[Tanaka,Hirose,Saito]

In preparation for the measurements of PRCL and SRCL, we measured the IR laser power and beam profile on the POS optical table.
The laser power was approximately 15μW.

• On the POS optical table, the IR laser passed through a periscope, a beam splitter that separates the green laser and IR laser, and a lens. It was then reflected by a mirror and finally directed toward PDs for S-polarization and P-polarization. We placed a power meter in front of the mirror to measure the laser power, which was approximately 15μW.
• Next, we placed an additional mirror in front of the original mirror (at a position approximately 450 mm from the lens)(Photo 1) and measured the beam profile of the IR laser. In this measurement, the origin was defined as the center of the hole on the optical table where the additional mirror was placed. A beam waist was observed at approximately 263 mm, but the beam split into two at around 300 mm (Photo 2). Even at 575 mm, the beam remained distorted (Photo 3). The measurement was initially performed in the single bounce configuration. Considering the possibility of birefringence effects, the Y-arm was then locked. However, almost no change was observed (Photo 4).

The fitting results of the beam profile will be posted later.