[Ushiba, Tanaka, Hirose, Fujimoto, Saito]

In the PLL optical path, the beam sampler (R:T = 1:9), where the main-laser and sub-laser beams are combined, was moved to match the waist positions of the two beams. As a result, the mode-matching ratio improved to approximately 85%. After aligning the PLL optical path, a beat signal was successfully observed. In the optical path that injects the sub-laser into the interferometer, a beam profiler was placed at the expected beam-waist location, and the lens positions were adjusted so that the waist occurred at that location. When the beam profile was examined from upstream toward the waist, the beam initially had a reasonably clean shape, but gradually became distorted into a vortex-like pattern. Around the waist position, however, the beam profile became clean again. Although the cause of this behavior remains unclear, the beam shape at the point where it enters the interferometer was clean, so we decided to proceed with alignment. After alignment, we attempted to observe flashes from the SRC, but none were detected. This was likely because the main laser was not properly aligned to the SRC. In the next experiment, we plan to realign the system and attempt to observe the flashes again.
 

• First, the beam profile of the main laser in the PLL optical path was measured and fitted (Fig. 1). 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.

  Main laser

  x direction: Waist position = −175.8 ± 2.6 mm, Waist radius = 0.0520 ± 0.0006 mm
  y direction: Waist position = −149.4 ± 2.2 mm, Waist radius = 0.0567 ± 0.0006 mm
  → Average: Waist position = −163 mm, Waist radius = 0.0544 mm

  Comparing these results with the sub-laser waist position measured previously (klog:37086), the waist positions differed by approximately 74 mm. Therefore, the beam sampler (R:T = 1:9) where the main-laser and sub-laser beams are combined was moved approximately 1.5 holes to the right (Fig. 2). The beam profile of the sub-laser was then measured and fitted (Fig. 3). The resulting waist positions and waist radii are listed below. The coordinate origin is defined at the new position of the beam sampler (R:T = 1:9).

  Sub-laser

  x direction: Waist position = −202.2 ± 2.9 mm, Waist radius = 0.0838 ± 0.0010 mm
  y direction: Waist position = −202.3 ± 2.4 mm, Waist radius = 0.0793 ± 0.0008 mm
  → Average: Waist position = −202 mm, Waist radius = 0.082 mm

  To compare the waist positions of the main and sub-laser beams, the main-laser waist position was shifted by the same amount that the beam sampler was moved (approximately 1.5 holes to the right), yielding an adjusted waist position of −200.5 mm. Based on these results, the mode-matching ratio was calculated to be approximately 85%. Next, two irises were installed, and the alignments of the main and sub-laser beams were adjusted so that both beams passed through them. The RFPD position was then adjusted while only the sub-laser beam was incident on the RFPD, and the mirror immediately before the RFPD was used to maximize the DC signal. When the main-laser beam was also directed onto the RFPD, a beat signal was observed. The alignment of the main laser was then optimized to maximize the beat-signal amplitude.
   

• Next, in the optical path that injects the sub-laser into the interferometer, a beam profiler was placed at the expected waist location, and the lens positions were adjusted so that the beam waist occurred there. However, the beam profile appeared distorted (Fig. 4). We first considered the possibility that the beam was being clipped somewhere in the optical path. However, no significant power loss was observed between the FI output and the beam waist. All mirrors and lenses downstream of the FI were inspected and adjusted to ensure that no clipping was occurring, but the beam profile remained distorted. The optical surfaces of the mirrors and lenses were also checked and cleaned, but no improvement was observed. Furthermore, the beam profiler was positioned near the waist, and the mirror angles were adjusted while observing the beam profile. The distorted beam shape persisted and merely shifted laterally, suggesting that the distortion was not caused by clipping. When the beam profile was observed while moving from upstream toward the waist, the beam initially appeared reasonably clean, then gradually developed a vortex-like distortion, and finally became clean again around the waist position. Although the cause of this behavior remains unknown, the beam shape at the point where it enters the interferometer was clean, so we decided to proceed with alignment. Regarding mode matching, since the beam waist was adjusted to occur at the intended waist location, the mode matching is expected to be reasonably good. The beam-waist radii were approximately 0.076 mm in the x direction and 0.074 mm in the y direction, corresponding to an average waist radius of approximately 0.075 mm. If the waist positions are matched, the mode-matching ratio is expected to be approximately 90%. Finally, alignment was performed using the two irises, and the main-laser beam was blocked so that only the sub-laser remained in the SRC. We then attempted to observe SRC flashes, but none were detected. However, because the main laser was not aligned to the SRC, the alignment of the sub-laser, which had been adjusted to match the main laser, was likely also incorrect. This is considered the most probable reason why no flashes were observed. In the next experiment, we plan to realign the system and continue searching for SRC flashes.