以光纖雷射建構相位光時域反射儀以供全域結構震動檢測

Abstract

本研究以光纖感測技術為基礎，探討結構環境振動監測等應用，並提出光纖布拉格光柵系統(FBG)及相位光時域反射儀系統(Phi-OTDR)，並搭配自製光纖雷射放大器以實現結構健康監測，同時針對光纖區域所存在之外部擾動進行評估，由於一般大型客機自然頻率介於 16 Hz 至 934 Hz，為了獲得實時結構擾動狀況以降低共振頻率所造成之風險，透過光纖感測系統提供實時結構振動訊號，此系統之應用可結合於地震監測、大型土木結構及離岸風電等領域。 首先研究使用光纖布拉格光柵針對結構靜態載重進行量測，布拉格光柵具有高靈敏度和抗干擾能力，能夠對結構物進行高精度監測。透過不同載重變化導致反射波長產生偏移現象，除了探討外力所產生之波長偏移變化外，更說明光柵在結構監測上之應用情境，以及量測上之限制與瓶頸。 其次，研究使用傳統光時域反射儀原理延伸至相位光時域反射儀並檢測結構表面振動所引起的光纖纖芯變化，系統透過由光纖單一端產生之瑞利背向散射進行分析，由纖芯變化所引起相位的改變，可獲得結構振動頻率等重要參數，實驗中藉由光時域反射儀針對弱反射點進行探測，以獲得 4.2 公尺之空間分辨率，後者則透過接收瑞利背向散射訊號，將訊號切割與重建，並由後端訊號處理以獲得光纖區段之頻率響應，並量測將近 800Hz之頻率擾動。 研究中存在光功率不足等問題，為了提升光強度，開發光纖雷射共振腔系統以放大光信號，透過系統內部之摻鉺光纖在系統中來回傳遞反覆增益並將信號光訊號大幅提升至 13.4dbm，並探討系統內部相關元件對於系統效能的提升及系統模擬，增強後之光訊號結合相位光時域反射儀系統同時提升實驗上之低信噪比問題，並將信噪比提升至 7.8db，藉由信噪比的提升增加系統穩定度，以及減少雜訊所帶來之干擾，並針對光纖區域所存在之外部擾動進行評估。

This study investigates the applications of fiber optic sensing technology in structural vibration monitoring and proposes the use of Fiber Bragg Grating (FBG) and Phase-Sensitive Optical Time-Domain Reflectometry (Phi-OTDR) systems, combined with a homemade fiber laser amplifier for structural health monitoring. The study focuses on evaluating external disturbances in the fiber optic region. The research aims to achieve real-time monitoring of structural vibrations to decrease the risks associated with resonance frequencies, considering that the natural frequencies of large aircraft range from 16 Hz to 934 Hz. The fiber optic sensing system provides real-time structural vibration signals and can be applied in areas such as earthquake monitoring, large-scale civil structures, and offshore wind power. The study first utilizes fiber Bragg gratings to measure static loading on structures. FBGs offer high sensitivity and interference resistance, enabling precise monitoring of structures. By analyzing the wavelength shift caused by different load variations, the study examines the wavelength shift induced by external forces, as well as the application scenarios, limitations, and bottlenecks of using gratings in structural monitoring. Next, the study extends the principles of conventional optical time-domain reflectometry to Phi-OTDR and detects changes in the fiber core caused by surface vibrations of the structure. By analyzing the Rayleigh backscattering generated at a single end of the fiber, the system measures important parameters such as structural vibration frequency based on the phase change caused by core variation. The experiment detects weak reflection points using optical time-domain reflectometry to achieve a spatial resolution of 4.2 meters. For Phi-OTDR, the study receives the Rayleigh backscattering signals, segments and reconstructs them and processes the signals to obtain the frequency response of the fiber section, measuring frequency disturbances of nearly 800 Hz. The study encounters issues such as insufficient optical power. To enhance the optical intensity, a fiber laser resonator system is developed to amplify the optical signal. By utilizing an erbium-doped fiber within the system to repeatedly amplify the signal, the signal power is significantly increased to 13.4 dBm. The study investigates the improvement of system performance by incorporating relevant components and conducting system simulations. The enhanced optical signal, combined with the Phi-OTDR system, addresses the low signal-to-noise ratio issue in experiments, increasing the signal-to-noise ratio to 7.8 dB. The improved signal-to-noise ratio enhances system stability, reduces interference from noise, and evaluates external disturbances in the fiber optic region.