[Fujimoto, Saito]

When injecting the sub-laser into SRY and aligning it using the RFPD DC signal, the maximum signal was about 3.7 times larger for P polarization than for S polarization. The signal was maximized for P polarization and minimized for S polarization.We also changed the polarization of the sub-laser and the polarization setting of the HWP at the detection port, but we did not observe the shift between the resonance peaks for S and P polarization that was observed in the previous measurement (klog:37445). However, when using the main laser and setting the HWP at the detection port to S polarization, a shift between the resonance peaks was observed. Since only the P-polarized signal showed a split into two peaks, and the maximum and minimum optical powers corresponded to P and S polarization, respectively, we think that the eigenpolarizations are S and P polarization.

Similarly, when injecting the sub-laser into SRX and aligning it using the RFPD DC signal, the maximum signal was about 5.9 times larger for S polarization than for P polarization. The signal was maximized for S polarization and minimized for P polarization. The shift between the resonance peaks was not clearly visible because the resonance peaks themselves were small. When using the main laser and setting the HWP at the detection port to S polarization, a small shift between the resonance peaks was observed. We think the different behavior from SRY is because the BS has a high transmission and a low reflectivity for P polarization.
 

• First, we calibrated the angular scales of the HWP and polarizer. We placed a PBS downstream of the HWP on the POS table and rotated the HWP to minimize the transmitted power through the PBS. The HWP scale was 82.2. Next, we placed a polarizer between the HWP and the PBS and rotated the polarizer to minimize the transmitted power through the PBS. The polarizer scale was 274.8. Then, we installed another HWP between the polarizer and the PBS and rotated the HWP to minimize the transmitted power through the PBS. The HWP scale was 90.8.
   
• Next, with the polarizer and ND filter in front of the RFPD removed, we injected P-polarized sub-laser light into SRY. We monitored the RFPD DC signal and adjusted the alignment, obtaining a maximum of -482 counts (Photo 1). We then changed the polarization to S polarization and obtained a maximum of -131 counts (Photo 2). When we rotated the HWP, the signal was maximized for P polarization and minimized for S polarization. Therefore, we think that the eigenpolarizations are S and P polarization. Next, we installed a mirror in front of the lens before the RFPD, and placed an HWP, a 150mm focal-length lens, a PBS, and two PDs to separately observe the P- and S-polarized components. We also checked the mode of the sub-laser at this point, and it was less clean than that of the main laser (Photo 3). We then injected S-polarized light and set the HWP at the detection port to S polarization. The S-polarized component had a higher optical power. When we rotated the HWP at the detection port so that the S- and P-polarized components had the same optical power, we did not observe the shift between the resonance peaks for S and P polarization that had been observed in the previous measurement (klog:37445) (Photo 4). Here, the yellow line corresponds to S polarization and the blue line corresponds to P polarization. We also rotated the HWP at the detection port to equalize the S- and P-polarized optical powers when the input polarization was P polarization or 45° polarization, but we did not observe any shift between the S- and P-polarized resonance peaks. In addition, even when the input polarization was set to 45° and the HWP at the detection port was set to S polarization, we did not observe any shift between the S- and P-polarized resonance peaks.
   
• Next, we injected S-polarized sub-laser light into SRX and adjusted the alignment while monitoring the RFPD DC signal. The maximum signal was -112 counts (Photo 5). We then changed the polarization to P polarization and obtained a maximum of -19 counts (Photo 6). When we rotated the HWP, the signal was maximized for S polarization and minimized for P polarization. Next, we injected S-polarized light and set the HWP at the detection port to S polarization. The S-polarized component had a higher optical power. When we rotated the HWP at the detection port so that the S- and P-polarized components had the same optical power, the resonance peaks themselves were too small to be clearly observed (Photo 7).
   
• Next, we injected the main laser into SRY, swept the SRM, and set the HWP at the detection port to S polarization. A shift between the resonance peaks for S and P polarization was observed (Photo 8). Since only the P-polarized signal showed a split into two peaks, we think that the eigenpolarizations are not rotated relative to the S/P axes, but are S and P polarization. We think that the P-polarized peak is caused by a small amount of P-polarized light that is already leaking into the signal. When we rotated the HWP at the detection port so that the S- and P-polarized components had the same optical power, the shapes of the S- and P-polarized peaks were slightly different (Photo 9).
   
• Next, we injected the main laser into SRX, swept the SRM, and set the HWP at the detection port to S polarization. A small shift between the resonance peaks for S and P polarization was observed (Photo 10). We think that the lower optical power of the P-polarized component is because the BS has a low reflectivity for P polarization. When we rotated the HWP at the detection port so that the S- and P-polarized components had the same optical power, the shapes of the S- and P-polarized peaks became consistent with each other (Photo 11).