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Title: Distributed dynamic optical fiber sensing systems based on light scattering techniques
Authors: Jin, Chao
Degree: Ph.D.
Issue Date: 2019
Abstract: Distributed optical fiber sensing (DOFS) techniques provide many unique advantages comparing with other alternative sensing techniques and have found many applications in different application areas. Majority of the research effort has been on the measurement of quasi-static process like temperature variation or applied strain at the beginning. In recent decades, the dynamic optical sensor techniques have developed rapidly for the purpose of providing dynamic information of non-static scenarios involving pipeline security monitoring, infrastructure health monitoring, etc. Motivated by demands for lower respond time applications, several advanced distributed fiber-optic sensing schemes for such dynamic measurements have been proposed and investigated. This thesis focuses on the studies of systems based on various kind of optical scatterings including the Rayleigh scattering based phase-sensitive optical time domain reflectometer (OTDR) system and Brillouin scattering based Brillouin optical time domain analysis (BOTDA) system. First, an integrated telecommunication system and phase sensitive-OTDR sensing system with shared optical bandwidth is studied. The incorporation of the sensing system into the communication system can potentially ensure the reliable operation of a communication system by detecting events that may result in disruption to the operation of a communication link. Optical Orthogonal Frequency Division Multiplexing (OFDM) modulation technique is used in modern telecommunication networks. In the proposed system, 16QAM-OFDM signal at 112Gbit/s data rate is transmitted over 20km single mode fiber (SSMF). Meanwhile, distributed phase sensitive-OTDR system is deployed to monitor the environmental vibration along the fiber. Experimental results show that about 1 dB optical signal-to-noise ratio (OSNR) penalty of bit-error-rate (BER) performance is observed in the communication link through the incorporation of the sensing signal. On the other hand, the influence of communication signal on sensing system is almost negligible as the power reflected by OFDM signal is extremely weak (more than 30 dBm lower than reflected phase-OTDR signal). 20 m spatial resolution with SNR of location information as high as 8.4 dB is obtained with 20 km fiber length. Detection of 1kHz vibration applied by Piezo-ceramic transducers (PZT) is successfully demonstrated.
In addition, a digitally generated optical frequency comb is proposed to realize ultra-fine and fast optical spectral analysis without frequency scanning and thus can be potentially used to realize a fast BOTDA. A fast BOTDA based on the digital optical frequency comb (DOFC) has been studied, which can locate the Brillouin frequency shift (BFS) without frequency scanning. The scanning-free BOTDA system has been demonstrated experimentally with 51m spatial resolution over 10km standard single mode fiber. The resolution and range are 1.5°C and 30°C for temperature measurement respectively. The resolution and range are 43.3µε and 900µε for strain measurement respectively. Finally, a single-measurement BOTDA based on coherent detection of the phase of DOFC probe signal without any averaging and frequency scanning process is investigated. The phase shift of each frequency component of DOFC probe, after transmitting through the fiber under test (FUT) is directly mapped by coherent demodulation in a single data acquisition without any averaging, leading to a sensing speed only limited by the sensing range. Compared with the previous techniques proposed, the measurement time is reduced by eliminating the need for averaging. Brillouin phase shift (BPS) detection instead of Brillouin gain spectrum (BGS) has enabled better measurement accuracy. The performance of our scheme is analyzed through simulation and experiment. Distributed temperature sensing over 10km FUT has been demonstrated by using the proposed single-measurement BOTDA with a BFS uncertainty of 1.5 MHz and spatial resolution of 51.2 m. With the advantage of high speed offered by the proposed scheme, dynamic measurement up to 1 kHz vibration frequency has been demonstrated.
Subjects: Hong Kong Polytechnic University -- Dissertations
Optical fiber detectors
Pages: 163 pages : color illustrations
Appears in Collections:Thesis

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