Short wavelength far infrared laser polarimeter with silicon photoelastic modulators

Akiyama, T.; Kawahata, K.; Tanaka, K.; Okajima, S.; Nakayama, K.
October 2008
Review of Scientific Instruments;Oct2008, Vol. 79 Issue 10, p10E720
Academic Journal
A short wavelength far infrared laser whose wavelength λ is about 50 μm 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 (∝λ2) 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 CH3OD laser (λ=57.2 and 47.7 μm). 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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