Ultrasonic sensitivity-improved fiber-optic Fabry–Perot interferometer using a beam collimator and its application for ultrasonic imaging of seismic physical models**Project supported by the National Natural Science Foundation of China (Grant Nos. 61735014, 61327012, and 61275088), the Scientific Research Program Funded by Shaanxi Provincial Education Department, China (Grant No. 08JZ58), and the Northwest University Graduate Innovation and Creativity Funds, China (Grant No.

Abstract

An ultrasonic sensitivity-improved fiber-optic Fabry–Perot interferometer (FPI) is proposed and employed for ultrasonic imaging of seismic physical models (SPMs). The FPI comprises a flexible ultra-thin gold film and the end face of a graded-index multimode fiber (MMF), both of which are enclosed in a ceramic tube. The MMF in a specified length can collimate the diverged light beam and compensate for the light loss inside the air cavity, leading to an increased spectral fringe visibility and thus a steeper spectral slope. By using the spectral sideband filtering technique, the collimated FPI shows an improved ultrasonic response. Moreover, two-dimensional images of two SPMs are achieved in air by reconstructing the pulse-echo signals through using the time-of-flight approach. The proposed sensor with easy fabrication and compact size can be a good candidate for high-sensitivity and high-precision nondestructive testing of SPMs.