[Smith, Tanaka, Hirose, Fujimoto, Saito]

The mount of a mirror located near the BS was found to be malfunctioning, so it was replaced. As a result, this mirror can now be used for alignment. The power of the main laser incident on the PLL RFPD was measured and found to have increased from approximately 19 μW (klog:37020) before the BS was replaced with a mirror in klog:37058 to approximately 69 μW. Mirrors and lenses were then installed according to the redesigned optical layout. In the PLL path, the mode-matching ratio between the main laser and the sub-laser was found to be approximately 33%. This is due to a mismatch in the waist positions. If the waist positions are matched, the mode-matching ratio is expected to improve to approximately 94%. Therefore, we plan to modify the sub-laser optical path length after the 50 mm focal-length lens. The mode-matching ratio for the optical path that injects the sub-laser into the interferometer will be evaluated in the next measurement.

• The mirror located near the BS, which is the last mirror encountered by the sub-laser before entering the interferometer, had a faulty mount, making alignment using this mirror impossible. Therefore, the mount was replaced. Before the replacement, the alignment of the PLL optical path was adjusted so that the DC signal at the RFPD was maximized. After replacing the mount, alignment was performed using only this mirror, again maximizing the DC signal at the RFPD.
   
• The power of the main laser incident on the RFPD was then measured. Before the BS was replaced with a mirror in klog:37058, the power was approximately 19 μW (klog:37020), whereas it is now approximately 69 μW.
   

• Next, the mirrors and lenses used in the PLL optical path were reinstalled according to the redesigned optical layout (Fig. 1). The beam profile was measured after the 50 mm focal-length lens and fitted (Fig. 2). The waist positions and waist radii obtained from the fitting are listed below. The coordinate origin is defined at the beam sampler (R:T = 1:9) where the main-laser and sub-laser beams are combined.

  Sub-laser

  x direction: Waist position= −236.3 ± 3.0 mm, Waist radius= 0.0835 ± 0.0011 mm
  y direction: Waist position= −237.6 ± 2.3 mm, Waist radius= 0.0786 ± 0.0008 mm
  →Average: Waist position= −237 mm, Waist radius= 0.081 mm

  For comparison, the beam-profile results of the main laser from klog:37020 were used:

  Main laser

  x direction: Waist position= −220.1 ± 3.7 mm, Waist radius= 0.0577 ± 0.0006 mm
  y direction: Waist position= −170.7 ± 7.6 mm, Waist radius= 0.0674 ± 0.0014 mm
  →Average: Waist position= −195.4 mm, Waist radius= 0.0626 mm

  Using these results, the mode-matching ratio was calculated to be approximately 33%. Since the waist positions differ by 41.5 mm, the beam sampler (R:T = 1:9), where the main and sub-laser beams are combined, needs to be moved 20.8 mm to the right in Fig. 1. If the waist positions are perfectly matched, the mode-matching ratio is expected to improve to approximately 94%. In addition, one of the mirrors in the main-laser optical path was moved slightly relative to its position in klog:37020. Therefore, before moving the beam sampler (R:T = 1:9), we plan to remeasure the beam profile of the main laser and determine the required adjustment based on the new measurement results. Once the mode-matching ratio has been improved, the beat signal will be investigated again.
   

• Finally, based on the optical layout shown in Fig. 1, four additional mirrors were installed to increase the optical path length of the sub-laser beam directed toward the interferometer. As a result, the 200 mm and 150 mm focal-length lenses require little or no repositioning. In the next measurement, beam profiles will be measured both before and after the waist to confirm whether the waist position is correct. If satisfactory mode matching is achieved, alignment of the sub-laser beam into the interferometer will then be performed.

[Smith, Hirose, Saito]

A 100 mm focal-length lens was placed before the FI to reduce the beam diameter, and the beam profile remained clean after passing through the FI. This indicates that the large beam diameter of the sub-laser was likely the cause of the distortion. Another 100 mm focal-length lens was then placed after the FI, and the beam profile after this lens was measured. Based on the results, the optical layout will be modified.
 

• First, a beam profiler was placed after the reflection from the beam sampler (R:T = 9:1), and the angle and height of the FI were adjusted to determine whether the beam shape could be improved. However, no improvement was observed. Next, the aperture of the FI was inspected and appeared clean. Then, with the beam profiler still positioned after the reflection from the beam sampler (R:T = 9:1), a 100 mm focal-length lens was placed before the FI. In this configuration, the beam profile remained clean (Fig. 1). Therefore, the large beam diameter of the sub-laser appears to have been the cause of the distorted beam profile.
   

• Next, another 100 mm focal-length lens was placed after the FI so that the beam divergence would be similar to that before the first lens was inserted. The beam profile after this second lens was then measured and fitted (Fig. 2). The waist positions and waist radii obtained from the fitting are as follows:

  x direction: Waist position= 952.2 ± 2.2 mm, Waist radius= 0.1782 ± 0.0014 mm
  y direction: Waist position= 960.8 ± 1.8 mm, Waist radius= 0.1790 ± 0.0012 mm
  →Average: Waist position= 957 mm, Waist radius= 0.179 mm
   

• In the next experiment, the optical layout will be modified based on these waist parameters. Specifically, we plan either to move the 200 mm focal-length lens by +270 mm and the 150 mm focal-length lens by +296 mm, or to increase the optical path length by 1600 mm.

[Saito, Hirose, Tanaka, Ushiba, Fujimoto]

Abstract

We built and tested a transimpedance circuit for an AC-coupled PD to be used for the SRCL and PRCL length measurements.
The designed parameters are a high-pass cutoff of 0.5 Hz, a low-pass cutoff of 100 kHz, and a transimpedance of 10 kOhm.
In the test using voltage injection from Moku:Lab, we obtained results consistent with the LTspice simulation.
Next, we plan to attach a photodiode to the board and build a mount to fix the board on the optical table.

Details

Background
For the SRCL and PRCL length measurements, the main laser is locked to the interferometer, and an auxiliary laser phase-locked to the main laser is scanned to measure the flashes.
During this measurement, the transmitted light of the main laser is also incident on the PD and disturbs the measurement. Therefore, the DC component and low-frequency fluctuations of this light need to be removed. For this purpose, an AC-coupled PD is required, and we built a transimpedance circuit for it.

Design of the transimpedance circuit
Fig. 1 shows the circuit diagram of the designed transimpedance circuit. The important parameters are as follows:

• High-pass cutoff: 0.5 Hz
• Low-pass cutoff: 100 kHz
• Transimpedance in the flat region: 10 kOhm

Fig. 2 shows the simulated frequency response of the transimpedance.

Assembly of the transimpedance circuit
Fig. 3 shows a photo of the transimpedance circuit built on a universal board.

Test of the transimpedance circuit
We tested the circuit by connecting Moku:Lab and measuring the transfer function from the input voltage to the output voltage.
Since Moku:Lab has a 50 Ohm output impedance, the circuit diagram when it is connected to the transimpedance circuit is as shown in Fig. 4.
The simulated gain expected in this configuration is shown in Fig. 5, where the gain in the flat region is expected to be 46 dB.

Fig. 6 shows the measurement result. The measured result agrees well with the simulation, indicating that the circuit is working properly.

Plan for tomorrow

Tomorrow, we plan to attach a photodetector (FGA21, Thorlabs) to the board we built. We also plan to make a mount so that the board can be fixed on the optical table.

I offloaded the F1 GAS with the FR.

I offloaded the BF GAS with the FR.

[Kimura and Yasui]
 On JUne 15, we had a final vacuum leak test for SRM.
Then we  detected a vacuum leak in the range of 1×10^-10 Pa·m³/s at the side flange on the +X side. 
We confirmed that there were no other leaking flanges besides this one.

[Ushiba, Smith, Saito]

We measured the upstream laser power, which had not been measured previously, and found that the transmission of the FI was approximately 86%, indicating that there was no unexplained laser power loss. The beam profile of the sub-laser itself was also measured and found to be clean. However, when the beam profile was measured after reflection from the beam sampler (R:T = 9:1), the beam shape became distorted. This suggests that the FI was causing the problem. Since the sub-laser beam diameter may be too large relative to the maximum allowable beam diameter of the FI, we plan to place a lens before the FI in the next measurement to reduce the beam diameter and investigate whether the beam profile improves after transmission through the FI.
 

• First, we measured the upstream laser power, which had not been measured previously. The measurements were performed using a power meter with an OD = 2.0 ND filter. The results were as follows:

  Immediately after the sub-laser output: 24 mW
  Reflected by the PBS: 3 mW
  Transmitted through the PBS: 21 mW
  Before the FI: 21 mW
  After the FI: 18 mW
  After reflection from the beam sampler (R:T = 9:1): 16.6 mW
  Before the BS: 16.5 mW

  Therefore, the transmission of the FI was approximately 86%, and there appears to be no significant laser power loss elsewhere in the optical path. In other words, clipping does not seem to be occurring.
   

• Next, we measured the beam profile near the beam waist using a beam profiler and observed a beam profile similar to that reported previously (klog:37065). To identify the source of the distortion, we first removed the HWP closest to the sub-laser and replaced it with a mirror, then measured the beam profile of the sub-laser itself (Fig. 1). The beam shape of the sub-laser appeared clean. Next, the mirror was replaced with the original HWP, and the beam profiler was placed after the reflection from the beam sampler (R:T = 9:1) to measure the beam profile (Fig. 2). In this case, the beam shape became distorted. We then removed the FI and measured the beam profile again, finding that the beam shape returned to a clean profile (Fig. 3). This indicates that the FI is likely responsible for the distortion. To investigate further, we measured the beam diameter at the FI input and output locations. The results were:

  At the FI input:

  x: 1.869 mm
  y: 1.687 mm

  At the FI output:

  x: 2.075 mm
  y: 2.121 mm

  The maximum beam diameter specified for the FI (IO-3-1064-HP) is 2.7 mm, suggesting that the beam diameter may be too large for proper operation. In the next measurement, we plan to install a lens before the FI to reduce the beam diameter and determine whether the beam profile becomes cleaner after passing through the FI.

• The front-end computer was moved from U32-33 to U20-21 of EY1 rack.
• Equipment in U34-40 (Timing Fanout, Timing IRIG-B, Network switches for DGS and DAQ networks, RFM switch, and a rack rail) was shifted to U33-39.
• U40 is now blank and assigned for the new MTP splice box.

I implemented the GAS modal damping control of SR2 into the guardian (fig1).
As discussed in klog36787, only M1 modal damping is engaged with IMV damping.
In addition, I disengaged the sensor correction of F0 GAS because it would conflict with the modal damping controls.

After implementation into the guardian, I tested and no issue was found.

I performed the regular alignment for PMC with PICO because PMC trans. power was lower than 20 W and PMC REFL camera image got brighter than before.

Now, PMC trans. power was back to 23.2 W and the REFL camera image got darker.

Joshua, Chiaki, Kenta

We tried to improve the SRC flash size on OMC REFL PD by tweaking the alignment of two mirrors and the two lens relative position. However, any improvement could not be seen even though the alignment and the mode matching seem to be optimized. Fig.1 shows the time series when we finished the adjustment. Before the adjustment, we implemented the calibration factor from DGS cnts to uW, which is estimated from PDA100A2 and DGS parameters, into FM2 in the K1:OMC-REFL_DC filter bank. And we subtracted the dark offset. So, the unit of K1:OMC-REFL_DC_OUT is uW if our calculation is correct. In Fig.1, the interval between Y cursors represents the fringe size before adjustment. As you can see, the fringe size seems to be not changed so much, unfortunately.

There are some mysterious points

1. fringe size seems to be breathed slowly
   During the adjustment, we could not restore the fringe size just by restoring an alignment of one mirror or a position of lens. At first, we used SRM free swinging to obtain the flash fringe. So, we suspected the SRM swing was too small to sweep the FSR. So, the size depends on whether SRM position is close to resonance. Therefore, we tried to drive laser frequency with laser PZT by injecting a sine wave (1Hz, 2 Vpp) so that the laser frequency acrosses the resonance point.  However, the situation was not changed. Fig. 2 shows the time series of the fringe. At the vertical cursor, we started the injection to PZT. Fringe size seems to be changed even though we did not touch anything. the interval of the breath seems to be ~60 secs. We are not sure of the cause of this breath for now.
2. Input Beam profile seems to be strange
   Since we moved the lens location, we attempted to check the beam waist positon after the adjustment. Then we found that the beam profile seems to be strange (fig.3). The beam profile was measured at the 13 holes far from the 1st beam splitter. We are not sure that the mode match ratio can be acheived to 80% with such an ugly beam profile. We measured the power in front of the 1st beam splitter. And it is still 1 W even though the beam profile is strange, and is the same value with the power after the reflection from R:T = 9:1 beam splitter.

We are not sure what shoud we do next to improve the situation.

Date: 2026/06/12

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.

[Ushiba, Smith, Hirose, Saito]

The optical layout was slightly modified to make the alignment procedure easier. The beam profile of the sub-laser was measured, and the beam waist position and waist radius were determined. Comparing these values with those of the main laser measured in klog:36730, the mode-matching ratio was estimated to be approximately 89%. In addition, the alignment of the sub-laser beam incident on the SRC was performed using two irises. When the sub-laser PZT was driven, flashes were observed with the OMC REFL PD (K1:OMC-REFL_DC_OUT_DQ). By adjusting one mirror to maximize the signal while the PD gain was set to 40 dB, the maximum signal reached approximately 478 counts. Further improvements in alignment and mode matching are expected to increase the signal to approximately 1700 counts.

• First, the optical layout was modified slightly to facilitate alignment (Figure 1). Using the sub-laser beam, the alignment was adjusted to avoid beam clipping. Next, the beam profile of the sub-laser was measured at locations far from the beam waist using a beam profiler, and a fit was performed (Figure 2). The resulting waist positions and waist radii were as follows:

  Sub-laser

  x-direction: Waist position = 290.1 ± 9.8 mm, Waist radius = 0.0728 ± 0.0025 mm
  y-direction: Waist position = 237.5 ± 8.9 mm, Waist radius = 0.0686 ± 0.0022 mm
  →Average: Waist position = 264 mm, Waist radius = 0.071 mm

  For comparison, the main-laser beam profile measured in klog:36730 was used. The results were:

  Main laser

  x-direction: Waist position = 264.3 ± 5.1 mm, Waist radius = 0.0526 ± 0.0025 mm
  y-direction: Waist position = 253.6 ± 2.3 mm, Waist radius = 0.0555 ± 0.0013 mm
  →Average: Waist position = 259 mm, Waist radius = 0.054 mm

  Using these results, the mode-matching ratio was calculated to be approximately 89%.
   

• Next, two irises were used to ensure that the optical paths of the main laser and the sub-laser coincided. The alignment was adjusted using the BS and a nearby mirror. The main laser was then blocked so that only the sub-laser beam was present in the SRC. The sub-laser PZT was driven with a sinusoidal signal generated by the Moku:Lab function generator, and the OMC REFL PD (K1:OMC-REFL_DC_OUT_DQ), which had been installed by Tanaka-san(klog:37057), was monitored for flashes. Initially, no flashes were observed. However, after adjusting the mirror associated with the BS, flashes became visible (Figure 3). By adjusting one mirror to maximize the signal, the signal amplitude oscillated between approximately 305 counts and 478 counts.
   

• Assuming that the optical power of the sub-laser incident on the SRC is 500 mW, the optical power reaching the OMC REFL PD at resonance is expected to be approximately 16 μW. The responsivity of the OMC REFL PD (Thorlabs PDA100A2) is approximately 680 V/W, and the PD gain was set to 40 dB. Therefore, the output voltage at resonance is expected to be approximately 1.1 V. Using the calibration factor of 610 μV/count, this corresponds to approximately 1700 counts at resonance. Therefore, by using the second mirror to further improve the alignment, the signal should be increased further.