Key techniques of reflective Sagnac interferometer-type fiber optic current transformers

The phase difference detected by a reflective Sagnac interferometer-type fiber optic current transformer (RS-FOCT) as a function of the Verdet constant and linear birefringence of the sensing fiber, phase retardation of quarter-wave retarder, pigtail polarization crosstalk of integrated optical phase modulator and polarization crosstalk of the polarization-maintaining delay fiber are given respectively. The temperature dependences of these parameters are determined to be major causes of the scale factor error of the RS-FOCT applied to the outdoor environment. Some error suppression measures are taken to improve the temperature performance of the RS-FOCT including winding the sensing fiber spirally, the self-compensation of the scale factor temperature error and lowering the polarization coupling intensity of the phase modulator and delay fiber. The polarization nonreciprocal error mechanism of the RS-FOCT is analyzed with the corresponding error suppression method. The method aligns the fast axis of the pigtail and integrated optical waveguide of the polarizer so that the coherence between the nonreciprocal waves can be eliminated. Over the temperature range from -40°C to 60°C, the scale factor error caused by the sensing loop is within 0.2%, and the scale factor changes caused by the temperature variation of the phase modulator and delay fiber are reduced from 1.828% to 0.283% and from 5.96% to 0.0531% respectively, in addition, the quasi-periodic oscillation of the RS-FOCT output with the temperature of the input pigtail of the polarizer is greatly suppressed.