Author: Guo, Nan
Title: Applications of fiber-optic communication techniques in brillouin scattering based distributed optical fiber sensing systems
Advisors: Lu, Chao (EIE)
Degree: Ph.D.
Year: 2018
Subject: Hong Kong Polytechnic University -- Dissertations
Optical fiber communication
Optical detectors
Department: Department of Electronic and Information Engineering
Pages: xxviii, 159 pages : color illustrations
Language: English
Abstract: Distributed optical fiber sensing (DOFS) techniques offer many unique advantages comparing with other alternative sensing techniques and have found many applications in different application areas. Therefore, significant amount of research effort has been devoted to the development of the techniques in order to enhance the system performance and to improve their flexibility when used in different applications. At the same time, various advanced optical fiber telecommunication techniques have been developed as enablers for high speed transmission systems for flexible and high capacity optical networks. As optical fiber telecommunication systems share a lot of similarities with distributed optical fiber sensing systems, there is a strong motivation to investigate the possible use of mature optical transmission techniques in distributed optical fiber sensing systems for better sensing performance. This thesis focuses on the studies of system performance enhancement of one of the most popular DOFS techniques, Brillouin optical time domain analysis (BOTDA) system, based on optical fiber telecommunication techniques. First, a novel scheme of bi-directional Brillouin time domain analyzer (BD¬≠BOTDA) is proposed and experimentally demonstrated to extend the sensing range based on wavelength division multiplexing (WDM) technique. By deploying two pump-probe pairs at two different wavelength channels, the Brillouin frequency shift (BFS) distribution over each half of the whole fiber can be obtained with simultaneous detection of Brillouin signals in both channels. Compared to the conventional unidirectional BOTDA system of the same sensing range, the proposed BD-BOTDA scheme provides better Brillouin signal¬≠to-noise (SNR) performance for both FUT deployment configurations in different paths or in a single path for the round trip.
In addition, we propose and experimentally demonstrate a scheme of coherent BOTDA system without any trace averaging. Assisted by a commercial integrated coherent receiver with a local oscillator (LO) generated through single sideband (SSB) modulation from the same laser source, the Brillouin signals carried on a stable intermediate frequency (IF) are extracted by electrical/digital filters and then recovered to baseband by digital signal processing (DSP). This increases the signal-to-noise ratio (SNR) and avoids the need of trace averaging and enables real-time signal acquisition. To eliminate Brillouin gain fluctuation, two adjacent Brillouin time-domain traces stimulated by two sequential polarization orthogonal pump pulses recovered after the IF signals are detected in a real-time manner. Finally, we modify the scheme above to realize a vector BOTDA system, which enables both distributed Brillouin gain spectrum (BGS) and Brillouin phase-shift spectrum (BPS) measurements without trace averaging. The proposed scheme generates and launches a reference probe light into the fiber under test (FUT) together with the frequency scanning probe light as a reference to eliminate phase noise and distortion in the probe. By processing and analyzing the receiver outputs at specific intermediate frequencies (IF), the amplitude and phase signals carried by the scanning and the reference probe can be resolved at the same time. In this way, both Brillouin gain and Brillouin phase shift can be obtained simultaneously. In addition, the scheme is not affected by chromatic dispersion (CD) of the FUT. The effectiveness of the configuration is verified through experiments carried out over an 18.3 km FUT with a spatial resolution of 4 m.
Rights: All rights reserved
Access: open access

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