Beam position changes on ETMY.

And optical layout in the Tx module. What we added were BSX11 and Dumper. Probably the leg of BS was touched when I fixed the BSX11.

[Kawakami, Tanaka, Saito]

Following the same procedure as in klog:37250, the lengths of SRX, PRX, and PRY were measured. The differences from the design values were found to be 2.80(13) cm for SRX, 0.677(11) cm for PRX, and −0.107(46) cm for PRY. Compared with the results reported in klog:37209 and klog:37223, the largest discrepancy is observed for PRX, but it corresponds to only about 1.25σ, indicating that the results are statistically consistent.

Furthermore, using these results together with the method described in klog:37248, the SRC length, PRC length, and Schnupp asymmetry were determined. The results are 66.61409(70) m for the SRC, 66.59425(24) m for the PRC, and 3.33791(45) m for the Schnupp asymmetry. Their differences from the design values are 2.279(70) cm for the SRC, 0.285(24) cm for the PRC, and 0.811(45) cm for the Schnupp asymmetry. Compared with the results of klog:37248, the largest discrepancy is found for the PRC, but it is only about 1.48σ, indicating that the two measurements are consistent.
 

• Following the same procedure as in klog:37250, the sub-laser PZT was driven directly for SRX, PRX, and PRY, and the Max Hold function of the spectrum analyzer was used to obtain transmission-power data as a function of beat frequency around 1.6 GHz, 800 MHz, −800 MHz, and −1.6 GHz. Here, the negative frequencies correspond to the case where the sub-laser frequency is lower than the main laser frequency. The peaks in the measured data were fitted both with and without a linear background. The fitting results for the individual peaks will be posted separately. The peak frequencies obtained from the fits and their uncertainties were used to determine the maximum and minimum values within the uncertainty range. The overall uncertainty range was taken as the union of the results with and without a linear background. The following data were therefore used to determine the lengths of SRX, PRX, and PRY.

  SRX
  Minimum (MHz)    Maximum (MHz)
  1543.4471    1543.4932
  884.8872    884.9360
  −856.1353    −855.8030
  −1654.9558    −1654.8394
  PRX
  Minimum (MHz)    Maximum (MHz)
  1631.4880    1631.4954
  867.3066    867.3270
  −874.0048    −873.9845
  −1646.9334    −1646.9256
  PRY
  Minimum (MHz)    Maximum (MHz)
  1627.6201    1627.6266
  868.0174    868.0423
  −882.0137    −881.9492
  −1632.3336    −1632.2710

• For each cavity, the midpoint frequency between the minimum and maximum values was divided by the FSR calculated from the design cavity length (68.2562 m for SRX, 68.2563 m for PRX, and 64.9265 m for PRY). The resulting values were rounded to the nearest integers, and the measured frequencies were fitted with the function AN+B, where A and B are fitting parameters and N is an integer. The fitting results are as follows.

  SRX (Fig. 1)
  A: 2.195180(42) MHz
  B: 0.256(25) MHz
  PRX (Fig. 2)
  A: 2.1958613(35) MHz
  B: −0.0348(25) MHz
  PRY (Fig. 3)
  A: 2.308744(17) MHz
  B: −0.042(11) MHz
   

• Since A corresponds to the FSR, the cavity lengths were calculated as follows.

  SRX
  Fitted length: 68.2842(13) m
  Design value: 68.2562 m
  Difference (fit − design): 2.80(13) cm
  PRX
  Fitted length: 68.26307(11) m
  Design value: 68.2563 m
  Difference (fit − design): 0.677(11) cm
  PRY
  Fitted length: 64.92543(46) m
  Design value: 64.9265 m
  Difference (fit − design): −0.107(46) cm

  Comparing these results with those reported in klog:37209 and klog:37223, namely, differences from the design values of 2.56(45) cm for SRX, 1.25(46) cm for PRX, and 0.15(33) cm for PRY, the largest discrepancy is found for PRX, but it corresponds to only about 1.25σ. Therefore, the results are considered to be consistent.
   

• Using these results together with the SRY measurement reported in klog:37250, the SRC length, PRC length, and Schnupp asymmetry were calculated following the same procedure as in klog:37248.

  SRC
  Calculated value: 66.61409(70) m
  Design value: 66.5913 m
  Difference (calculated − design): 2.279(70) cm
  PRC
  Calculated value: 66.59425(24) m
  Design value: 66.5914 m
  Difference (calculated − design): 0.285(24) cm
  Schnupp asymmetry
  Calculated from the SRC: 3.34023(139) m
  Calculated from the PRC: 3.337640(473) m
  Weighted average: 3.33791(45) m
  Design value: 3.3298 m
  Difference (weighted average − design): 0.811(45) cm

  Comparing these values with those reported in klog:37248, namely 66.6120(47) m for the SRC, 66.5984(28) m for the PRC, and 3.3405(48) m for the Schnupp asymmetry, the largest discrepancy is found for the PRC, but it corresponds to only about 1.48σ. Therefore, the present results are considered to be consistent with the previous measurements.

With Seiya Matsuo

We found that the beam was not perfectly aligned to the LPD used to monitor the beam power just after the laser source. This condition had probably existed since before O3GK.

We also confirmed that the LPD became saturated when the beam was fully aligned to it. To avoid this saturation, we installed a BSX11 beam splitter, with 90% reflection and 10% transmission, just before the LPD and dumped the 90% reflected beam. After this modification, the beam could be fully aligned to the LPD without saturating it. The LPD threshold in the Pcal GRD was also adjusted accordingly.

However, this work appears to have affected the beam alignment. A change in the beam alignment was observed inside the Rx module. On the other hand, the beam position on ETMY did not change significantly, so we believe that the alignment can be recovered using the picomotors.

We will correct this beam alignment the next time ETMY can be aligned.

Until this alignment correction is completed, we think that Pcal-Y should not be brought to high power.

With Yokozawa-san

We performed the initial alignment Xarm, Yarm, OMC, and DRMI.

The SRM GRD says "GAS is close to saturation (F0)".

[Kawakami, Komori, Tanaka, Saito]

The SRY cavity was scanned by directly driving the sub-laser PZT while monitoring the beat signal in the OMC REFL. Using the Max Hold function of the spectrum analyzer, transmission power as a function of beat frequency was obtained around 1.6 GHz, 800 MHz, −800 MHz, and −1.6 GHz. The measurement data are stored in:

Dropbox → All files/Dropbox KAGRA/Measurements/IFO/SRCL/SRCL_OMC_REFL_beat_signal_PZT_sweep/2026_07_23_SRY

The data were fitted both with and without a linear background, and the maximum and minimum values within the fitting uncertainties were used to estimate the uncertainty. From these results, the SRY cavity length was determined by fitting. The measured length differs from the design value by 1.757(50) cm, which is consistent with the previous result of 1.58(82) cm reported in klog:37209.
 

• First, a new spectrum analyzer was installed to monitor the beat signal at the OMC REFL. A PLL was established using the down-converted beat signal when the original beat frequency was approximately 450 MHz, and it was confirmed that the SRY cavity could be scanned by frequency-modulating the LO. Next, although the mixer used for down-conversion has a nominal upper frequency limit of 500 MHz, it was expected that PLL operation would still be possible at higher frequencies with only increased conversion loss. Therefore, the beat frequency was increased to approximately 800 MHz, and it was confirmed that the SRY cavity could again be scanned by frequency-modulating the LO while using the down-converted signal for the PLL.
   
• Furthermore, since the PLL is not required for the OMC REFL beat-signal measurement itself, the PLL was turned off and the sub-laser PZT was driven directly to scan the SRY cavity. This worked successfully when the beat frequency was around 800 MHz. Therefore, the sub-laser PZT was driven directly, and the Max Hold function of the spectrum analyzer was used to obtain transmission-power data as a function of beat frequency around 1.6 GHz, 800 MHz, −800 MHz, and −1.6 GHz (Fig. 1). Here, the negative frequencies correspond to the case in which the sub-laser frequency is lower than the main laser frequency. In addition, while observing the maximum beat-signal level in the OMC REFL during the PZT sweep, the alignment was optimized using a single mirror. As a result, the peak shapes became noticeably cleaner. This suggests that the relatively large uncertainty of the SRY measurement in klog:37209 was likely caused by imperfect alignment. After acquiring the data around 1.6 GHz and 800 MHz, the temperature of the sub-laser was increased to reduce its frequency. The beat signal disappeared at approximately 29.8°C. According to klog:36777, the sub-laser undergoes a mode hop between 29.5°C and 30°C, so the disappearance of the beat signal is attributed to this mode hop. The temperature was then increased further, and at approximately 31.64°C, data around −800 MHz were acquired. Before and after this point, data around 1.6 GHz and −1.6 GHz were also obtained.
   

• The peaks in the measured data were fitted with and without a linear background, following the same procedure as in klog:37209. The fitting results for the individual peaks are listed below. The measurement data are also stored in:

  Dropbox → All files/Dropbox KAGRA/Measurements/IFO/SRCL/SRCL_OMC_REFL_beat_signal_PZT_sweep/2026_07_23_SRY

  Fig. 2: Around 1.6 GHz, without a linear background. Using data from 1518.25 MHz to 1519.1 MHz, the fitted peak frequency is 1518.7773(42) MHz.
  Fig. 3: Around 1.6 GHz, with a linear background. Using data from 1518.25 MHz to 1519.1 MHz, the fitted peak frequency is 1518.725(13) MHz.
  Fig. 4: Around 800 MHz, without a linear background. Using data from 872 MHz to 873.6 MHz, the fitted peak frequency is 872.8561(45) MHz.
  Fig. 5: Around 800 MHz, with a linear background. Using data from 872 MHz to 873.6 MHz, the fitted peak frequency is 872.8274(81) MHz.
  Fig. 6: Around −800 MHz, without a linear background. Using data from 823.3 MHz to 824.25 MHz, the fitted peak frequency is 823.7654(15) MHz.
  Fig. 7: Around −800 MHz, with a linear background. Using data from 823.3 MHz to 824.25 MHz, the fitted peak frequency is 823.7819(48) MHz.
  Fig. 8: Around −1.6 GHz, without a linear background. Using data from 1555 MHz to 1556 MHz, the fitted peak frequency is 1555.3774(33) MHz.
  Fig. 9: Around −1.6 GHz, with a linear background. Using data from 1555 MHz to 1556 MHz, the fitted peak frequency is 1555.3867(99) MHz.
   

• From the fitted peak frequencies and their uncertainties, the maximum and minimum frequencies within the uncertainty range were determined. The overall uncertainty range was taken as the union of the results obtained with and without a linear background. The following values were therefore used to determine the SRY cavity length:

  Minimum (MHz)    Maximum (MHz)
  1518.7120    1518.7815
  872.8193    872.8606
  −823.7866    −823.7639
  −1555.3967    −1555.3768

  The midpoint frequency between the minimum and maximum values was divided by the FSR calculated from the SRY design length of 64.9264 m. The resulting values were rounded to the nearest integers, and the measured frequencies were fitted with the function AN + B, where A and B are fitting parameters and N is an integer. The fitting results are shown in Fig. 10:

  A: 2.308085(18) MHz
  B: 0.2480(98) MHz

  Since A corresponds to the FSR, the SRY cavity length was calculated from this value:

  Fitted length: 64.94397(50) m
  Design value: 64.9264 m
  Difference (fitted − design): 1.757(50) cm

  This result is consistent with the previous result reported in klog:37209, which gave a difference from the design value of 1.58(82) cm.

Abstract:

Followings are the summary of PRC length, SRC length, and Schnupp asymmetry calculated from the PRX/PRY/SRX/SRY length measurements reported in klog37209 and klog37223.

$L_\mathrm{PRC}=66.5984(28)$ m

$L_\mathrm{SRC}=66.6120(47)$ m

$L_\mathrm{Schnupp}=3.3405(48)$ m

Detail:

According to  klog37209 and klog37223, the cavity length of PRX/PRY and SRX/SRY can be summarized as follows:

Based on these results, we can calculate the PRC/SRC length (average length of PRX/SRX and PRY/SRY) and Schnupp asymmetry.

PRC length:

PRC length can be obtained by averaging PRX and PRY length, so the PRC length can be calculated as follows:

$L_\mathrm{PRC}=\frac{L_\mathrm{PRX}+L_\mathrm{PRY}}{2} = \frac{68.2688+64.9280}{2}\pm\frac{\sqrt{0.0046^2+0.0033^2}}{2}=66.5984(28)$

SRC length:

SRC length can be calculated as the similar way of PRC.

$L_\mathrm{SRC}=\frac{L_\mathrm{SRX}+L_\mathrm{SRY}}{2} = \frac{68.2818+64.9422}{2}\pm\frac{\sqrt{0.0045^2+0.0082^2}}{2}=66.6120(47)$

Schnupp asymmetry:

Schnupp asymmetry can be calculated by subtracting PRY/SRY length from PRX/SRX length as follows.

$L_\mathrm{Schnupp from PR}=L_\mathrm{PRX}-L_\mathrm{PRY} = (68.2688-64.9280)\pm\sqrt{0.0046^2+0.0033^2}=3.34080(566)$
$L_\mathrm{Schnupp from SR}=L_\mathrm{SRX}-L_\mathrm{SRY} = (68.2818-64.9422)\pm\sqrt{0.0045^2+0.0082^2}=3.33960(935)$

Above 2 values are independent results, so we can calculate the weighted average of these values. So, the Schnupp asymmetry can be calculated as follows:

$L_\mathrm{Schnupp}=\frac{3.3408/0.00566^2+3.3396/0.00935^2}{1/0.00566^2+1/0.00935^2}\pm\frac{1}{\sqrt{1/0.00566^2+1/0.00935^2}}=3.3405(48)$

We performed the initial alignment for PRMI and SRY. 

With Hirose-san and Yokozawa-san

Summary

The initial alignment through the OMC was completed successfully. During the subsequent DRMI ADS investigation, we found that the BS ADS loop was likely destabilizing the DRMI lock. Disabling the BS ADS improved the lock stability. Since the BS injection was more clearly observed in AS DC than in POP 90 and AS RF34, we started investigating AS DC as an alternative sensing signal and began transfer-function measurements for phasing.

Initial alignment

We performed the initial alignment through the OMC. The X arm, Y arm, and OMC alignment were completed without any significant issue. The OMC transmission was approximately 33.

DRMI ADS investigation

We then investigated the instability of the DRMI ADS during ALIGNING_DRMI. The DRMI lock appeared to be disturbed by the BS ADS loop. After disabling the BS ADS, the lock became significantly more stable, indicating that this loop was likely the main source of the instability.

With the BS ADS injection kept on, we checked several possible sensing signals. A small response was visible in AS RF34, while a much clearer response was observed in AS DC. Based on this result, we started investigating whether AS DC could be used as the sensing signal for the BS ADS loop.

We briefly closed the loop with a low gain, but the BS alignment continued to drift and the loop did not appear to control the alignment properly. We therefore started preparing a transfer-function measurement to determine the appropriate demodulation phase using AS DC.

Current status

• Initial alignment through the OMC was completed successfully.
• Disabling the BS ADS improved the DRMI lock stability.
• AS DC showed the clearest response to the BS ADS injection.
• Transfer-function measurements were started to investigate the BS ADS phasing.

Ushiba, Komori, Tanaka (original idea by Fujimoto-kun?)

Ushiba-san modified the script of TOF measuremet for schnupp asymmentry in order to be able measure the PRC length. We ran the script and tried to measure the REFL PDA3 RF45 optimal demod. phases for Xarm and PRMI, respectively. This time, we locked PRMI with the 1f signal (VERTEX guardian state is PRMI_1F_LOCKED). The basic procedure is the same as in klog36343. 

Each optimal demod. phase for PRMI and Xarm is written as follows,

$\theta_{\mathrm{PRMI}} = \frac{\omega_{\mathrm{mod}}}{c} (L_{\mathrm{EOM2PRM}} + L_{\mathrm{PRM2PD}})$

$\theta_{\mathrm{Xarm}} = \frac{\omega_{\mathrm{mod}}}{c} (L_{\mathrm{EOM2PRM}} + 2 (L_{\mathrm{PRM2BS}} + L_{\mathrm{BS2ITMX}}) + L_{\mathrm{PRM2PD}})$

where, omega_{mod} is a modulation angular frequency, L_{A2B} is a distance from A to B (A,B: EOM, PD, PRM, BS, ITMX), and c is a speed of light. These phase difference is derived as

$\theta_{\mathrm{Xarm}} - \theta_{\mathrm{PRMI}} = \frac{\omega_{\mathrm{mod}}}{c} \times 2(L_{\mathrm{PRM2BS}}+ L_{\mathrm{BS2ITMX}}) = \frac{\omega_{\mathrm{mod}}}{c} \times 2(L_{\mathrm{PRM2BS}}+ \frac{L_{\mathrm{BS2ITMX}} + L_{\mathrm{BS2ITMY}}}{2} + \frac{L_{\mathrm{BS2ITMX}} - L_{\mathrm{BS2ITMY}}}{2}) = \frac{\omega_{\mathrm{mod}}}{c} \times 2(L_{\mathrm{PRCL}} + \frac{L_{\mathrm{asym}}}{2})$

we can derive

$L_{\mathrm{PRCL}} + \frac{L_{\mathrm{asym}}}{2} = \frac{c}{2\omega_{\mathrm{mod}}} (\theta_{\mathrm{Xarm}} - \theta_{\mathrm{PRMI}})$

## Results

Fig.1 and Fig.2 show the results of measurements for Xarm and PRMI, The optimal demod. phases of REFL PDA3 RF45 for Xarm and PRMI were 230.14 degrees and 117.80 degrees, respectively. (I forgot to note the error value...)

If I assumed 20*360 degrees additional rotation because 45 MHz is 20 times FSR of PRMI, L_PRCL + L_asym/2 = 2.9979e8 * (20*360 + 230.14 -117.80) / (2*360 * 8* 5.624365513 MHz) ~ 67.67 m

It seems to be inconsistent with the design value (L_PRCL + L_asym/2 = 66.591 + (26.6649 - 23.3351)/2 = 68.2559 m (ref. JGWwiki)).

We are not sure of the cause of this discrepancy. We suspect that the RF45 sideband is not exactly at the anti-resonance point of the PRMI. As a result, its reflection phase deviates from 180°. This effect is expected to be more noticeable in the low-finesse PRMI than in the high-finesse X arm cavity.

But we did not confirm whether the SNR is enough or not with PRMI configration. We need more investigation.

[Kawakami, Komori, Tanaka, Saito]

A mixer and a 140 MHz low-pass filter were inserted between the 20 dB RF amplifier and the 45 dB RF amplifier. By setting the mixer's LO frequency to 350 MHz, the beat signal at approximately 450 MHz was successfully down-converted to approximately 100 MHz. PLL operation was successfully achieved using this down-converted signal. In the next experiment, a spectrum analyzer will be installed at the OMC REFL port, and since the mixer supports input frequencies up to 500 MHz, beat signals up to approximately ±500 MHz will be analyzed. The cavity lengths of SRX, SRY, PRX, and PRY will then be measured by applying the same fitting procedure used in klog:37209.
 

• First, the signal after the 20 dB RF amplifier was monitored with a spectrum analyzer, and the sub-laser temperature was adjusted so that the beat frequency became approximately 450 MHz. A mixer and a 140 MHz low-pass filter were then inserted between the 20 dB RF amplifier and the 45 dB RF amplifier. In addition, the 10 dB attenuator located before the PFD was removed. The current control chain from the RFPD to the sub-laser PZT is as follows:

  RFPD → 12 MHz high-pass filter → 20 dB RF amplifier → mixer → 140 MHz low-pass filter → 45 dB RF amplifier → power splitter → PFD → 100 kHz low-pass filter → Moku:Lab (100 Hz integrator, 10 kHz low-pass filter) → SR560 (gain = 200, 1 Hz low-pass filter) → 20 dB high-voltage amplifier → sub-laser PZT
   

• With the mixer's LO frequency set to 350 MHz, the signal taken from the power splitter before the PFD was monitored using the Moku:Lab spectrum analyzer, and the beat signal was observed at approximately 100 MHz. When the mixer's LO frequency was decreased, the beat frequency increased, confirming that the original beat signal was at approximately 450 MHz. Since the PFD locks with an offset when the input frequency is too low, the sub-laser temperature was adjusted so that the down-converted signal became approximately 160 MHz. The mixer's LO frequency was then changed to 340 MHz, resulting in a down-converted beat frequency of approximately 135 MHz. Finally, by setting the PFD LO frequency to 135 MHz, stable PLL lock was successfully achieved.

• In the next experiment, a spectrum analyzer will be installed at the OMC REFL port. Since the mixer can process signals up to 500 MHz, beat signals up to approximately ±500 MHz will be measured. The same fitting procedure as in klog:37209 will then be applied to determine the lengths of the SRX, SRY, PRX, and PRY cavities.

[Kawakami, Tanaka, Hirose]

We performed the initial alignment for Xarm, Yarm, PRMI, and SRY.

• We attempted DRMI initial alignment partway through, but since the BS ADS isn’t working correctly, adjustments are needed to separate the signal degrees of freedom.
• During the SRY initial alignment, the amplitude of the SRCL control feedback signal was saturating, so following Ushiba-san's advice, we turned on FM7 (Elptrick Filter 500 Hz). We also modified the settings so that it turns on every time in the “LOCKING_SRC_3F” state of the VERTEX Guardian.

[Tanaka, Saito]

The sub-laser was injected into the SRY cavity, and the PLL was locked. The LO frequency was then frequency-modulated by ±10 kHz at a modulation frequency of 1 kHz. As a result, a signal appeared at 1 kHz in the power spectrum of the OMC REFL DC PD. By adjusting the LO frequency to minimize this signal, its amplitude was reduced to approximately the noise floor. To further improve the frequency resolution, the frequency modulation amplitude was increased to ±30 kHz. However, the amplitude of the 1 kHz signal changed in response to spontaneous fluctuations in both the frequency and amplitude of the beat signal observed at the OMC REFL RF PD, making it difficult to determine the beat frequency with 1 kHz-level precision. Therefore, it is considered that the beat frequency can be determined with a precision on the order of 10 kHz.
 

• First, the SRY cavity was locked, and the main-laser intensity noise measured by the OMC REFL DC PD was reduced to a level comparable to that reported in klog:37144. The sub-laser was then injected into SRY, and the PLL was locked. A Moku:Lab was used as the LO source. Using its frequency modulation function, the LO frequency was swept by ±2 MHz at 10 mHz, and the LO frequency was adjusted to approximately maximize the beat signal observed at the OMC REFL RF PD.
   
• Next, the LO frequency was modulated by ±10 kHz at 1 kHz. A spectral peak approximately five times higher than the noise floor appeared at 1 kHz in the power spectrum of the OMC REFL DC PD. The LO frequency was then adjusted so as to minimize this peak, and at a certain frequency the peak became comparable to the noise floor. To improve the frequency accuracy further, the modulation amplitude was increased to ±30 kHz. This again produced a peak at 1 kHz, and the LO frequency was adjusted in 1 kHz increments to minimize it. However, the amplitude of the 1 kHz peak varied together with spontaneous changes in the frequency and amplitude of the beat signal observed at the OMC REFL RF PD. As a result, determining the beat frequency with 1 kHz-level precision proved difficult. It is therefore concluded that the beat frequency can be determined with a precision on the order of 10 kHz.
   
• The final power spectra of the OMC REFL DC PD are shown in Figures 1 and 2. In Figure 1, the red trace corresponds to the present measurement. Its higher noise level at high frequencies compared with the other traces is attributed to the additional intensity noise from the sub-laser. In Figure 2, the green trace was measured with the sub-laser injected but without LO frequency modulation; the blue trace was measured after initially adjusting the LO frequency to approximately maximize the OMC REFL beat signal; and the red trace was measured after increasing the modulation amplitude to ±30 kHz and fine-tuning the LO frequency with approximately 1 kHz resolution.

[ Kimura, Nakagaki ]

We tested the device for acquiring the open/closed status of the PRM-PR3 gate valve and confirmed that it operates correctly.