Study On The Fiber Optical Distributed Wide-frequency Vibration Sensing Based On φ-OTDR

With the development of economy and technology, China has entered the era of massive construction of large-scale infrastructures, which mainly include underground transmission systems(such as water/gas pipelines, coal handing system), rail transport systems(such as high-speed rail, metro and Light rail) and large-scale civil construction(such as large bridges), etc.. It is necessary to monitor health status of these long-distance infrastructures, and quickly identify and alert when faults occur. Faults often accompanied by changes in multiple physical parameters, such as strain, temperature and vibration. Vibration is an important parameter in monitoring and often happens as a serious fault, such as the cracks and leaks of pipelines and the rupture of materials in civil structures, and the vibration frequency range can be up to Mega Hz. Currently, monitoring is mainly based on electrical sensors, which have the defects of point sensing, sensitive to electromagnetic and require real-time power. Distributed fiber sensing can achieve remote monitoring of each point along the sensing fiber, and has the advantages of anti-electromagnetic interference, small size and multi-point measurement. Up to now, the real distributed measurement systems are mainly based on backscattering in fiber, however, the frequency response range is limited by the sensing distance, especially for ultra-long sensing distance that is up to tens or even hundreds of kilometers. In this thesis, with aim to realize wide-band frequency response and high spatial resolution in long-haul distributed vibration sensing, we analyze the theoretical models of phase-sensitive optical time domain reflectometry(φ-OTDR), and study the limitations of frequency response. On this basis, we propose three methods to effectively broaden the frequency response in distributed vibration sensing system, and related work in the theory of systems, physical implementations and sensing performance testing have been carried out.The core contents of this thesis include:① We have introduced physical mechanisms of Rayleigh backscattering in fiber, and studied the vibration measurement and limitations of frequency response in φ-OTDR system. We have established the discrete sampling model of vibration measurements in φ-OTDR, and studied the uniform sampling model in distributed vibration measurements. Three methods to broaden the detected frequency range are proposed.② We have proposed and built a distributed vibration sensing system based on merged Mach-Zehnder interferometer(MZI) and φ-OTDR, which combines the merits of broadband frequency response of MZI and high-precision location of φ-OTDR. A novel modulated optical pulse signal is used to achieve the integration of the two sensing systems. The frequency information of external vibration signals is demodulated by MZI, while the location is detected by φ-OTDR. Precise location and wide frequency response can be achieved in the proposed system. With aim to solve the trade-off between the detected highest frequency response and signal to noise ratio of location signal, another system using time-division multiplexing technology based on the combination of MZI and φ-OTDR has been proposed. By using a time-multiplexed narrow pulse and a wide pulse, the precise location and further broadened frequency response in distributed vibration sensing in realized.③ We have proposed a φ-OTDR system based on frequency-division multiplexing to enhance the highest frequency response. In order to break the limitation of sensing fiber length on frequency response range, the frequency-multiplexed probe optical pulses are injected into the sensing fiber in one measurement interval. On the basis of non-uniform sampling model of vibration signals, the effective sampling rate for each measurement point along the sensing fiber is increased, so that the vibration frequency that is higher than the Nyquist frequency can be detected. Therefore, the frequency response range is broaden. The essence of the method is to optimize the input signal of the sensing system.④ We have proposed φ-OTDR system using non-uniform sampling method to enhance the detected highest vibration frequency. In order to achieve a simple and cost-effective configuration, the system is based on the traditional φ-OTDR system. On the basis of the theory of non-uniformly periodical sampling, the non-uniformly distributed probe pulses are injected into the sensing fiber to detect the external vibration. The frequency response range is widen by the sub-Nyquist sampling. The essence of the method is the optimization of the output signal of the sensing system.The main innovations of the thesis are as follows:① We have proposed and built two merged MZI and φ-OTDR based distributed vibration sensing system. The modulated pulses and time- multiplexed pulses are used to achieve the integration of MZI and φ-OTDR. The frequency response is demodulated by MZI, while the accurate vibration position is obtained by φ-OTDR. By combining distributed interferometer with φ-OTDR, a precise location and wide frequency response range are achieved in vibration measurements.② We have proposed and built a system based on frequency-division multiplexing to enhance the φ-OTDR system frequency response. The number of probe light pulses is increased by using frequency-division multiplexing, such that the effective sampling rate of each measuring point along the sensing fiber is enhanced. The proposed system is able to break the trade-off between sensing fiber length and detected vibration frequency response.③ We have proposed and built a non-uniformly periodical sampling based φ-OTDR system. A set of periotic non-uniformly distributed probe optical pulses are used to sample the external vibration signals, realizing sub-Nyquist frequency measurement. The system can break the limitation of sensing fiber length on the maximum frequency response, and has the merits of simple structure and low cost...