Remote fiber Bragg grating strain sensing by self-mixing interferometry

An innovative investigation on optical feedback or self- mixing interference within the cavity of a single-longitudinal- mode laser is described as an integral part of a novel fiber Bragg grating interrogation for strain measurement in the domain of structural health monitoring. The entire sensor device simply consists of a laser diode with an integrated photodiode which is coupled to a fiber Bragg grating under strain. A small percentage of the injected lightwave resonantly reflected off the grating structure re-enters the laser cavity and modifies the emission properties of the laser, resulting in the formation of characteristic sawtooth fringes which contain the embedded strain information. The feasibility of demodulating the induced small wavelength shifts of fiber Bragg gratings under dynamic mechanical loading by self-mixing interferometry is thus presented analytically and experimentally verified.