Recovering signals from optical fiber interferometric sensors

Abstract

This dissertation examines three methods of recovering signals cheaply from one class of highly sensitive Optical Fiber Interferometric Sensors (OFIS). This class of sensors consists of a laser light source; a 2X2 optical fiber coupler to split the beam in two; a differential transducer which converts a signal of interest into optical phase shift in the laser light transmitted through the two optical fibers in the interferometer; and a 3X3 optical fiber coupler which recombines the two beams, producing interference which can be detected electronically. The three outputs can be operated symmetrically or asymmetrically to recover the signal of interest. The use of the 3X3 coupler permits Passive Homodyne Demodulation of the phase-modulated signals provided by the interferometer without feedback control or modulation of the laser itself and without requiring the use of electronics within the interferometer. One of the three methods discussed in this dissertation performs symmetric demodulation with analog electronics. Another uses analog-to-digital conversion of the signals and performs asymmetric fringe rate demodulation in digital hardware. The third method discussed used asymmetric fringe-rate demodulation. The three methods are characterized by their harmonic distortion, minimum detectable signal, bandwidth, dynamic range, noise, complexity and approximate cost.