A short wavelength laser whose wavelength is about 50 μm is preferable for a polarimeter and an interferometer in large fusion devises. This paper reports the development of a polarimeter with a photoelastic modulator(PEM) for a CH3OD laser (wavelengths of 57.2 and 47.6 μm). The PEM with a high-resistive silicon as a photoelastic element has been newly developed. The transmissivity of the high-resistive silicon is high in a far infrared region. The polarimeter with the Si PEM has been tested and the polarization angle is successfully measured. Noise sources (a multi-reflection of the laser beam in the photoelastic element, a measurement error of amplitude of a detector output and an estimation error of the retardation) of the measured angle are also discussed.
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