A short wavelength far infrared laser whose wavelength lambda is about 50 mu m is preferable for a polarimeter and an interferometer for high density operations in the Large Helical Device (LHD) and on future large fusion devices such as ITER. This is because the beam bending effect (propto lambda^2) in a plasma, which causes fringe jump errors, is small and the Faraday and the Cotton?Mouton effects are moderate.We have developed a polarimeter with highly resistive silicon photoelastic modulators (PEMs) for the CH_3OD laser (lambda =57.2 and 47.7 mu m) . We performed bench tests of the polarimeter with a dual PEM and demonstrated the feasibility for the polarimeter. Good linearity between actual and evaluated polarization angles is achieved with an angular resolution of 0.05° and a temporal resolution of 1 ms. The baseline drift of the polarization angle is about 0.1° for 1000 s.
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