Interrogation of Fiber Bragg Grating Dynamic Strain Sensors by Self-Mixing Interferometry

An innovative investigation of optical feedback or self-mixing interference within the cavity of a single-longitudinal-mode laser is described as an integral part of a novel interrogation scheme to be employed in a fiber Bragg grating-based sensor for strain measurement. 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 reenters 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. A relatively good corroboration has been achieved.