[Takano, Tanaka, Fujimoto, Saito]

To improve the mode matching between the main laser beam and the sub-laser beam incident on the RFPD, the optical path length of the sub-laser was adjusted. The resulting mode-matching ratio was approximately 76%. The alignment was performed using two irises, and the RFPD was installed. The alignment of the main laser was then finely adjusted to maximize the beat signal amplitude, and the polarization of the sub-laser was also optimized. As a result, the beat signal level increased from -58.67 dBm in the previous measurement (klog:36974) to -45.59 dBm.
 

• In klog:37004, the waist positions of the two beams differed by 352 mm. Therefore, part of the optical setup was modified to increase the optical path length of the sub-laser by approximately 350 mm. The beam profiles of both the main laser and the sub-laser were then measured at locations far from the beam waist on both sides of the waist, and fitting was performed (Figure 1). The green and orange curves represent the fitting results for the main laser, while the red and blue curves represent those for the sub-laser. The origin of the coordinate system is defined at the beam sampler where the two beams are combined.

  The waist positions and waist radii obtained from the fitting are summarized below.

  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

  Sub-Laser

  x-direction: Waist position = -522.6 ± 1.5 mm, Waist radius = 0.0929 ± 0.0003 mm
  y-direction: Waist position = -522.6 ± 6.3 mm, Waist radius = 0.1077 ± 0.0015 mm
  →Average: Waist position = -522.6 mm, Waist radius = 0.1003 mm

  The waist position and waist radius of the main laser, which should not have changed, were found to differ significantly from the values reported previously. In klog:37004, the fitting had been performed using measurements taken only after the waist, which likely resulted in a large fitting error. If the sub-laser waist position is assumed to be -172.6 mm while all other parameters are taken from the present measurement, the calculated mode-matching ratio is 62%. Therefore, the mode-matching ratio of the optical setup in klog:37004 was likely around 62%, rather than the previously estimated 5%.

  Since the waist positions were still offset by 327.2 mm, part of the optical setup was modified again to reduce the sub-laser optical path length by approximately 325 mm. The beam profiles of the main laser and sub-laser were then measured only at locations far beyond the waist. The waist radii were fixed to the values obtained in Figure 1, and fitting was performed only for the waist positions (Figure 2). The green and orange curves represent the fitting results for the main laser, while the red and blue curves represent those for the sub-laser. The origin is again defined at the beam sampler where the two beams are combined.

  The fitted waist positions are summarized below.

  Main Laser

  x-direction: Waist position = -199.0 ± 13.1 mm
  y-direction: Waist position = -148.7 ± 14.8 mm
  →Average: Waist position = -173.9 mm

  Sub-Laser

  x-direction: Waist position = -133.0 ± 10.4 mm
  y-direction: Waist position = -194.1 ± 3.9 mm
  →Average: Waist position = -163.6 mm

  Using these results, the mode-matching ratio was calculated to be approximately 76%.
   

• Next, two irises were installed, and the alignments of both the main laser and sub-laser were adjusted so that the beams passed through both irises. A 50 mm focal-length lens used to focus the beams onto the RFPD, the steering mirror located immediately before the RFPD, and the RFPD itself were then installed. The current optical layout is shown in Figure 3.
   

• When both beams were directed onto the RFPD using the steering mirror, the DC output was -4.825 V. When only the main laser beam was incident on the RFPD, the DC output was -54.88 mV. The optical power incident on the RFPD was approximately 19 μW for the main laser and 1.638 mW for the sub-laser. After confirming the presence of the beat signal, the alignment of the main laser was finely adjusted to maximize the beat signal amplitude. In addition, the polarization of the sub-laser was adjusted using a HWP to further maximize the beat signal (Figure 4). As a result, the beat signal level increased from -58.67 dBm in the previous measurement (klog:36974) to -45.59 dBm. Furthermore, the beat signal amplitude exhibited very little fluctuation over time.