A novel FBG interrogation method for potential structural health monitoring applications

We propose an improved innovative interrogation scheme for fiber Bragg grating-based strain sensors using the optical feedback coupled into the cavity of a DFB laser diode. Strain-induced shifts of the Bragg grating peak reflectivity, which are back-reflected into the laser cavity when both the Bragg and laser wavelengths are in resonance, result in sawtooth-type fringes typical of optical feedback interference being produced. By introducing a modulation scheme to the laser drive current, the laser wavelength is scanned slowly across the extremities of the FBG spectrum, allowing the dynamic strain range to be extended by a factor of approximately half the width of the FBG spectrum. A resolution of ~6 μe is currently obtained via basic fringe counting of the detected sawtooth interference signals.