Research On Ultrasonic Sensing Demodulation System Based On ?-phase Shifted Fiber Bragg Grating

Fiber Bragg grating(FBG)has the merits of simple structure,strong electromagnetic immunity,high sensitivity and multiplexing capability,it has great development prospect in ultrasonic structural health detection.How to accurately measure the ultrasonic wave is the key to ultrasonic structural health monitoring.It is crucial for the design of a demodulation system which can monitor the ultrasonic signal in real time.This paper focuses on the design and improvement of FBG ultrasonic sensing demodulation system,the main works are as follows,(1)To solve the trade-off between the measurement sensitivity and range of the FBG ultrasonic sensor,a demodulation scheme based on Fabry-Perot(F-P)cavity and ?-phase-shifted fiber Bragg grating(?-FBG)is proposed.The principle of demodulation based on F-P cavity is analyzed and numerically simulated in detail.A group of demodulators consisting of two longitudinal modes of the F-P cavity can double the system sensitivity,and has good linearity.A demodulator array can be constructed when extended to multiple modes,which can greatly increase the system range,and effectively solve the trade-off between measurement sensitivity and range.(2)To solve the problem that the enhance of the sensitivity of the demodulation system will cause the increase of the F-P cavity length,the photonic crystal waveguide(PCW)is used as the slow light medium into the F-P cavity to solve the trade-off between the sensitivity and cavity length.Analysis results show that under appropriate parameters,using the larger group index can effectively shorten the actual cavity length,so as to further improve the sensitivity on the basis of the stability of the system.(3)To solve the problems of high noise in the broadband light source system,an ultrasonic demodulation system equipped with a narrow band tunable laser and a F-P cavity as the input is proposed.The analysis results show that this scheme can not only reduce the system noise but also realize the multiplexing function of the sensor.