Effect of three-beam interference on phase-generated carrier demodulation of a fiber-optic interferometric sensor

Homodyne demodulation using a phase-generated carrier (PGC) has been applied in fiber-optic interferometric sensors to overcome the signal fading and distortion due to the drift of the operating point. An assumption needed for the PGC method to be valid is that the sensor output is a sinusoidal function of the phase delay between the arms of the interferometer, which is readily achieved by a two-beam interferometer. In this work, we theoretically and experimentally study the effect of three-beam interference, whose output deviates from a sinusoidal function of the phase delay, on the performance of the PGC scheme. The results show that the deviation could lead to additional undesirable terms in the in-phase and quadrature components in PGC implementation, which may result in significant signal fading with the drift of the operating point. The theoretical analysis leads to two strategies for eliminating these undesirable terms so that the PGC scheme is valid for three-beam interference. The analysis and the strategies were validated experimentally using a fiber-coil Fabry-Perot sensor with two fiber Bragg grating mirrors, each having a reflectivity of 26%.