Fiberoptic heterodyne magnetic field sensor for long-pulsed fusion devices

A heterodyne polarimeter which measures the Faraday effect has been under d evelopment as a magnetic field sensor for long-pulsed fusion devices. The w avelength 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 rotat ion 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 degrees by the digital complex demodulation and digital bandpass filter ing of beat signals. Although the Verdet constant of heavily Tb3+-doped par amagnetic 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. Wi th a 40-mm-long ZnSe sensing rod, resolutions of the magnetic field strengt h of about 5 and 2 G were found to be possible with time responses of 1 and 10 ms, respectively. (C) 2001 American Institute of Physics.