Temperature Compensation of Interferometric and Polarimetric Fiber-Optic Current Sensors With Spun Highly Birefringent Fiber

We theoretically and experimentally investigate intrinsic temperature compensation of interferometric and polarimetric fiber-optic current sensors with a coil of spun highly hirefringent fiber operated in reflection mode. The interferometric sensor recovers the differential magneto-optic phase shift of the left- and right-handed circular (or slightly elliptical) polarization states in the fiber by means of non-reciprocal phase modulation, whereas the polarimetric sensor employs a simple passive polarization analyzer. We show that the two sensor types exhibit substantial differences in their response to temperature changes. The main parameters that determine the sensors' sensitivity to temperature are, besides the fiber's Verdet constant, the intrinsic birefringence of the spun fiber, the retardation of the fiber retarder at the coil entrance that generates the elliptical polarization states, and the angle alpha between the principal axes of the retarder and spun fiber. In particular, fringe contrast changes at varying fiber birefringence make the polarimetric sensor version significantly more sensitive to temperature. Furthermore, whereas in the interferometric sensor, the contribution of the fiber birefringence to the temperature dependence disappears for special fiber orientations (alpha = 0 degrees, 90 degrees), such an arrangement is not possible for the polarimetric sensor. Also, the response to changes in the retardation of the retarder shows different patterns for the two sensor types. In spite of the differences, we achieve intrinsic temperature compensation well within +/- 0.2% between -40 degrees C and 85 degrees C for both sensors by using the retarder contribution to balance the contributions from the Verdet constant, the fiber birefringence, and potential further effects of temperature.