[Tanaka, Hirose, Saito]

We matched the polarization of the sub-laser beam to that of the beam coming from the interferometer. Next, we increased the power of the beam coming from the interferometer by opening the iris. Then, we adjusted the alignment so that the sub-laser beam overlapped with the beam coming from the interferometer at the mirror just before the RFPD. As a result, we were able to observe the beat signal between the beam coming from the interferometer and the sub-laser beam.
 

• First, we checked the polarization of the sub-laser by inserting a PBS after the FI and found that it was mostly P-polarized. Since the beam coming from the interferometer is S-polarized, we installed a HWP after the FI and used the PBS to adjust the polarization to S-polarization. Next, the iris used for alignment had been partially closed; by opening it, the power of the beam coming from the interferometer incident on the RFPD increased from about 2 μW to about 15 μW. Then, with both the beam coming from the interferometer and the sub-laser beam incident on the RFPD, we found that their spot positions differed at the mirror just before the RFPD. We adjusted the alignment so that the sub-laser beam overlapped with the beam coming from the interferometer. At this time, observing the DC output of the RFPD with the oscilloscope on Moku:Lab, we found an offset of about 12 mV; the beam coming from the interferometer contributed about 29 mV and the sub-laser beam about 83 mV. Next, we attempted to observe the beat signal using the spectrum analyzer on Moku:Lab. When we set the sub-laser temperature to 31.57 ℃ to match the main laser frequency, a beat signal was observed (Photo 1). The amplitude of the beat signal fluctuated significantly, and its frequency changed by about 3.5 MHz over the course of approximately one minute.