光柵光纖之超音波動態響應與感測系統的研製

Abstract

本文探討光柵光纖偵測超音波的動態響應及感測系統的研製，對於光柵光纖與不同材料試片的結構耦合問題及試片製造作一歸納整理。光纖感測系統包括光源、光纖干涉儀、光纖感測器、解調裝置及光偵測器等。實驗以脈衝雷射生成板波及壓力波照射黏置於碳纖環氧樹酯表面及內埋於冷埋樹酯試片之光柵光纖，超音波的擾動改變光柵光纖的間距，造成光纖內的反射光波長發生微小的偏移。以不等光臂之Mach-Zehnder光纖干涉儀將波長偏移量解調為相位差，再以光偵測器將光照度轉換成電壓訊號，感測系統運作於干涉儀quadrature相位點附近，保持系統的線性關係。本文對於各元件的工作原理、系統架設及實驗結果予以討論與建議。

The thesis investigates responses of fiber Bragg grating sensors due to ultrasound and reports the experience in development of a sensing system. The influences resulted from various coupling conditions between optic fibers and the plate-like host materials as well as their corresponding manufacturing problems are reported. A fiber optic sensing system includes a broadband light source, interferometer-based fiber optic sensors, demodulation devices, optic detectors, and so on. In the experiment, laser-induced plate waves and acoustic bulk wave were impinged upon one fiber grating sensor mounted on the surface of a carbon/epoxy composite plate and another embedded in a plate made of acrylic resin. Wavelength of the reflected light from fiber grating is dynamically changed once upon the pitches of Bragg grating are disturbed by the incident acoustic waves. The infinitesimal shift in optical fiber wavelength is demodulated to phase change using an unbalanced Mach-Zehnder fiber optic interferometer. The demodulation is performed near the quadrature point to keep a linear relationshop between the phase shift and light intensity output of the interferometer. Working principles of each fiber-optic device, various experimental setups and their results are discussed and commented.