Bragg grating sensor interrogation using in-line, dual-mode fiber demodulators

Abstract

Optical fiber Bragg gratings, first demonstrated by Hill in 1978 [1], will see increasing deployment as sensors in smart structure applications due to their minimal insertion loss and small profile. Bragg grating sensors provide narrowband reflections from a periodic index discontinuity. Strain and temperature changes occurring on the grating induce a shift in the reflected wavelength and monitoring the wavelength shift provides a indication of the strength of the perturbation. Currently, the most basic Bragg grating demodulation system consists of monitoring the reflected wavelength with an optical spectrum analyzer (OSA), but use of these devices is limited due to high costs and poor frequency response. In this thesis, recent reported demodulation schemes are compared and a novel interrogation system using dual-mode optical fiber as an intensity-based demodulator is proposed. The modal birefringence of the dual-mode fiber is wavelength dependent and therefore, the modal phase relationship changes as a function of the Bragg grating reflected wavelength. Monitoring the spatially-filtered output from the dual-mode fiber produces a real-time, sinusoidally varying intensity signal. The optical architecture for this demodulation system is described and experimental data for temperature, strain, and vibration response is provided. It is predicted that with an optical source operating at 300 microwatts, less than 1 microstrain resolution is achievable.