週期分佈應變對於光纖光柵的動態響應

Abstract

內質式光纖感測器尺寸很小，感測訊號的頻寬不受到結構尺寸效應的影響，但是光纖光柵的有限長度卻限制了其所能偵測超音波的波長範圍。當光纖光柵感測器受到超音波的照射，光纖光柵會產生週期性的應變。本文的研究目的在於探討週期性應變對於有限長度光纖光柵之光譜特徵的影響，其特徵參數為中心波長的平移與光譜頻寬的改變，並建立數值模擬方法。在實驗量測上，以不等臂長之Mach-Zehnder (MZ)干涉儀解調光纖光柵受超音波照射產生之週期應變引致的光波長改變量，以兩個相位調制器將MZ干涉儀系統控制在quadrature相位點附近，使得光干涉的訊號強度與光纖光柵的應變保持線性關係。實驗試片係將一光纖光柵埋藏於一個圓形環氧樹脂厚試片的中心，探討頻率約1MHz的超音波自不同角度入射於光纖光柵的動態響應。

Having relatively small dimension, the optic fiber grating can be used as intrinsic fiber optic sensors applying in a broadband frequency range without limitation due to structure size. However, the finite length of fiber grating confines its application in detection of low-frequency dynamic response. When ultrasonic waves impinge upon embedded fiber Bragg grating sensors, periodic strains will be induced in the grating. The optical spectra of fiber Bragg gratings are studied theoretically and experimentally in this thesis to understand the feasibility of detecting ultrasonic waves using fiber Bragg grating. In this experiment, the shifts of center-wavelength in spectra of reflection from finite length of fiber Bragg grating are characterized by an unbalanced Mach-Zehnder (MZ) fiber interferometer. The wavelength shift of fiber Bragg grating is demodulated to be a variation of light intensity at terminals of the MZ interferometer. Two phase-shifters were employed in two arms of the MZ interferometer to hold the system at a quadrature point. A fiber Bragg grating was embedded in a thick, circular block made of epoxy in experiment to detect ultrasound at frequency around 1 MHz and in various incident angles.