Interrogation of a Birefringent Fiber Sensor by Nonreciprocal Phase Modulation

The technique of nonreciprocal phase modulation, known from Sagnac-type interferometers, is adopted to interrogate the differential phase shift of the orthogonal light waves in a birefringent fiber sensor. The sensing section of the novel interferometer consists of a series of two birefringent fibers with balanced differential group delay. A key element is a 45°-Faraday rotator which swaps the polarization states of the orthogonal waves after their round trip through the two fiber sections. An integrated-optic birefringent phase modulator introduces the nonreciprocal phase modulation at the detector end of the sensor. The interferometer is applied to voltage sensing.