Demodulation of fiber Bragg grating wavelength shifts by optical feedback interferometry

This work reports a novel technique for demodulating fiber Bragg gratings subject to mechanical loading. The small wavelength shifts induced in the grating reflection spectrum of the fiber grating under loading caused the central or peak reflected Bragg wavelength to be displaced from its original position. A small percentage of the strain-induced spectrum from the fiber Bragg grating was then retro-reflected into the active cavity of the emitting laser diode where the optical feedback led to perturbation to the active cavity. An equivalent cavity model is also presented to validate the resulting modification to the emission properties of the laser device as measured by a photodetector integrated into the laser package, leading to sawtooth-like interference fringes typical of self-mixing interferometry being generated. Consequently, the embedded strain information contained in these fringes can then be extracted as a function of the applied loading strength and frequency to give the desired measurand without any complicated signal processing schemes. With the laser diode serving as the source and detector, a relatively simple and cost-effective scheme can thus be designed for interrogating fiber Bragg grating-based sensors for strain measurements.