| Author: | Wang, Yuyao |
| Title: | Applications of distributed optical fiber sensing based on Rayleigh scattering spectrum demodulation |
| Advisors: | Lu, Chao (EEE) Yu, Changyuan (EEE) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Optical fiber detectors Optical detectors Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Electrical and Electronic Engineering |
| Pages: | xxii, 126 pages : color illustrations |
| Language: | English |
| Abstract: | Distributed optical fiber sensors (DOFS) have garnered significant research interest due to their extensive sensing range, immunity to electromagnetic interference, and multi-parameter measurement capabilities. Among these, phase-sensitive optical time domain reflectometers (Φ-OTDR) and optical frequency domain reflectometers (OFDR), which are based on Rayleigh scattering, offer high sensitivity and the advantage of single-ended measurements, presenting substantial application potential. However, practical implementations still face challenges. For example, in spectral demodulated Φ-OTDR, the dynamic range is often constrained by the frequency range of the Rayleigh spectrum, limiting the sensor's application in large parameter measurements. Moreover, traditional DOFS using standard single-mode fibers (SMF) can only measure temperature, strain, and vibration, failing to meet the requirements of diverse parameter measurements in practical applications. This thesis delves into various applications and performance enhancements of spectral demodulated DOFS. The specific research contributions are as follows: Firstly, we proposed and demonstrated a novel salinity sensor based on frequency scanning Φ-OTDR and polyimide-coated SMF. Leveraging the hygroscopic property of polyimide material and the high sensitivity of Φ-OTDR, we successfully measured salinity with a sensitivity of 782 MHz/(mol/L). To address the cross-sensitivity issue between temperature and salinity unresolved by SMF, we confirmed the feasibility of simultaneous temperature and salinity measurements using polyimide-coated polarization-maintaining fiber (PMF). The fast and slow axes of the PMF were separately interrogated with pulse sequences with orthogonal polarization states, enabling the decoupling of the two parameters with a single fiber. The proposed sensor achieved temperature sensitivities of -1407.8 MHz/K and -1348.9 MHz/K on the slow and fast polarization axes, and salinity sensitivities of 1028 MHz/(mol/L) and 1008.6 MHz/(mol/L) on the slow and fast polarization axes, respectively. The successful discrimination between salinity and temperature was also verified. In terms of performance enhancement, we proposed a large dynamic range coherent optical time domain reflectometer (COTDR) based on dual-sideband modulation and a sub-chirped-pulse extraction algorithm (SPEA). We employed a dual-sideband linear frequency modulation (LFM) signal generated by an intensity modulator (IM). The two frequency spectrum sidebands can be recovered with in-phase/quadrature (I/Q) detection and SPEA, thereby doubling the dynamic range of COTDR without compromising the system's bandwidth. Lastly, we proposed a highly sensitive pressure sensor based on OFDR, assisted by a dual-sideband LFM probe signal and an SMF with a thick acrylic resin coating. Due to the opposite slopes of the LFM signal sidebands, opposite frequency shifts appear in the positive and negative frequency spectrum sidebands. The sensitivity is doubled by subtracting the two frequency shifts. A thick acrylic resin coating enhances the fiber's compressibility, enabling high pressure sensitivity without the need for a special fiber design. A sensitivity of 3979 MHz/MPa and a measurement accuracy of 0.97 KPa are achieved on a 500 m fiber with a 35 cm spatial resolution (SR). The system's rapid responsiveness was also validated through the dynamic measurement of a pressure release process. |
| Rights: | All rights reserved |
| Access: | open access |
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