[Tanaka,Hirose,Saito]
Abstract:
We fitted the beam profile of the IR laser on the POS optical table measured in klog: 36724. The waist position was found to be 259 mm, and the waist size was 0.054 mm. Based on these results and the fitted beam profile of the sub-laser, we calculated the positions of two lenses required for mode matching. Furthermore, we developed an optical layout plan for the measurements of PRCL and SRCL.
Detail:
First, we fitted the beam profile of the IR laser on the POS optical table measured in klog: 36724 (Fig.1). The points represent the measurement results, and the lines represent the fitting results. The green and orange points were used for the fitting, while the blue and red points were excluded because their behavior differed from the other measurement points.
The measurement points around 300 mm deviate significantly from the fitting curve because, as shown in Photo 2 of klog: 36724, the beam split into two, and both spots were simultaneously fitted with a Gaussian. The apparently reasonable behavior around 350 mm is because only one of the split beams was successfully fitted with a Gaussian. However, since the beam was distorted as shown in Photo 3 of klog:36724, it should have been compared with the beam between 100 mm and 250 mm to determine whether to include it in the fitting; unfortunately, no photos were taken in that range. In addition, when the mesurement points around 350 mm are included in the fitting (Fig.2), the mesurement points between 100 mm and 250 mm deviate from the fitting curve compared to Fig.1. Therefore, we decided to use the results from Fig.1 to determine the positions of the two lenses for mode matching. (For the measurements of PRCL and SRCL, it is sufficient to observe the resonance of the TEM00 mode, so a slight mismatch in mode matching is considered acceptable.)
The waist position and waist size obtained from the fitting in Fig.1 are as follows:
Major : waist position = 264.3 ± 5.1 mm, waist size = 0.0526 ± 0.0025 mm
Minor : waist position = 253.6 ± 2.3 mm, waist size = 0.0555 ± 0.0013 mm
→ Average: waist position = 259 mm, waist size = 0.054 mm
The laser to be newly installed on the POS optical table must be mode-matched to this waist position and size. Therefore, its beam profile was measured in advance in Kashiwa and fitted (Fig.3). The origin is defined as the point where the laser is emitted.
The waist position and waist size obtained from the fitting in Fig.3 are as follows:
Major : waist position = −118.8 ± 1.9 mm, waist size = 0.1469 ± 0.0008 mm
Minor : waist position = −127.8 ± 1.8 mm, waist size = 0.1562 ± 0.0008 mm
→ Average: waist position = −123 mm, waist size = 0.1516 mm
Next, based on these waist positions and waist sizes, we determined the positions of two lenses for mode matching (Fig.4). The origin is defined as the point where the laser is emitted. The orange line represents the beam profile of the laser, the green line represents the beam profile after passing through the first lens, the red line represents the beam profile after passing through the second lens, and the blue line represents the target beam profile of the infrared laser on the POS optical table.
The focal lengths and positions of the two lenses are as follows:
Lens 1: focal length = 200 mm, position = 1426 mm
Lens 2: focal length = 500 mm, position = 2363 mm
Finally, based on the calculated lens positions, we designed an optical layout for a phase-locked loop (PLL) used in the measurements of PRCL and SRCL (Fig.5). The right-hand side of Fig.5 corresponds to the optical system designed in this work.
