Abstract
A heterodyne polarimeter which measures the Faraday effect has been under development as a magnetic field sensor for long-pulsed fusion devices. The wavelength of laser beam was chosen to be 850 nm since the radiation-induced loss in optical fibers is significant below 800 nm while the Faraday rotation angle decreases with increasing wavelength. By the use of a distributed Bragg reflector laser diode and a Faraday isolator, excellent stability of phase measurement was attained. The phase resolution was raised to up to 0.02° by the digital complex demodulation and digital bandpass filtering of beat signals. Although the Verdet constant of heavily Tb3+-doped paramagnetic glass is larger than that of ZnSe by about a factor of 1.4, ZnSe is superior to the former in terms of negligible temperature dependence. With a 40-mm-long ZnSe sensing rod, resolutions of the magnetic field strength of about 5 and 2 G were found to be possible with time responses of 1 and 10 ms, respectively.
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