Numerical analysis of second-order polarization effects in a Sagnac current sensor

The authors present two aspects of a novel, stabilized Sagnac optical-fiber current sensor. The sensor system and its compensation are described, and a numerical analysis of the variation of polarization along the optical fiber is presented. These results are used to scale the optimal parameters of the sensor and to define theoretical limits of accuracy. Linear birefringence, fiber launching efficiency, laser intensity, and, in particular, their variations limit the accuracy of the system. Linear birefringence can influence both the phase and the amplitude of the two interfering beams, whereas launching efficiency and the laser intensity can only affect the amplitude. The sensor output is stabilized by electronically combining the output signal with a fraction of the two counterpropagating signals before their interferometric reconstruction. This renders possible the compensation of the sensor for the effect of all perturbations which can affect the intensity of one mode by causing loss or coupling into another mode. Results of the numerical analysis of the variation of polarization are shown for an optical fiber containing both linear and circular birefringences and are applied to the Sagnac optical-fiber current sensor to determine the theoretical accuracy and the limits to be expected using the proposed compensation scheme