Highly Stabilized Phase-Shifted Fiber Bragg Grating Sensing System for Ultrasonic Detection

A phase-shifted fiber Bragg grating (PS-FBG) sensing system based on Pound–Drever–Hall (PDH) technique is proposed and experimentally demonstrated for ultrasonic detection. The sensing system is highly stabilized as the laser is frequency locked to the PS-FBG resonance using PDH technique. This scheme reduces the effect of laser intensity noise and enables high-sensitivity ultrasonic detection by monitoring the error signal of frequency detuning between the laser and the PS-FBG resonance. Response of the frequency-locked PS-FBG to ultrasound is done by testing the ultrasonic refection of a 0.4-mm plastic film, immersed in water. The sensor exhibits a high signal-to-noise ratio of 48 dB at 200 kHz and a noise-limited detectable strain of 8.7 above 10 kHz. Frequency response of PS-FBG from 100 kHz to 1 MHz was also characterized.