Optimal demodulation of wavelength shifts in a fiber Bragg grating sensor using an adaptive two wave mixing photorefractive interferometer

Abstract

Recent work by our research group on the dynamic demodulation of strain-induced wavelength shifts in fiber Bragg grating (FBG) sensors show that these sensors are suitable for the detection of high frequency ultrasonic waves produced by impact loading. A FBG sensor is incorporated into an optical detection system that uses a broadband tunable laser source in the C-band, a two wave-mixing photorefractive interferometer, and a high-speed photodetector. When an ultrasonic wave interacts with the FBG sensor, the wavelength of the reflected light in the fiber is dynamically shifted due to strain-induced perturbation of the index of refraction and/or the period of the grating in the fiber. The wavelength shift is converted into an intensity change by splitting the light into signal and pump beams and interfering the beams in an InP:Fe photorefractive crystal (PRC). The resulting intensity change is measured by a photodetector. The two-wave mixing (TWM) photorefractive interferometer allows for several FBG sensors to be wavelength multiplexed in one PRC and it also actively compensates for low frequency signal drifts associated with unwanted room vibrations and temperature excursions. In this work, we present preliminary experimental results on the detection of impact signals using a low power (1 mW) TWM PRC based demodulation system. The response time of the PRC is optimized by focusing the signal and pump beams into the crystal allowing for adaptivity of the demodulation system to quasi-static strains or temperature drifts. The TWM intensity gain of the system is optimized for efficient wavelength demodulation through resonant enhancement of the space charge electric field formed in the PRC. The low power demodulation system would facilitate significant reduction in the overall cost of the system.