resonance in SRC (Fig. 3). We implemented the appropriate servo settings in the VERTEX guardian. We could lock SRMI by requesting it. The measured open loop transfer functions are as attached (Fig. 1 and Fig. 2).

During the trial, we noticed that PR3 often moved quite largely, sometimes more than 10 urad. It looked like big glitches pushed it. These glitches sometimes unlocked the interferometers during the initial alignment (Fig. 4).

Attached figures show the latest open-loop transfer functions of PRCL and MICH in PRMI 3f locked after changing the REFL HWP angle. The overall gain of each loop was tuned to match the old measurement.

[Takano, Dan, Hirose, Saito]

Following klog:36916, we continued our attempts to achieve PLL lock. We tested both flat filters and low-pass filters. Although loose locking was achieved, we were unable to add an integrator while maintaining lock. In addition, we were unable to measure the open-loop transfer function successfully.
 

• First, we confirmed the beat signal between the sub-laser beam and the beam coming from the interferometer. Next, the 100 kHz low-pass filter after the mixer was replaced with a 1.9 MHz low-pass filter. A control filter was then implemented using Moku:Lab, and lock acquisition was attempted by matching the LO frequency to the beat signal frequency. For a flat control filter, the system appeared to be most stable with a gain of -10 dB. Increasing or decreasing the gain from this value resulted in poor locking performance. For a low-pass control filter, higher gains could be used compared to the flat-filter case while maintaining lock. However, as the cutoff frequency was reduced, lock acquisition became increasingly difficult.

  The control filters and corresponding beat signals are shown below. In the filter screenshots, the red trace represents the error signal and the blue trace represents the feedback signal.

  Flat filter with a gain of -20 dB (Photo 1), corresponding beat signal (Photo 2)
  Flat filter with a gain of -10 dB (Photo 3), corresponding beat signal (Photo 4)
  Flat filter with a gain of 0 dB (Photo 5), corresponding beat signal (Photo 6)
  Low-pass filter with a gain of 0 dB and a cutoff frequency of 100 kHz (Photo 7), corresponding beat signal (Photo 8)
  Low-pass filter with a gain of 40 dB and a cutoff frequency of 10 kHz (Photo 9), corresponding beat signal (Photo 10)
  Low-pass filter with a gain of 30 dB and a cutoff frequency of 1 kHz (Photo 11), corresponding beat signal (Photo 12)

  In addition, we attempted to add an integrator while the system was locked using either the flat filter or the low-pass filter. However, the lock was lost after the integrator was enabled. Since the integrator cutoff frequency was set directly to a relatively high value such as 1 kHz, it is possible that the lock could have been maintained if the integrator had been introduced gradually, starting from a much lower frequency. We also attempted to measure the open-loop transfer function while the PLL was locked in order to determine the UGF. However, we were unable to obtain a successful measurement.

[Takano]

Summary

Many updates on the initial alignment for DRMI were made. They are implemented in the initial alignment guardian, but no documents for them have been written so far. Also, PRMI ADS problem was solved (I hope).

Detail

When we tried SRMI locking, it seemed that the alignment to AS port looked bad. Until then, we aligned the beam up to OMMT1 using the OMMT2 trans QPD, but left the alignment to OMC as is. We already have the guardian state for the alignment to OMC using OMMT2 and OSTM, but they didn't work because the power after the SRM is now attenuated by 15%. To fit the guardian to the current situation, we added new states for OMC alignment, including OMMT1 alignment using the OMMT2 trans QPD.

The revised procedure is as follows:

1. Lock one of the arm cavities (Y arm is easier, because normally we align the beam to OMC after Yarm alignment)
   1. Set PRM to MISALIGNED_BF in order to increase the input power later
   2. Because of this, these states are different from the normal IR_LOCKED state, in which PRM is in MISALIGNED_FOR_LOCK_ACQ state.
2. Increase the input power to 8.5 W while keeping the arm cavity locked
   1. When the input power is 8.5 W, OMC transmission is 30 mW.
   2. The HWPs on both PSL table and REFL table are rotated in iterations. When the input power is increased, the power going to REFL PD gets increased; to compensate for this change, the HWP on REFL table is rotated.
3. Engage OMMT2 transmission QPD control loop for OMMT1 alignment
4. Lock OMC and engage OMC QPD control loop for OMMT2 and OSTM loop

These actions were written in INITIAL_ALIGNMENT guardian.

While locking OMC, we faced the guardian error again, which was once reported here. The previous procurement didn't work well unfortunately, but YamaT-san finally found a solution and now it's resolved.

After OMC alignment, I tried to lock PRMI, but it failed even with a decent alignment (POP90I buildup of 0.5). Looking at the error signal, REFL51Q had an offset. I subtracted the dark offset, but still an offset of 2.6 was there. I subtracted it manually by adding an offset in MICH1 filter, then I could lock PRMI, but ADS for it didn't work. Through many many investigations, I found that the angle of REFL HWP was different: when ADS for PRMI worked well, the angle was 125; when it did not, it was 146. The angle depended on which guardian we called to lock PRMI; when we didn't use INITIAL_ALIGNMENT guardian, the angle was 125 and ADS worked successfully, but INITIAL_ALIGNMENT guardian set the HWP angle to 146 when it was called. I changed the guardian so that we set the angle to 125 initially, then we could successfully engage the ADS loop for PRMI. This also removed the REFL51Q offset and PRMI was locked more easily than before.

I offloaded the F1 and F2 GAS filters with the FR. The standalone stepper motor driver for the F3 GAS didn't work. 

I offloaded the F0 and F3 GAS filters with the FRs.

I offloaded the BF GAS with the FR.

[Yokozawa, Chen, Hirose]

We performed the initial alignment for Xarm, Yarm, OMMT and SRY.
Before Xarm initial alignment, SRM was in LOCK AQ state. So we request SRM GRD to Misalign state.
After that, we completed the Xarm initial alignment.

Subsequently, when we requested ‘IRY LOCK state’ in the initial alignment GRD(Guardian) for the Yarm initial alignment, the IMC’s alignment deteriorated and the lock was lost. To lock Yarm using IR light, we feed feedback to the laser at a high frequency. However, due to the IMC's L2A coupling, it caused the IMC to become misaligned and loss lock.
At that time, the ASC feedback signal was no longer at the normal level, so when we tried to recover the lock again, it became unstable. Therefore, we adjusted the OPLEV on the MCs and the PZT on the IMC’s IP to the values used when the lock was stable. When we tried to lock again, it locked stably. (We reset the PZT offset value to the standard 75V.)

We performed the initial alignment of Yarm. If the transmission power of GRY is low, adjust the PITYAW of SR3; if the transmission power of IRY is low or the flash light is not visible, adjust the PITYAW of BS. This time, we moved BS.

We performed the initial alignment of the OMMT. For the OMMT initial alignment, we referred to klog36759. We also recorded the OMMT1 alignment in the ‘PRFPMI’ of the MEDM screen ‘all oplev’.

For SRY, we request the “Aligning the SRY” state, which is currently in the initial alignment guardian. If the lock is unstable, we adjust the SRM’s PITYAW while monitoring the POPSpol camera and the AS RF34 PD power or AS DC PDA1 OUT power. 
Furthermore, once it was confirmed that the “Aligning the SRY” state had been reached (i.e. SRY lock was complete and the SRM’s ADS was operating stably), the SRY initial alignment was finished. The SRM alignment was recorded and offloaded in the “PRFPMI” on the MEDM “all oplev” screen.

[Fujimoto, Tanaka, Hirose, Takano]

Summary

We tried locking SRMI. After tuning the filter gain, we easily succeeded in locking it, but further investigation is necessary for good stability.

Detail

For locking DRMI, we tried the SRMI configuration. The idea is to know a hint for a good servo gain of SRCL and MICH in DRMI.

We chose the locking condition as follows:

1. The Carrier is at the dark fringe in MICH.
   • The f1 sidebands are transmitting MICH by 85 %
2. The f1 sidebands are on resonance in SRC
   • The carrier would be on anti-resonance, but before entering SRC most carriers are reflected by MICH
3. Error signals are taken from POP17
   • SRCL: POP17I
   • MICH: POP17Q
   • No guarantee that these two are well degenerated

The conditions for the light fields are different from those in DRMI, but the signal conditions are similar. We expect that, from the servo gain for locking SRMI in these conditions, we would have a good estimate of them for locking DRMI, especially for locking SRCL and MICH in DRMI.

Once we finished the initial alignment of PRMI with ADS, we misaligned PRM and aligned SRM. First we tried the manual locking by tuning the servo gain of MICH and SRCL. After several trials and errors, we found good numbers:

• SRCL2 gain: 0.5
• MICH2 gain: -40

We could also enable the integrator on each servo. The measured open-loop transfer functions are shown in Fig.1 and Fig. 2. While locking SRMI, we saw many glitches and they disturbed locking (Fig. 3), resulting in lock loss occasionally. We first suspected the reflection back from PRM, so we misaligned it more by requesting MISALIGNED_BF state, but the situation didn't improve at all.

After succeeding in locking, we implemented the guardian state and tested it. However, every time the guardian failed to lock it when the servos were engaged or the integrators were engaged. After other trial and errors, it turned out that the order of engaging servo gains and the integrators was important: without the integrators, we first need to engage the MICH servo first, then SRCL servo. Also, we found that we couldn't engage the integrator in SRCL servo cannot be enabled without breaking locking, nor increase the SRCL servo gain either. It could be that the demodulation phase is not optimised for SRMI and now POP17I and POP17Q are mixed, which makes the control loops unstable. Further investigation is necessary for stable locking.

[Fujimoto, kTanaka, Hirose]

We performed the initial alignment in order to perform PLL work. However, as the initial alignment took longer than expected, I ended up working on the SRMI instead of the PLL. The initial alignment work done was on the Xarm, Yarm, OMMT, SRY and PRMI. I will post the details later.