The experimental setup of Fig. 2 was constructed and installed. The specification of experimental apparatus is the same as Table 1. Figure 4 shows photographs of the optics (left) and the square mirror in the discharge chamber (right). The sides of the square mirror were 28cm in length, and its reflectance was about 95%. The number of the reflections on a single mirror was fixed at 12-14. The total round-trip optical path length was
about 20 m. The incident laser beam power of 150 mW was attenuated to less than 100 /j,W at the output because of the reflection loss of the mirrors and divergence of the beam. The discharge gap was placed at the center of the chamber. The discharge gap length was 0-10 cm.
The rotation angle of the beam polarization was estimated as 0.21 degrees (max) in the above situation, so the polarization ratio will become 1:1.007. It means that the differential detection of the dynamic range of 30 dB enables to distinguish the difference.
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