【作者】 杨双收；

【导师】 吴重庆；

【作者基本信息】 北京交通大学 ， 光学工程， 2010， 博士

【摘要】 分布式光纤传感技术利用外界参量对光纤中信号的影响来测量它沿光纤传输路径的空间分布和时间分布。偏振敏感光时域反射技术(POTDR)是分布式光纤传感器的一种,它通过测量光纤中背向瑞利散射光偏振态的变化来检测外界物理量的变化。在POTDR提出后的30年中,虽然有了一定的进展,但距离成功应用还有很大距离。本文围绕POTDR技术的一些关键问题,首先对POTDR测量系统本身进行改进和完善,以便精确测定光纤中偏振态(本地双折射)的分布；然后对本地双折射与外界应力或者弯曲的关系进行了研究,将POTDR的测量结果与被传感的物理量直接联系起来。本文的工作对于完善POTDR分布式传感的理论有重要的学术价值,对于推动POTDR分布式传感的应用有重要的指导意义。本文的创新成果主要有：1.提出了一种利用挤压光纤式偏振控制器的POTDR系统(PPC-assisted POTDR),该系统中使用偏振态可以精确控制的PPC光纤偏振态发生器改变输入被测光纤的偏振态,输入光与散射光都经过PPC光纤偏振态发生器,光纤偏振器保证了测量系统的坐标一致性。通过改变PPC的驱动电压可以得到多条不同背向散射曲线,通过背向散射光信号可以分析光纤的偏振参数；通过该方法测得了光纤的双折射的空间分布。2.基于PPC-assisted POTDR系统对外应力和弯曲进行了传感实验。光纤受外力作用时,对外力的定位十分准确。提出了使用相关系数来计算光纤弯曲半径的方法,结果表明,对于确定弯曲半径,双折射分布的相关系数近似为一常数,弯曲半径变化时,相关系数呈指数形式变化。该方法对弯曲采样点足够少的情况十分有效,有利于推进POTDR的实用化。3.提出了一种将光纤在外应力和弯曲作用下的本地双折射分为固有双折射和感应双折射叠加的波片模型,利用该模型进行了仿真,结果表明,在光纤长度足够小的时候,两种双折射的叠加随主轴夹角的变化呈现一个“8”字型；在固有双折射与感应双折射接近的时候,双折射的叠加在很小范围内波动,叠加形成一个很小的“8”字；当感应双折射足够大的时候,固有双折射对叠加结果的影响十分小,叠加结果会在邦加球上形成一个大的“8”字。4.针对以往的POTDR信噪比低、分辨率差的问题,提出了一种干涉型POTDR测量系统,它采用两路光纤(参考光纤与被测光纤)中的背向瑞利散射光进行偏振态的相干检测,利用琼斯矩阵对双折射进行了理论计算,该矩阵的迹描述了被测光纤双折射的大小。与以往的POTDR技术相比在相同的脉宽的条件下,干涉型POTDR的分辨率明显提高,信噪比也显著提高。更多还原

【Abstract】 Distributed optical fiber sensors make use of the changing of optical signals to measure the space and time distribution information of the physical parameter along the fiber. Polarization-sensitive optical time domain reflectometry (POTDR) is a kind of distributed optical fiber sensors, POTDR can measure the external physical changes by detecting the evolution of state of polarization (SOP) of Rayleigh scattering light. In 30 years after POTDR presented, the successful application of POTDR is difficult although many studies have been done. This dissertation focuses on some key issues of POTDR; the improving and perfecting of POTDR can exactly measure the distribution of SOP (local birefringence); the relationship between birefringence and stress or bending is studied, which connect the measurement results of POTDR and sensing parameters. This work has important academic value for improving the sensing theory of POTDR, and important guidance for promoting the application of POTDR. The main innovative points of this dissertation are presented as following:1. A piezoelectric-polarization-controller-assisted POTDR (PPC-assisted POTDR) system is proposed. In this system, PPC-SOP generator which can accurately control the SOP is used to change the input SOP of fiber under test, the input and scattering light all pass through the PPC-SOP generator, and the fiber polarizer ensures the consistency of measurement system. Many POTDR traces can be got by changing the driving voltage of PPC, then the polarization parameters can be analyzed; the spatial distribution of fiber birefringence is calculated.2. Sensing experiment of stress and bend have been done based on PPC-assisted POTDR. The location of stress can be detected exactly. Correlation coefficient is proposed to calculate the bend radius. The results show that the correlation coefficient is approximately a constant if the bend radius is determined, the correlation coefficient exponentially changes when the bend radius changes. This method is effective for few sampling points, and conducive to promoting the practical POTDR.3. A fiber waveplate model is proposed for the superposition of intrinsic birefringence and induced birefringence under stress and bend. The local birefringence contains intrinsic and induced birefringence when the fiber is bend, the superposition is derived by matrix multiplication. Simulation results show that when fiber length is small enough, the superposition of two kinds of birefringence shows a’8’with the change of axis angle; if intrinsic birefringence and induced birefringence are close, the superposition with a small’8’fluctuate in a very small range; when induced birefringence is large enough, the effect of intrinsic birefringence is very small for superposition, the superposition will form a large’8’on the Poincare sphere.4. Because of low signal to noise ratio (SNR) and poor resolution of previous POTDR, a coherent POTDR system is proposed. The evolution of SOP is detected utilizing the coherent Rayleigh scattering light in two fibers (Reference fiber and fiber under test). The birefringence is calculated based on Jones matrix, and the trace of Jones matrix can describe the magnitude of birefringence. Compared with the previous POTDR in the same pulse, the resolution and SNR of coherent POTDR is significantly improved.更多还原