Research on Rayleigh-Scattering-based Distributed Optical Fiber Vibration Sensing Systems

This thesis studies the distributed acoustic sensing (DAS) technology that utilizes the Rayleigh backscattering (RBS) light within optical fiber as the sensing signal. Because it simultaneously offers the advantage of long coverage range and high spatial resolution while remaining compatible with existing optical fiber communication infrastructures, DAS technology is becoming increasingly important over recent years, especially in the oil and gas industry. However, conventional DAS systems generally require the use of hardware of large bandwidth and high sampling rate for signal detection, and expensive signal processing unit for real-time acoustic wave demodulation. The large-scale industrial adoption of the DAS technology is therefore limited by its hardware requirements and computational complexity. This thesis is focused on developing signal demodulation algorithms and sensing principles that reduces computational complexity and hardware requirements, eventually realizing distributed acoustic sensing with cost-effective configurations.

Phase-Sensitive Optical Time-Domain Reflectometry (Φ-OTDR) is the key sensing technique widely adopted for DAS system. To investigate the acoustic sensing algorithms and configurations of Φ-OTDR, this thesis first establishes the theoretical model of Φ-OTDR, including the physical process of RBS light generation, the conventional sensing configurations, and signal demodulation algorithms.

Based on the theoretical analysis, this thesis then proposes a signal demodulation algorithm based on spatial phase shifting (SPS) technique for heterodyne Φ-OTDR. By exploiting the phase differences between the heterodyne signal from adjacent spatial sampling channels, the orthogonal signal can be directly generated from the original signal with simple algebra calculations. The computational complexity of the signal demodulation process is significantly decreased by using the SPS method compared to conventional methods. Experimental results confirm that the proposed SPS method is at least 100% faster than the conventional methods with negligible performance tradeoff.

To further decrease the required detection bandwidth and sampling rate, this thesis then investigates the sensing principle of self-homodyne Φ-OTDR. Phase diversity technique is integrated into two schemes of self-homodyne Φ-OTDR in either the transmitter side or the detection side for low computational complexity signal demodulation. Both proposed schemes satisfy the requirements of low system sampling rate and low system cost, offering cost-effective DAS solutions for possible on-site system deployments and large-scale installations.

This thesis then studies the compensation method for the I/Q imbalance issue in self-homodyne Φ-OTDR. The Lissajous figure fitting (LFF) method is proposed and proved effective in determining the imbalance parameters and correcting the signal distortions caused by the I/Q imbalance. By employing this approach, the advantage of cost-effectiveness is preserved for self-homodyne Φ-OTDR system, while also improving its performance of DAS signal demodulation.

Considering the overall theoretical models, numerical simulations and experimental demonstrations, this thesis ultimately aims to accelerate the signal demodulation process and simplify the system configuration for Φ-OTDR based DAS systems, in an effort to realize a cost-effective DAS solution and promote its industrial potentials.

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