The fitting results for SRY in klog:37466 are shown below.

The measurement data are saved in the following Dropbox folder:

All files/Dropbox KAGRA/Measurements/IFO/SRCL/SRCL_OMMT2_REFL_beat_signal_PZT_sweep/2026_09_07_SRY
 

• Fig. 1: Around 1.6 GHz, without a linear background. Using data from 1610.59 MHz to 1611.02 MHz, the fitted peak frequency was 1610.8297(27) MHz.
  Fig. 2: Around 1.6 GHz, with a linear background. Using data from 1610.59 MHz to 1611.02 MHz, the fitted peak frequency was 1610.893(13) MHz.

  Fig. 3: Around 800 MHz, without a linear background. Using data from 802.82 MHz to 803.27 MHz, the fitted peak frequency was 803.0799(23) MHz.
  Fig. 4: Around 800 MHz, with a linear background. Using data from 802.82 MHz to 803.27 MHz, the fitted peak frequency was 803.147(27) MHz.

  Fig. 5: Around -800 MHz, without a linear background. Using data from 800.72 MHz to 801.16 MHz, the fitted peak frequency was 800.9089(24) MHz.
  Fig. 6: Around -800 MHz, with a linear background. Using data from 800.72 MHz to 801.16 MHz, the fitted peak frequency was 800.839(13) MHz.

  Fig. 7: Around -1.6 GHz, without a linear background. Using data from 1592.3195 MHz to 1592.7295 MHz, the fitted peak frequency was 1592.4944(25) MHz.
  Fig. 8: Around -1.6 GHz, with a linear background. Using data from 1592.3195 MHz to 1592.7295 MHz, the fitted peak frequency was 1592.4459(93) MHz.

[Fujimoto, Saito]

We adjusted the HWP and polarizer for SRY to obtain a clean peak and acquired the beat signal data. After fitting the data, the deviation of the SRY length from the design value was found to be 2.332(93) cm. This result is consistent with the previous measurements reported in klog:37455: the deviations from the design value were 2.168(80) cm with the HWP scale at 14 and the polarizer scale at 295, and 2.319(71) cm with the HWP scale at 59 and the polarizer scale at 250. However, it is slightly different from the result obtained before moving the lens to improve the mode-matching ratio (klog:37250), where the deviation of the SRY length from the design value was 1.757(50) cm.

Using the SRX result from klog:37450 and the results from klog:37458, I calculated the SRC length and Schnupp asymmetry as follows:

SRC length: 66.61860(51) m
Schnupp asymmetry calculated from the SRC: 3.33775(102) m
Schnupp asymmetry from the weighted average: 3.33842(49) m

These results are consistent with the previous measurements reported in klog:37455 and klog:37458. With the HWP scale at 14 and the polarizer scale at 295, the SRC length was 66.61778(45) m, the Schnupp asymmetry calculated from the SRC was 3.339390(904) m, and the Schnupp asymmetry from the weighted average was 3.33884(48) m. With the HWP scale at 59 and the polarizer scale at 250, the corresponding values were 66.61853(41) m, 3.337880(825) m, and 3.33839(47) m, respectively. Comparing these results with the measurements before moving the lens to improve the mode-matching ratio (klog:37260), where the SRC length was 66.61409(70) m, the Schnupp asymmetry calculated from the SRC was 3.34023(139) m, and the Schnupp asymmetry from the weighted average was 3.33791(45) m, the SRC length and the Schnupp asymmetry calculated from the SRC are somewhat different. However, the Schnupp asymmetry obtained from the weighted average is consistent with the previous result.
 

• First, we set the HWP scale at the sub-laser to 82 (S polarization) and set the polarizer in front of the RFPD to S polarization. With the main laser turned off, we adjusted the alignment of the sub-laser to SRY. Next, we locked the main laser to SRY, adjusted the alignment to the RFPD, and checked the beat signal. However, as in the previous measurement (klog:37440), the peak was split. Therefore, we adjusted the alignment of the sub-laser to maximize the beat signal, and rotated the HWP and polarizer to maximize the beat signal as well. The HWP scale was then 124 (P polarization), and the polarizer scale was 2 (P polarization). However, the peak was split into three peaks.

  Next, we turned off the main laser and adjusted the alignment again using the sub-laser. When the polarizer in front of the RFPD was removed, the maximum signal was -16 counts for S-polarized injection and -52 counts for P-polarized injection, giving a difference of about a factor of three. The peak also appeared cleaner as a single peak for S polarization, whereas it was split into about three peaks for P polarization.

  Next, with the HWP scale at 82 (S polarization), we set the polarizer scale to 272 (S polarization). Peaks separated by approximately 1 MHz were observed (Photo 1). we then set the polarizer scale to 317, and one of the two peaks became larger (Photo 2). When I set the polarizer scale to 227, the other peak appeared to become larger (Photo 3). The three cases are plotted together in Fig. 1.
   

• Next, we acquired beat signal data around 1.6 GHz (Photo 4), 0.8 GHz (Photo 5), -0.8 GHz (Photo 6), and -1.6 GHz (Photo 7) with the HWP scale at 82 and the polarizer scale at 317, where the peak appeared the cleanest. I fitted the obtained SRY data both with and without a linear background. I will post the fitting results for the individual peaks later. From the peak frequencies obtained from the fits and their uncertainties, I calculated the minimum and maximum values within the uncertainty ranges. I then took the overall minimum and maximum values from the two fitting models, with and without a linear background, as the uncertainty range.
  Therefore, I used the following data to determine the SRY length:

  Minimum (MHz)    Maximum (MHz)
  1610.8271    1610.9060
  803.0776    803.1743
  -800.9112    -800.8258
  -1592.4969    -1592.4366

  I set the FSR index of the data around -1.6 GHz to 0. For each pair of nearby peak frequencies, I calculated the frequency difference and divided it by the design value of the FSR. I then rounded the resulting value to the nearest integer to determine the FSR index. I fitted the measured peak frequencies as a function of the FSR index using the linear function AN+B, where A and B are fitting parameters and N is the FSR index.
  The fitting results are shown below (Fig. 2):

  A: 2.307881(33) MHz
  B: -1592.467(27) MHz

  Since A corresponds to the FSR, I calculated the SRY length from the fitted value of A, obtaining:

  Fitted value: 64.94972(93) m
  Design value: 64.9264 m
  Difference from the design value (fitted value − design value): 2.332(93) cm

  This result is consistent with the previous measurements reported in klog:37455: the deviations from the design value were 2.168(80) cm with the HWP scale at 14 and the polarizer scale at 295, and 2.319(71) cm with the HWP scale at 59 and the polarizer scale at 250. However, it is slightly different from the result obtained before moving the lens to improve the mode-matching ratio (klog:37250), where the deviation of the SRY length from the design value was 1.757(50) cm.
   

• Using the SRX result from klog:37450 and the results from klog:37458, I calculated the SRC length and Schnupp asymmetry as follows:

  SRC
  Calculated value: 66.61860(51) m
  Design value: 66.5913 m
  Difference from the design value (calculated value − design value): 2.730(51) cm

  Schnupp asymmetry calculated from the SRC
  Calculated value: 3.33775(102) m
  Design value: 3.3298 m
  Difference from the design value (calculated value − design value): 0.795(102) cm

  Schnupp asymmetry
  Weighted average: 3.33842(49) m
  Design value: 3.3298 m
  Difference from the design value (weighted average − design value): 0.862(49) cm

  These results are consistent with the previous measurements reported in klog:37455 and klog:37458. With the HWP scale at 14 and the polarizer scale at 295, the SRC length was 66.61778(45) m, the Schnupp asymmetry calculated from the SRC was 3.339390(904) m, and the Schnupp asymmetry from the weighted average was 3.33884(48) m. With the HWP scale at 59 and the polarizer scale at 250, the corresponding values were 66.61853(41) m, 3.337880(825) m, and 3.33839(47) m, respectively. Compared with the measurements before moving the lens to improve the mode-matching ratio (klog:37260), where the SRC length was 66.61409(70) m, the Schnupp asymmetry calculated from the SRC was 3.34023(139) m, and the Schnupp asymmetry from the weighted average was 3.33791(45) m, the SRC length and the Schnupp asymmetry calculated from the SRC are somewhat different. However, the Schnupp asymmetry obtained from the weighted average is consistent with the previous result.

I offloaded the BF GAS with the FR.

I offloaded the F0 GAS with the FR.

The fitting results for PRX and PRY in klog:37458 are shown below.
The measurement data are saved in the following Dropbox folders:

All files/Dropbox KAGRA/Measurements/IFO/PRCL/PRCL_OMMT2_REFL_beat_signal_PZT_sweep/2026_09_04_PRX
All files/Dropbox KAGRA/Measurements/IFO/PRCL/PRCL_OMMT2_REFL_beat_signal_PZT_sweep/2026_09_04_PRY
 

• PRX
  Fig. 1: Around 1.6 GHz, without a linear background. Using the data from 1591.51 MHz to 1592.0 MHz, the fitted peak frequency was 1591.7750(12) MHz.
  Fig. 2: Around 1.6 GHz, with a linear background. Using the data from 1591.51 MHz to 1592.0 MHz, the fitted peak frequency was 1591.8014(56) MHz.

  Fig. 3: Around 800 MHz, without a linear background. Using the data from 807.62 MHz to 808.15 MHz, the fitted peak frequency was 807.8942(17) MHz.
  Fig. 4: Around 800 MHz, with a linear background. Using the data from 807.62 MHz to 808.15 MHz, the fitted peak frequency was 807.8842(49) MHz.

  Fig. 5: Around -800 MHz, without a linear background. Using the data from 821.12 MHz to 821.59 MHz, the fitted peak frequency was 821.3504(11) MHz.
  Fig. 6: Around -800 MHz, with a linear background. Using the data from 821.12 MHz to 821.59 MHz, the fitted peak frequency was 821.3352(35) MHz.

  Fig. 7: Around -1.6 GHz, without a linear background. Using the data from 1563.29 MHz to 1563.82 MHz, the fitted peak frequency was 1563.5494(14) MHz.
  Fig. 8: Around -1.6 GHz, with a linear background. Using the data from 1563.29 MHz to 1563.82 MHz, the fitted peak frequency was 1563.5525(32) MHz.
   

• PRY
  Fig. 9: Around 1.6 GHz, without a linear background. Using the data from 1599.85 MHz to 1600.57 MHz, the fitted peak frequency was 1600.1828(18) MHz.
  Fig. 10: Around 1.6 GHz, with a linear background. Using the data from 1599.85 MHz to 1600.57 MHz, the fitted peak frequency was 1600.1637(33) MHz.

  Fig. 11: Around 800 MHz, without a linear background. Using the data from 810.32 MHz to 810.95 MHz, the fitted peak frequency was 810.6120(14) MHz.
  Fig. 12: Around 800 MHz, with a linear background. Using the data from 810.32 MHz to 810.95 MHz, the fitted peak frequency was 810.5939(29) MHz.

  Fig. 13: Around -800 MHz, without a linear background. Using the data from 816.67 MHz to 817.23 MHz, the fitted peak frequency was 816.9934(28) MHz.
  Fig. 14: Around -800 MHz, with a linear background. Using the data from 816.67 MHz to 817.23 MHz, the fitted peak frequency was 817.0274(72) MHz.

  Fig. 15: Around -1.6 GHz, without a linear background. Using the data from 1596.96 MHz to 1597.66 MHz, the fitted peak frequency was 1597.3557(27) MHz.
  Fig. 16: Around -1.6 GHz, with a linear background. Using the data from 1596.96 MHz to 1597.66 MHz, the fitted peak frequency was 1597.4093(82) MHz.

[Fujimoto, Ye, Saito]

We adjusted the HWP and polarizer to obtain clean peaks and acquired beat signal data for PRX and PRY. After fitting the data, the deviations from the design values were 0.926(22) cm for PRX and 0.043(52) cm for PRY. Compared with the previous measurements before moving the lens to improve the mode-matching ratio (klog:37260), which gave deviations of 0.677(11) cm for PRX and -0.107(46) cm for PRY, both results are slightly different.

We also calculated the PRC length and the Schnupp asymmetry from the PRC. Using the Schnupp asymmetry calculated from the SRC in klog:37455, we then calculated the Schnupp asymmetry by taking a weighted average of the two values. The results were as follows:

PRC length: 66.59625(28) m
Schnupp asymmetry calculated from the PRC: 3.338630(565) m
Schnupp asymmetry from the weighted average:
3.33884(48) m when SRY was measured with the HWP scale at 14 and the polarizer scale at 295
3.33839(47) m when SRY was measured with the HWP scale at 59 and the polarizer scale at 250

Compared with the previous measurements before moving the lens to improve the mode-matching ratio (klog:37260), which gave a PRC length of 66.59425(24) m, a Schnupp asymmetry calculated from the PRC of 3.337640(473) m, and a Schnupp asymmetry from the weighted average of 3.33791(45) m, the PRC length is slightly different. However, there is no inconsistency between the Schnupp asymmetry calculated from the PRC and that calculated from the weighted average.
 

• To acquire the beat signal data for PRX, we first turned off the main laser and adjusted the alignment of the sub-laser. We then turned on the main laser and adjusted the alignment to the RFPD. When we checked the beat signal with the HWP scale at 82 and the polarizer scale at 50 using a spectrum analyzer, the peak was not clean (Photo 1). We therefore rotated the HWP and polarizer. The peak became clean when the HWP scale was set to 70 and the polarizer scale was set to 254. We then acquired data around 1.6 GHz (Photo 2), 0.8 GHz (Photo 3), -0.8 GHz (Photo 4), and -1.6 GHz (Photo 5).
   
• Similarly, for PRY, we first turned off the main laser and adjusted the alignment of the sub-laser. We then turned on the main laser and adjusted the alignment to the RFPD. We set the HWP scale to 16 and the polarizer scale to 117 to obtain clean peaks, and acquired data around 1.6 GHz (Photo 6), 0.8 GHz (Photo 7), -0.8 GHz (Photo 8), and -1.6 GHz (Photo 9).
   

• We then fitted the obtained PRX and PRY data with and without a linear background. I will post the fitting results for the individual peaks later. From the peak frequencies obtained from the fits and their uncertainties, we calculated the minimum and maximum values within the uncertainty ranges. We then took the overall minimum and maximum values from both fitting models, with and without a linear background, as the uncertainty range. The following data were therefore used to determine the PRX and PRY lengths.

  PRX
  Minimum (MHz)    Maximum (MHz)
  1591.7738    1591.8070
  807.8792    807.8958
  -821.3516    -821.3318
  -1563.5557    -1563.5479

  PRY
  Minimum (MHz)    Maximum (MHz)
  1600.1604    1600.1846
  810.5910    810.6133
  -817.0346    -816.9907
  -1597.4175    -1597.3530

  We set the FSR index of the data around -1.6 GHz to 0. For each pair of nearby peak frequencies, we calculated the frequency difference and divided it by the design value of the FSR. We then rounded the resulting value to the nearest integer to determine the FSR index. We fitted the measured peak frequencies as a function of the FSR index using the linear function AN+B, where A and B are fitting parameters and N is the FSR index.

  The fitting results were as follows.

  PRX (Fig. 1)
  A: 2.1957810(72) MHz
  B: -1563.5483(38) MHz

  PRY (Fig. 2)
  A: 2.308691(19) MHz
  B: -1597.363(21) MHz

  Since A corresponds to the FSR, we calculated the PRX and PRY lengths from the fitted values of A. The results were as follows.

  PRX
  Fitted length: 68.26556(22) m
  Design value: 68.2563 m
  Difference from the design value (fitted length − design value): 0.926(22) cm

  PRY
  Fitted length: 64.92693(52) m
  Design value: 64.9265 m
  Difference from the design value (fitted length − design value): 0.043(52) cm

  Compared with the previous measurements before moving the lens to improve the mode-matching ratio (klog:37260), which gave deviations of 0.677(11) cm for PRX and -0.107(46) cm for PRY, both results are slightly different.
   

• We also calculated the PRC length and the Schnupp asymmetry from the PRC. Using the Schnupp asymmetry calculated from the SRC in klog:37455, we calculated the Schnupp asymmetry from the weighted average as follows.

  PRC
  Calculated value: 66.59625(28) m
  Design value: 66.5914 m
  Difference from the design value (calculated value − design value): 0.485(28) cm

  Schnupp asymmetry calculated from the PRC
  Calculated value: 3.338630(565) m
  Design value: 3.3298 m
  Difference from the design value (calculated value − design value): 0.8830(565) cm

  Schnupp asymmetry
  When SRY was measured with the HWP scale at 14 and the polarizer scale at 295:
  Weighted average: 3.33884(48) m
  Design value: 3.3298 m
  Difference from the design value (weighted average − design value): 0.904(48) cm

  When SRY was measured with the HWP scale at 59 and the polarizer scale at 250:
  Weighted average: 3.33839(47) m
  Design value: 3.3298 m
  Difference from the design value (weighted average − design value): 0.859(47) cm

  Compared with the previous measurements before moving the lens to improve the mode-matching ratio (klog:37260), which gave a PRC length of 66.59425(24) m, a Schnupp asymmetry calculated from the PRC of 3.337640(473) m, and a Schnupp asymmetry from the weighted average of 3.33791(45) m, the PRC length is slightly different. However, there is no inconsistency between the Schnupp asymmetry calculated from the PRC and that calculated from the weighted average.

The fitting results for the SRY measurements in klog:37455, with the HWP scale at 14 and the polarizer scale at 295, and with the HWP scale at 59 and the polarizer scale at 250, are shown below.

The measurement data are saved in the following Dropbox folders:

All files/Dropbox KAGRA/Measurements/IFO/SRCL/SRCL_OMMT2_REFL_beat_signal_PZT_sweep/2026_09_01_SRY_hwp14_pol295
All files/Dropbox KAGRA/Measurements/IFO/SRCL/SRCL_OMMT2_REFL_beat_signal_PZT_sweep/2026_09_01_SRY_hwp59_pol250
 

• HWP scale: 14, Polarizer scale: 295

  Fig. 1: Around 1.6 GHz, without a linear background. Using the data from 1611.21 MHz to 1612.42 MHz, the fitted peak frequency was 1611.809(10) MHz.
  Fig. 2: Around 1.6 GHz, with a linear background. Using the data from 1611.21 MHz to 1612.42 MHz, the fitted peak frequency was 1611.766(45) MHz.

  Fig. 3: Around 800 MHz, without a linear background. Using the data from 810.37 MHz to 811.56 MHz, the fitted peak frequency was 810.962(11) MHz.
  Fig. 4: Around 800 MHz, with a linear background. Using the data from 810.37 MHz to 811.56 MHz, the fitted peak frequency was 810.993(45) MHz.

  Fig. 5: Around -800 MHz, without a linear background. Using the data from 845.47648 MHz to 846.683146 MHz, the fitted peak frequency was 846.1233(96) MHz.
  Fig. 6: Around -800 MHz, with a linear background. Using the data from 845.47648 MHz to 846.683146 MHz, the fitted peak frequency was 846.033(45) MHz.

  Fig. 7: Around -1.6 GHz, without a linear background. Using the data from 1655.6 MHz to 1656.77 MHz, the fitted peak frequency was 1656.233(13) MHz.
  Fig. 8: Around -1.6 GHz, with a linear background. Using the data from 1656.6 MHz to 1656.77 MHz, the fitted peak frequency was 1656.2322(79) MHz.
   

• HWP scale: 59, Polarizer scale: 250

  Fig. 9: Around 1.6 GHz, without a linear background. Using the data from 1599.24 MHz to 1600.04 MHz, the fitted peak frequency was 1599.5509(77) MHz.
  Fig. 10: Around 1.6 GHz, with a linear background. Using the data from 1599.24 MHz to 1600.04 MHz, the fitted peak frequency was 1599.507(11) MHz.

  Fig. 11: Around 800 MHz, without a linear background. Using the data from 789.2 MHz to 789.96 MHz, the fitted peak frequency was 789.4894(61) MHz.
  Fig. 12: Around 800 MHz, with a linear background. Using the data from 789.2 MHz to 789.96 MHz, the fitted peak frequency was 789.449(11) MHz.

  Fig. 13: Around -800 MHz, without a linear background. Using the data from 814.0 MHz to 814.82 MHz, the fitted peak frequency was 814.4756(78) MHz.
  Fig. 14: Around -800 MHz, with a linear background. Using the data from 814.0 MHz to 814.82 MHz, the fitted peak frequency was 814.541(15) MHz.

  Fig. 15: Around -1.6 GHz, without a linear background. Using the data from 1582.56 MHz to 1583.35 MHz, the fitted peak frequency was 1583.0282(66) MHz.
  Fig. 16: Around -1.6 GHz, with a linear background. Using the data from 1582.56 MHz to 1583.35 MHz, the fitted peak frequency was 1583.0585(99) MHz.

We analyzed the SRY data obtained in klog:37440. The deviation from the design value was 2.168(80) cm when the HWP scale was 14 and the polarizer scale was 295, and 2.319(71) cm when the HWP scale was 59 and the polarizer scale was 250. Compared with the previous measurement (klog:37393), which gave a deviation of 2.385(42) cm from the SRY design value, the result with the HWP scale at 59 is consistent, while the result with the HWP scale at 14 appears to be slightly different. Both results differ more significantly from the result before moving the lens to improve the mode-matching ratio (klog:37250), which gave a deviation of 1.757(50) cm from the design value.

Using the SRX result from klog:37450, we also calculated the SRC length and the Schnupp asymmetry. The SRC lengths were 66.61778(45) m for the HWP scale of 14 and polarizer scale of 295, and 66.61853(41) m for the HWP scale of 59 and polarizer scale of 250. The Schnupp asymmetries calculated from the SRC lengths were 3.339390(904) m and 3.337880(825) m, respectively. Compared with the previous measurement (klog:37393), which gave an SRC length of 66.61828(92) m and a Schnupp asymmetry calculated from the SRC of 3.33605(185) m, both SRC lengths are consistent, while the Schnupp asymmetry with the HWP scale at 14 appears to be slightly different. Compared with the results before moving the lens to improve the mode-matching ratio (klog:37260), which gave an SRC length of 66.61409(70) m and a Schnupp asymmetry calculated from the SRC of 3.34023(139) m, both SRC lengths are slightly different, while the Schnupp asymmetry with the HWP scale at 59 appears to be slightly different.
 

• We fitted the SRY data obtained in klog:37440 with and without a linear background. I will post the fitting results for the individual peaks later. From the peak frequencies obtained from the fits and their uncertainties, we calculated the minimum and maximum values within the uncertainty ranges. We then took the overall minimum and maximum values from both fitting models, with and without a linear background, as the uncertainty range. The following data were therefore used to determine the SRY length.

  HWP scale: 14, Polarizer scale: 295
  Minimum (MHz)    Maximum (MHz)
  1611.7206    1611.8190
  810.9479    811.0371
  -846.1329    -845.9886
  -1656.2458    -1656.2199

  HWP scale: 59, Polarizer scale: 250
  Minimum (MHz)    Maximum (MHz)
  1599.4960    1599.5586
  789.4376    789.4956
  -814.5555    -814.4679
  -1583.0683    -1583.0217

  We set the FSR index of the data around -1.6 GHz to 0. For each pair of nearby peak frequencies, we calculated the frequency difference and divided it by the design value of the FSR. We then rounded the resulting value to the nearest integer to determine the FSR index. We fitted the measured peak frequencies as a function of the FSR index using the linear function AN+B, where A and B are fitting parameters and N is the FSR index.

  The fitting results were as follows.

  HWP scale: 14, Polarizer scale: 295 (Fig. 1)
  A: 2.307939(29) MHz
  B: -1656.230(13) MHz
  HWP scale: 59, Polarizer scale: 250 (Fig. 2)
  A: 2.307886(26) MHz
  B: -1583.043(22) MHz

  Since A corresponds to the FSR, we calculated the SRY length from the fitted value of A. The results were as follows.

  HWP scale: 14, Polarizer scale: 295
  Fitted length: 64.94808(80) m
  Design value: 64.9264 m
  Difference from the design value (fitted length − design value): 2.168(80) cm
  HWP scale: 59, Polarizer scale: 250
  Fitted length: 64.94959(71) m
  Design value: 64.9264 m
  Difference from the design value (fitted length − design value): 2.319(71) cm

  Compared with the previous measurement (klog:37393), which gave a deviation of 2.385(42) cm from the SRY design value, the result with the HWP scale at 59 is consistent, while the result with the HWP scale at 14 appears to be slightly different. Both results differ more significantly from the result before moving the lens to improve the mode-matching ratio (klog:37250), which gave a deviation of 1.757(50) cm from the design value.
   

• Using the SRX result from klog:37450, we calculated the SRC length and the Schnupp asymmetry as follows.

  HWP scale: 14, Polarizer scale: 295
  SRC
  Calculated value: 66.61778(45) m
  Design value: 66.5913 m
  Difference from the design value (calculated value − design value): 2.648(45) cm

  Schnupp asymmetry calculated from the SRC
  Calculated value: 3.339390(904) m
  Design value: 3.3298 m
  Difference from the design value (calculated value − design value): 0.9590(904) cm

  HWP scale: 59, Polarizer scale: 250
  SRC
  Calculated value: 66.61853(41) m
  Design value: 66.5913 m
  Difference from the design value (calculated value − design value): 2.723(41) cm

  Schnupp asymmetry calculated from the SRC
  Calculated value: 3.337880(825) m
  Design value: 3.3298 m
  Difference from the design value (calculated value − design value): 0.8080(825) cm

  Compared with the previous measurement (klog:37393), which gave an SRC length of 66.61828(92) m and a Schnupp asymmetry calculated from the SRC of 3.33605(185) m, both SRC lengths are consistent, while the Schnupp asymmetry with the HWP scale at 14 appears to be slightly different. Compared with the results before moving the lens to improve the mode-matching ratio (klog:37260), which gave an SRC length of 66.61409(70) m and a Schnupp asymmetry calculated from the SRC of 3.34023(139) m, both SRC lengths are slightly different, while the Schnupp asymmetry with the HWP scale at 59 appears to be slightly different.

The fitting results for the individual peaks after changing and then restoring the SRX length in klog:37450 are shown below.

The measurement data are saved in the following Dropbox folders:

All files/Dropbox KAGRA/Measurements/IFO/SRCL/SRCL_OMMT2_REFL_beat_signal_PZT_sweep/2026_09_03_SRX_moved
All files/Dropbox KAGRA/Measurements/IFO/SRCL/SRCL_OMMT2_REFL_beat_signal_PZT_sweep/2026_09_03_SRX_restored
 

• After changing the SRX length
  Fig. 1: Around 1.6 GHz, without a linear background. Using the data from 1630.48 MHz to 1631.57 MHz, the fitted peak frequency was 1630.9316(29) MHz.
  Fig. 2: Around 1.6 GHz, with a linear background. Using the data from 1630.48 MHz to 1631.57 MHz, the fitted peak frequency was 1630.9322(38) MHz.

  Fig. 3: Around 800 MHz, without a linear background. Using the data from 789.76 MHz to 790.84 MHz, the fitted peak frequency was 790.2061(23) MHz.
  Fig. 4: Around 800 MHz, with a linear background. Using the data from 789.76 MHz to 790.84 MHz, the fitted peak frequency was 790.2147(25) MHz.

  Fig. 5: Around -800 MHz, without a linear background. Using the data from 798.46 MHz to 799.51 MHz, the fitted peak frequency was 799.0342(30) MHz.
  Fig. 6: Around -800 MHz, with a linear background. Using the data from 798.46 MHz to 799.51 MHz, the fitted peak frequency was 799.0380(41) MHz.

  Fig. 7: Around -1.6 GHz, without a linear background. Using the data from 1588.38 MHz to 1589.78 MHz, the fitted peak frequency was 1589.2838(23) MHz.
  Fig. 8: Around -1.6 GHz, with a linear background. Using the data from 1588.38 MHz to 1589.78 MHz, the fitted peak frequency was 1589.2831(38) MHz.
   

• After restoring the SRX length
  Fig. 9: Around 1.6 GHz, without a linear background. Using the data from 1608.77 MHz to 1609.66 MHz, the fitted peak frequency was 1609.0959(28) MHz.
  Fig. 10: Around 1.6 GHz, with a linear background. Using the data from 1608.77 MHz to 1609.66 MHz, the fitted peak frequency was 1609.0650(63) MHz.

  Fig. 11: Around 800 MHz, without a linear background. Using the data from 796.61 MHz to 797.44 MHz, the fitted peak frequency was 796.9280(26) MHz.
  Fig. 12: Around 800 MHz, with a linear background. Using the data from 796.61 MHz to 797.44 MHz, the fitted peak frequency was 796.898(10) MHz.

  Fig. 13: Around -800 MHz, without a linear background. Using the data from 835.71 MHz to 836.57 MHz, the fitted peak frequency was 836.2367(26) MHz.
  Fig. 14: Around -800 MHz, with a linear background. Using the data from 835.71 MHz to 836.57 MHz, the fitted peak frequency was 836.2598(71) MHz.

  Fig. 15: Around -1.6 GHz, without a linear background. Using the data from 1577.79 MHz to 1578.53 MHz, the fitted peak frequency was 1578.1716(42) MHz.
  Fig. 16: Around -1.6 GHz, with a linear background. Using the data from 1577.79 MHz to 1578.53 MHz, the fitted peak frequency was 1578.168(10) MHz.

[Fujimoto, Ye, Saito]

After Takahashi-san and Ushiba-san changed the length of SRX (klog:37449), and again after they restored the length, we acquired beat signal data around 1.6, 0.8, -0.8, and -1.6 GHz. We then calculated the SRX length. The deviations from the design value were 3.050(11) cm after the length was changed and 3.127(42) cm after the length was restored. Comparing the result after restoring the length with the previous measurement (klog:37393), which gave a deviation of 3.01(18) cm from the design value, we consider the results to be consistent. However, the result differs slightly from the result before moving the lens to improve the mode-matching ratio (klog:37260), which gave a deviation of 2.80(13) cm. According to klog:37449, the SRX length decreased by 1 mm after the length was changed. The two measurements also show that the SRX length decreased by approximately 1 mm after the length was changed. Therefore, we confirmed that this measurement method can detect changes in the cavity length.
 

• After Takahashi-san and Ushiba-san changed the SRX length (klog:37449), we removed the mirror that had been placed in the optical path to the RFPD at the reflection port of OMMT2 in the previous measurement (klog:37445). We then injected the light into the RFPD and adjusted the alignment to maximize the DC output. Next, we injected the sub-laser with the HWP scale set to 82, corresponding to S-polarization. We observed the beat signal with a spectrum analyzer and rotated the polarizer to maximize the beat signal. The scale reading was then set to 50. We acquired data when the beat signal was around 1.6 GHz (Photo 1), 0.8 GHz (Photo 2), -0.8 GHz (Photo 3), and -1.6 GHz (Photo 4).
   
• After Takahashi-san and Ushiba-san restored the SRX length, we adjusted the alignment to maximize the DC output of the RFPD. When we checked the beat signal around 1.6 GHz, the peak appeared to be split into two peaks (Photo 5). We therefore turned off the main laser and adjusted the alignment of the sub-laser. However, the alignment already appeared to be sufficiently optimized, and there was almost no change. We then turned the main laser back on and checked the beat signal again. The peak had become clean, so we acquired data when the beat signal was around 1.6 GHz (Photo 6), 0.8 GHz (Photo 7), -0.8 GHz (Photo 8), and -1.6 GHz (Photo 9).
   

• We fitted the data around each peak with and without a linear background. I will post the fitting results for these peaks later. From the peak frequencies obtained from the fits and their uncertainties, we calculated the minimum and maximum values within the uncertainty ranges. We then took the overall minimum and maximum values from both fitting models, with and without a linear background, as the uncertainty range. The following data were therefore used to determine the SRX length.

  After changing the length
  Minimum (MHz)    Maximum (MHz)
  1630.9283    1630.9360
  790.2038    790.2172
  -799.0422    -799.0312
  -1589.2869    -1589.2793

  After restoring the length
  Minimum (MHz)    Maximum (MHz)
  1609.0587    1609.0987
  796.8883    796.9306
  -836.2669    -836.2342
  -1578.1785    -1578.1583

  We set the FSR index of the data around -1.6 GHz to 0. For each pair of adjacent peak frequencies, we calculated the frequency difference and divided it by the design value of the FSR. We then rounded the resulting value to the nearest integer to determine the FSR index. We fitted the measured peak frequencies as a function of the FSR index using the following linear function AN+B where A and B are fitting parameters and N is the FSR index.

  The fitting results were as follows.

  After changing the length (Fig. 1)
  A: 2.1951014(35) MHz
  B: -1589.2804(34) MHz

  After restoring the length (Fig. 2)
  A: 2.195077(14) MHz
  B: -1578.1721(92) MHz

  Since A corresponds to the FSR, we calculated the SRX length from the fitted value of A. The results were as follows.

  After changing the length
  Fitted length: 68.28670(11) m
  Design value: 68.2562 m
  Difference from the design value (fitted length − design value): 3.050(11) cm

  After restoring the length
  Fitted length: 68.28747(42) m
  Design value: 68.2562 m
  Difference from the design value (fitted length − design value): 3.127(42) cm

  Comparing the result after restoring the length with the previous measurement (klog:37393), which gave a deviation of 3.01(18) cm from the design value, we consider the results to be consistent. However, the result differs slightly from the result before moving the lens to improve the mode-matching ratio (klog:37260), which gave a deviation of 2.80(13) cm.
  According to klog:37449, the SRX length decreased by 1 mm after the length was changed. The two measurements also show that the SRX length decreased by approximately 1 mm after the length was changed. Therefore, we confirmed that this measurement method can detect changes in the cavity length.

[Ushiba, Takahashi]

We moved three SR mirrors to change SRCL. Each setpoint for the IP L LVDT was changed by +200 [um]. SRCL was decreased by 1mm in Total. Moving order was SR3, SRM, and SR2. After changing the setpoints, GRY was locked and aligned. The setpoints for the Oplev yaw were changed from 37 to 31 for SR3 and from -49 to -45 for SR2. Changed values are summarized below. Oplev L was different from IP L by a factor of 2. After the length measurements, all setpoints were recovered.

Note that: OMMT1 equips LIGO-OSEMs, and the OSEMs are using LEDs, whose wavelength is nearly infrared (but not 1um), and the IR camera (without proper filtering) senses the infrared light clearly. Very brighter than stars which the IR camera can detect.

To distinguish the light source, it might be helpful to turn off the OSEM LEDs, if possible.