Date of this Version
Tongqing Liu, Analysis of Fiber Bragg Gratings For Ultrasonic Detection. MS Thesis, University of Nebraska-Lincoln, 2012.
Fiber optic sensors became an emerging technique to detect ultrasonic emissions in the last decade. Their light weight, immunity to electromagnetic interference and capacity of being multiplexed make them defeat their electronic counterparts in various applications. In this thesis we presented a novel fiber optic ultrasonic sensor based on pi phase shift fiber Bragg gratings. Numerical simulations were performed to study the characteristics of pi phase shift fiber Bragg gratings impinged by ultrasonic waves. The coupling theory was introduced to analyze the change of fiber Bragg gratings when impinged by ultrasonic waves, and the transfer matrix method was utilized to implement the simulation. In addition, the effect of the grating length and grating refractive index modification depth on the wavelength sensitivity and intensity sensitivity of the pi phase shift fiber Bragg grating sensors were investigated. The responses of pi phase shift fiber Bragg gratings under ultrasonic pressure waves were also compared with that of uniform fiber gratings. Finally, the responses of pi FBGs when impinged by ultrasonic longitudinal waves and shear waves were provided. Our analysis revealed several unique characteristics of pi phase shift fiber Bragg gratings used for ultrasonic detection and will be useful for design and optimization of fiber optic ultrasonic sensors with pi phase shift fiber Bragg grating as the sensing element.
Adviser: Ming Han