After the calibration work, we moved the RxPD back to its nominal position and found that its output had changed from the pre-work value. We investigated the cause and found that the alignment between the integrating sphere and the plate underneath it had slightly shifted. We realigned the integrating sphere with the plate, and the RxPD output returned almost to its original value.

[Tanaka, Hirose, Saito]

A 10 dB attenuator was added, reducing the beat signal level from 23 dBm to 10 dBm. The open-loop transfer function of the PLL was then measured. The results suggest that the sub-laser PZT responds with a frequency change to the frequency-difference signal from the PFD, rather than responding with a frequency change to a phase-difference signal. In addition, the alignment of the sub-laser beam incident on SRY was performed using two irises. The sub-laser PZT was then driven, but no flashes were observed at the AS port. This is likely because one of the optics expected to be a mirror was actually a BS, the beam was clipping on the edge of a mirror, and the observation was being made at the AS port. In the next experiment, the BS will be replaced with a mirror, the alignment will be redone, and a PD will be installed at the OMC reflection port to observe the flashes.
 

• First, a 10 dB attenuator was added to reduce the input power to the PFD. As a result, the beat signal level decreased from 23 dBm to 10 dBm. Next, the 1 MHz low-pass filter in the SR560 used for feedback to the sub-laser PZT was replaced with either a 100 kHz or a 10 kHz low-pass filter. Under these conditions, the approximately 260 kHz oscillation previously observed in the error signal (klog:37046) disappeared. The open-loop transfer function was then measured, revealing a flat region. This was unexpected. We had assumed that the feedback loop operated by changing the sub-laser frequency via the PZT to eliminate the phase difference, which should behave as an integrator. However, the observation is consistent with the PFD producing a frequency-difference signal and the sub-laser PZT responding with a frequency change. Therefore, a 10 kHz low-pass filter was applied in the SR560 feedback path to the sub-laser PZT, and the gain and integrator cutoff frequency in Moku:Lab were varied. The measured open-loop transfer function then closely matched the designed filter response (Photo 1).
   

• Next, the mirrors were adjusted so that the main laser beam followed the optical path intended for the sub-laser beam entering the interferometer. Two irises were then installed along this path and aligned to the main laser beam. The sub-laser alignment was subsequently adjusted so that it also passed through the two irises. The main laser was then blocked, leaving only the sub-laser beam incident on SRY. The sub-laser PZT was driven with a triangular waveform generated by the Moku:Lab function generator, and the AS port was monitored for flashes. However, no flashes were observed. This is likely because the optical power incident on SRY was too low. The laser powers measured at Spots 1~5 in Photo 2 were as follows:

  Spot 1: approximately 130 mW
  Spot 2: approximately 390 mW
  Spot 3: approximately 780 mW
  Spot 4: approximately 1.0 W
  Spot 5: approximately 1.1 W

  From these measurements, the laser power incident on SRY is estimated to be approximately 130 mW, which corresponds to only about 0.25 μW at the AS port. Such a small signal is likely buried in noise. The reduction in laser power between Spot 2 and Spot 1 occurred because the optic between them was a BS rather than a mirror. In addition, the reduction in power between Spot 4 and Spot 3 may be due to the beam clipping on the edge of the mirror.
   

• Therefore, in the next experiment, the BS between Spot 2 and Spot 1 will be replaced with a mirror, and the alignment will be adjusted so that the beam does not clip on the edge of the mirror between Spot 4 and Spot 3. However, even after these modifications, the power at the AS port is expected to increase only to approximately 0.97 μW. Therefore, a PD will be installed at the OMC reflection port, and flashes will be monitored there instead.

KAGRA Pcal-X updates (2026/06/09)

Workers: Dan Chen, Jiahui Xiong, Misato Onishi

We performed monthly Pcal-X calibration on 2026/06/09.

After the calibration, we updated EPICS parameters related to the Pcal-X system. No issues were found.

A CAL Tcam session was performed to obtain beam position information necessary for Pcal. The parameters have already been updated, and SDF has been accepted.

Operator: Dan Chen

Update Time: 2026/06/09 05:54:18

Update Time: 2026/06/09 05:54:58

[Tanaka, Hirose, Saito]

Using the 20 dB RF amplifier and the 40 dB RF amplifier, the beat signal amplitude was increased to 23 dBm. After replacing the phase detector with a Phase Frequency Discriminator (PFD), it was confirmed that the beat signal frequency followed changes in the LO frequency. However, the error signal appeared to oscillate at approximately 260 kHz. This oscillation may be caused by a resonance of the sub-laser PZT.
 

• First, the alignment was adjusted to maximize the beat signal. The signal level under various RF amplifier configurations was as follows:

  No RF amplifier: -51 dBm
  40 dB RF amplifier only: -5 dBm
  20 dB RF amplifier only: -20 dBm
  40 dB RF amplifier followed by the 20 dB RF amplifier: 8 dBm
  20 dB RF amplifier followed by the 40 dB RF amplifier: 23 dBm
   

• Therefore, lock acquisition was attempted using the phase detector with the 20 dB RF amplifier followed by the 40 dB RF amplifier. A 1 MHz low-pass filter was applied in the SR560 used for feedback to the sub-laser PZT, while a flat filter was implemented in Moku:Lab. Various gains and integrator settings were tested. However, no behavior indicating that the beat frequency was being pulled toward the LO frequency was observed.
   

• Next, the phase detector was replaced with a PFD. With a filter consisting of an overall gain of -6 dB and an integrator providing 6 dB of gain at 1 Hz (Photo 1), it was observed that the beat signal frequency followed changes in the LO frequency. In the lower plot of Photo 1, the red trace represents the error signal and the blue trace represents the feedback signal. Since the feedback signal changed when the LO frequency was varied, the LO frequency and beat signal frequency were measured for feedback signal levels of approximately 0 V and ±1 V. The beat signal spectrum exhibited a shape with a dip at the center and peaks on both sides (Photo 2). Therefore, the frequencies of the two peaks were measured, and their average was taken as the beat signal frequency. In Photo 2, the light red trace corresponds to a feedback signal of approximately +1 V, while the red trace corresponds to a feedback signal of approximately -1 V.

  The results are summarized below:

  Feedback Signal           LO Frequency    Beat Signal Frequency
  Approximately -1 V      85.066 MHz         85.16 MHz
  Approximately  0 V      88.066 MHz         87.99 MHz
  Approximately +1 V    89.666 MHz         89.74 MHz
   

• In addition, when using a flat filter and gradually increasing the gain, the error signal began oscillating at approximately 260 kHz, and the width of the beat signal increased. This oscillation may have been caused by a resonance of the sub-laser PZT. The corresponding error signals and beat signals are shown below:

  Flat filter with a gain of -6 dB (Photo 3), corresponding beat signal (Photo 4)
  Flat filter with a gain of 0 dB (Photo 5), corresponding beat signal (Photo 6)
  Flat filter with a gain of +6 dB (Photo 7), corresponding beat signal (Photo 8)

Date: 2026/06/08

Member: Dan Chen, Misato Onishi, Seiya Matsuo

We performed our usual WSK calibration at UToyama.

The results look no problem.

Results

Comparison with Previous Results

Comparing with previous results, no significant issues were found.
Attached graph is the result summary including the latest measured data.

I offloaded the BF GAS with the FR.

[Tanaka, Fujimoto, Saito]

The alignment was adjusted to maximize the beat signal, resulting in a beat signal level of approximately -55 dBm, which was the same as in the previous measurement (klog:37031). It was also confirmed that the sub-laser PZT was operating properly. The mixer currently in use (ZX05-1-S+) was then replaced with a phase detector (ZRPD-1+) purchased by Dan-san, and lock acquisition was attempted. However, no lock was achieved. Furthermore, when an RF amplifier was inserted after the high-pass filter while using the phase detector, the signal was amplified without waveform distortion, unlike the behavior observed in the previous experiment (klog:37031).
 

• First, the alignment was adjusted to maximize the beat signal. The resulting beat signal level was approximately -55 dBm. Next, a signal from the Moku:Lab function generator was applied directly to the sub-laser PZT, and the corresponding movement of the beat signal was confirmed. Therefore, the PZT was verified to be functioning properly.
   

• Next, the 1.9 MHz low-pass filter located between the mixer and the SR560 was moved from immediately before the SR560 to immediately after the mixer. PLL lock acquisition was then attempted using Moku:Lab with a 10 kHz low-pass filter while varying the gain and adding an integrator. However, no behavior indicating that the beat frequency was being pulled toward the LO frequency was observed. Lock acquisition was also attempted without using Moku:Lab by varying the gain of the SR560 low-pass filter, but no lock was achieved. In addition, a 10 kHz low-pass filter was applied to the SR560 used in the feedback path to the sub-laser PZT, while a flat filter was implemented in Moku:Lab. However, lock acquisition was still unsuccessful. The mixer currently in use (ZX05-1-S+) was then replaced with the phase detector (ZRPD-1+) purchased by Dan-san. Various Moku:Lab filter configurations were tested in the same manner as before, but lock acquisition was again unsuccessful.

• Next, while continuing to use the phase detector, an RF amplifier was inserted after the high-pass filter. Unlike the result obtained in the previous experiment (klog:37031), the signal amplitude increased while maintaining a clean waveform (Photo 1). The error signal also increased in amplitude and became saturated. Therefore, the gain of the SR560 was reduced from 200 to 5. Under these conditions, the error signal amplitude was approximately 384 mVpp (Photo 2).

[Fujimoto, Tanaka, Takano]

Summary

We diagonalised the SRM oplev. Now, the coupling from other DoFs is below 1% at the resonance frequencies.

Detail

We noticed that when we shake SRM TM in Pitch we observed motion in Yaw as well. At first, we suspected the actuator couplings, but the health check results obviously told us the coupling of the sensor (the tilt oplev), see Fig. 1 (Pitch) and Fig. 2 (Yaw) (blue: measurement in 2022, green: before the diagonalisation, red: after the diagonalisation). The 2x2 matrix about the coupling between pitch and yaw is as follows:

After normalisation:

Using this matrix, we calculated its inverse and multiplied it by the current SENSALIGN matrix from the left. The health check results using the new SENSALIGN matrix are shown in the figure: in both Pitch and Yaw, the coupling is below 1% at the resonance peak frequencies. 

[Fujimoto, Tanaka, Takano]

Summary

We continued the DRMI optimisation. We could stably lock DRMI with POP17Q for MICH, instead of REFL51Q. REFL51 signals looked strange, probably due to a bad demodulation phase. ADS implementation was tried, but not succeeded yet.

Detail

We continued the DRMI locking trial. This time, we investigated the REFL51 signals and discussed whether they could be used for locking. As reported here, there is an offset on both REFL51I and REFL51Q, but we don't understand its mechanism. In addition, the locking with REFL51Q for MICH looked unstable, so we decided to change the MICH control signal from REFL51Q to POP17Q for better stability. With POP17Q,  stability seemed to be improved so much. The lock can now be kept for more than 10 minutes.

The measured open loop transfer function for each loop are shown in Fig. 1 - 3. They looked stable enough, but sometimes we observed that MICH control got unstable and started oscillation at apparently a random frequency in 20 - 40 Hz. It may be that because of the alignment, MICH sensing gain increased, and the phase margin got smaller, or another control loop sucked the phase margin in the MICH loop via unknown couplings. Anyway, MICH control seems the key to better stability of DRMI control.

Once DRMI got stable, we injected the dithering signal in SRM and tried the phasing of SRM ADS with AS34I signal. The phase of the transfer function from the dithering LO to the signal (AS34I) is shown in Fig. 4. The measured phase was put in ADS, and tried to engage the control for SRM. It was not obvious how it worked, because the buidup level of AS34I looked almost the same due to its large fluctuation. Then, ADS of other mirrors, PRM, BS and IMMT2 were engaged using POP90I signal, but it seemed that the feedback signal made the alignment worse. We should tune the phases of the other mirrors as well for full ADS in DRMI.

With DRMI locked with POP17I&Q and REFL135I, the REFL51 signals were quickly examined. Obviously, both REFL51I and REFL51Q had a large offset. When the demodulation phase was rotated, the offset value also changed in sine curve for I phase and in cosine curve for Q phase (Fig.5). We don't know the source of these offsets, probably either MICH or SRCL has this offset.

Now that we can lock DRMI without REFL51 signals, the investigation is easier than before. I hope these issues will be solved soon and DRMI will be locked using fully 3f signals.

Optical table keeps the temperature around 297K even when HPBD temperature reaches around 365K.

[Tanaka, Fujimoto, Saito]

Following klog:36995, a high-pass filter with a cutoff frequency of approximately 12 MHz was added to suppress the 900 kHz noise originating from the sub-laser. In addition, the beat signal was found to be approximately 10 dB lower than in the previous measurement (klog:37020), although the cause remains unclear. Lock acquisition was then attempted, but no lock was achieved. An RF amplifier was also inserted after the high-pass filter; however, although the beat signal amplitude increased, the signal waveform became distorted.
 

• To suppress the 900 kHz noise from the sub-laser identified in klog:36995, a high-pass filter with a cutoff frequency of approximately 12 MHz was constructed using a 270 pF capacitor (Photo 1). The measured transfer function of the high-pass filter is shown in Photo 2. The high-pass filter was then installed immediately after the output of the RFPD, and the spectra with and without the filter were compared. When the RFPD output was passed through the high-pass filter, the signal level decreased by approximately 20 dBm compared with the unfiltered case (Photo 3). The red trace corresponds to the case without the high-pass filter, the light red trace corresponds to the case with the high-pass filter, and the blue trace corresponds to the measurement with nothing connected. Next, the high-pass filter was tested with the power splitter and mixer used in the PLL setup connected. In this configuration, the signal level decreased by approximately 11 dBm when the high-pass filter was inserted (Photo 4).
   
• To determine the conversion between dBm and Vpp in the Moku:Lab spectrum analyzer, a signal generated by the function generator of one Moku:Lab was measured using the spectrum analyzer of another Moku:Lab. When a 3 mVpp signal was applied, the measured level was -46.35 dBm. Therefore, the beat signal observed in the previous experiment (klog:37020) corresponds to approximately 3 mVpp.
   

• The beat signal was then re-examined and found to be smaller than in the previous measurement (klog:37020). To investigate this, the alignment was readjusted, the RFPD was reinstalled, and the polarization was varied. After reinstalling the RFPD, the DC output became saturated, so the sub-laser power was reduced to 0.825 mW. Ultimately, the beat signal level reached approximately -55 dBm (Photo 5). This is about 10 dB lower than in the previous measurement (klog:37020). The reduction cannot be explained solely by the decrease in sub-laser power, and the cause remains unclear. However, very little fluctuation in the beat signal amplitude was observed.

• Next, lock acquisition was attempted using Moku:Lab with a 10 kHz low-pass filter while varying the gain and adding an integrator. However, no behavior indicating that the beat frequency was being pulled toward the LO frequency was observed. Lock acquisition was also attempted using the low-pass filter of an SR560 instead of Moku:Lab while varying the gain, but no lock was achieved. In addition, when the sub-laser temperature was lowered from 31.51 ℃, a beat signal was still observed at 29.74 ℃. Based on the absolute frequency measurement reported in klog:36760, this is believed to be due to a mode hop. PLL operation was also tested at this temperature, but lock acquisition was again unsuccessful.
   

• Finally, an RF amplifier was inserted after the high-pass filter in an attempt to increase the beat signal amplitude. Although the beat signal became larger, its waveform became significantly distorted (Photo 6). When the mixer was removed, the waveform returned to a clean shape, indicating that the mixer was responsible for the distortion. Therefore, this RF amplifier is not suitable for the present application.

One of possibilties why SRCL build up is lower than our expected is detuned from the resonance point. Actually, When DRMI was locked, RF51 I was not zero even though SRCL error signal by 1f was zero (fig.1). This offset is often caused by RFAM. So I checked current RFAM status of each demodulated signal from each REFL RFPD when the single bounced beam from PRM is on each PD.

Fig.2 shows that REFL demodulated signals and DC powers on REFL PDs. As you can see, RF56, RF45 seems to have some RFAMs. On the other hands, 3f PDs seems to be zero. Therefore, The offset on SRCL error signal cannot be explained by RFAM.