Study on Phase-sensitive Optical Time-domain Reflectometry and its Fading Noise

相位敏感光时域反射仪(Phi-OTDR)是一种新型的分布式光纤传感器，在对光纤线路范围内的振动信号进行远程和实时监测方面，具有传统的传感器所不能替代的优势。目前国内外关于Phi-OTDR 的研究中，大部分检测的是瑞利散射信号的强度信息，但是对外界扰动引起光相位的变化并没有定量的测量。最近，本课题提出了相位解调的 Phi-OTDR 系统，通过解调瑞利散射光的相位差信息来获得外界的扰动，相位解调精度达到0.1rad，保证了后续模式识别的准确性。 然而，由于系统采用高相干性的激光光源，这使得瑞利后向散射光发生干涉衰落，即相干瑞利噪声。同时采用相干探测的方式，也不可避免会带来偏振衰落噪声的影响。干涉衰落和偏振衰落的共同作用会使得瑞利散射波形在一些位置处幅度接近于零，从而导致相位解调出现较大的误差，严重时将导致系统出现误报，直接影响 Phi-OTDR 系统工作的可靠性。 为了提高相位解调的准确性，本文主要围绕 Phi-OTDR 系统中出现的干涉衰落和偏振衰落问题展开一系列理论和实验研究，所涉及到的具体内容如下： (1) 研制了一套 Phi-OTDR 分布式光纤传感系统样机，详细介绍了整个 Phi-OTDR 系统，包括其基本原理、系统结构、信号处理方法。为了显示实时性的要求，我们在原有基础上进行了算法改进和程序优化。同时在此实验平台上开展了PZT振动实验和声传感实验。最后对整个系统的性能进行了测试，各项性能指标如下：传感距离10km，空间分辨率5m，相位解调精度0.1rad，振动信号探测带宽为25 Hz-5 kHz。 (2) 为了消除干涉衰落噪声对 Phi-OTDR 相位解调的影响，我们从瑞利散射的统计分布规律出发，对干涉衰落噪声的机理进行研究。针对现有模型不能够准确和定量解释干涉衰落现象的问题，提出了改进模型，并对其进行了数值仿真。研究了光纤折射率分布、随机散射系数、激光器频率以及脉冲宽度等参数对干涉衰落波形的影响，从而为实验研究奠定了理论基础。 (3) 开展基于 Phi-OTDR 系统中衰落噪声的实验研究。针对干涉衰落噪声问题，提出了采用多频率光注入的方法；针对偏振衰落噪声问题，采用了分偏振接收的方法；实验结果表明，基于三频率光注入的方法是可以综合鉴别干涉衰落导致的误报，基于分偏振接收方法可以有效地排除偏振衰落的影响。通过二者结合，我们可以有效地提高 Phi-OTDR 系统相位解调的准确性。Phase-sensitive optical time-domain reflectometry (Phi-OTDR ) is a new kind of distributed optical fiber sensor. In the domain of on-line vibration monitoring, it has more advantages than the traditional sensors in dynamic and real-time measurement. At present, the typical Phi-OTDR systems detect the intensity of Rayleigh scattering signal，but they can not quantitatively detect of optical phase change caused by external vibration. Recently, our group has proposed a phase-demodulated Phi-OTDR system, where the phase difference information of Rayleigh scattering light induced by external disturbance was demodulated. Phase demodulation accuracy of 0.1 rad can be achieved, which ensures the accuracy of the subsequent pattern recognition. However, as highly coherent laser source is adopted in the sensing system, the interference fading occurs in the Rayleigh backscattered light，and it is also called coherent Rayleigh noise. At the same time, with the method of coherent detection, the effects of polarization fading are inevitable. Combination of interference fading and polarization fading make the Rayleigh scattering waveform lower to near zero at some place. This will result in large errors of phase demodulation and even seriously false alarms occur in security system，which directly affects the reliability of the Phi-OTDR system. In order to improve the accuracy of the phase demodulation, this paper is mainly focused on the theoretical and experimental studies of interference fading and polarization fading in the Phi-OTDR system. The detailed contents involved are as followed： (1) A prototype of the Phi-OTDR distributed optical fiber sensor system is developed. The whole system is described in detail, including its basic principle, system structure, signal processing method. On the basis of the original, we conducted algorithm improvement and process optimization for real-time display. Additionally, the PZT vibration and acoustic sensing experiments are carried out on this experimental platform. At last, the performance of our system are tested below: the sensing distance is 10 km, the spatial resolution is 5 m, the precision of phase demodulation is 0.1 rad, and the detection bandwidth of vibration signal is 25 Hz-5 kHz。 (2) In order to eliminate the influence of interference fading on the phase-demodulated Phi-OTDR system, the mechanism of interference fading is studied based on the statistical distribution theory of Rayleigh scattering. As the existing models can not accurately and quantitatively explain the phenomenon of interference fading, an improved model is proposed and related numerical simulation is carried out. It is seen that the interference fading waveforms depend on multiple parameters, including the index distribution of the fiber, random scattering coefficient, the laser frequency and the probe pulsewidth, which laid a theoretical foundation for experimental research. (3) The fading noise in the Phi-OTDR system is experimentally investigated. In the view of interference fading, a multi-frequency method is proposed; For polarization fading noise, the polarization-diversity detection technique is put forward. The experimental results show that the three-frequency method is effective for discriminating the false alarms caused by interference fading and the polarization-diversity detection technique can effectively reduce the influence of polarization fading. Combining the two schemes, the accuracy of the phase demodulation could be improved