Performance test of dual modulator polarimeters in two different configurations for magneto-optic measurement of fusion devices

We proposed and tested a new configuration of a magneto-optic polarimeter with a pair of photoelastic modulators (PEMs). In the new configuration, the number of optical components including PEMs can be much smaller than that in a conventional one with PEMs. This paper describes the results of performance test and comparisons to the conventional configuration. The dependences on the light source (a He–Ne laser, 632.8 nm and a superluminescent diode, SLD, 822 nm) are also discussed. The polarization angle can be measured and angle resolution comparable to those in the conventional one was obtained. Angle resolution of 0.002° and 0.07° with a response time of 10 ms was achieved at an incident polarization angle of about 0° and 21°, respectively. The resolution of 0.07° corresponds to 7 G when a 40-mm-long ZnSe sensing rod is used. Linearity of the measured angle against the real one degraded, especially with He–Ne laser, increasing the numbers of the mirrors for beam transmission. On the other hand, the resolution is insensitive to the number of mirrors. Good long-time stability was confirmed with the SLD but a little deterioration was found with the He–Ne laser in the two configurations.