波前相位校正抑制模间串扰实现长距离高精度FMF-BOTDR传感

In virtue of the Few Mode Fiber (FMF) with the characteristic that different modes have different temperature and strain sensitivity, Few Mode Fiber Brillouin Optical Time-Domain Reflectometry (FMF-BOTDR) has been demonstrated as one of the most effective approaches to solve the cross sensitivity of temperature and strain of the traditional BOTDR. However, because of the serious light field crosstalk aliasing in modes of long distance FMF that affects the sensitivity coefficients of temperature and strain, which reduces the simultaneous measurement accuracy of the temperature and strain. In brief, this approach still has the problem of short sensing distance and low measurement accuracy, which restricts its applications in large scale and high accuracy measurement situations. .Therefore, based upon the principle that light field propagation in FMF is determined by its wavefront phase, this proposal proposes a wavefront encoding method to suppress the light field crosstalk aliasing in FMF modes, in order for significantly enhancing sensing distance and accuracy of the FMF-BOTDR. This proposal will investigate and establish the light field propagation model of FMF based on random matrix theory and clarify modulation mechanism of the propagating light field in FMF. The light field crosstalk aliasing in FMF modes is suppressed by a wavefront correction using a spatial light modulator (SLM) codded and controlled by an improved phase retrieval algorithm. Meanwhile, mode conversion module is designed by multiplexing the SLM. Optimal modes are chosen to establish a high performance dual modes dual parameter matrix sensing model for significantly enhancing sensing distance and accuracy of the FMF-BOTDR. .Then, the Brillouin Scattering threshold is increased, by a sub-portfolio optimization of the FMF using a multi parameter optimization model, for further improving the sensing distance..Implementation of this proposal can provide a novel method to promote the performances of FMF-BOTDR and develop a long distance and high accuracy temperature and strain simultaneous measurement technology, which has the potential to solve the key problem restricted applications of the FMF-BOTDR.

少模光纤（FMF）不同模式对温度与应变敏感不同的特性，使FMF-BOTDR成为解决二者交叉敏感的有效方案之一，然而长距离FMF的模间串扰严重，影响敏感系数，降低传感精度，制约其在大范围和高精度测量场合应用。.为此，本项目依据FMF中光场波前相位决定其横向分布和轴向传输的原理，提出一种FMF传输光波前相位编码方法，抑制模间串扰，提升FMF-BOTDR传感距离和精度。项目研究用随机矩阵理论建立FMF传输模型，分析FMF模式传输光场调控机理，设计波前相位编码方法，编码空间光调制器，校正光波前相位，抑制其他高阶模对选取模式的串扰混叠；研究分段FMF提高布里渊散射阈值（BST）机理，匹配并协调优化BST，保证传感精度前提下，进一步提升传感距离。.项目实施将提供一种提升FMF-BOTDR传感性能的新方法，发展一种长距离高精度的温度应变同时测量技术，解决制约FMF-BOTDR应用的关键问题，拓展其应用。

本项目依据传感光纤中光场波前相位决定其横向分布与轴向传输的原理， 提出基于空间光调制器编码波前相位，抑制FMF光纤模式串扰，提升传感距离和精度等性能；研究光纤分段结合系统光源线宽、光功率等参数优化，提高布里渊散射阈值，进一步提升BOTDR传感性能。我们总体上按照原定计划开展了研究，具体包括：研究FMF光纤模式传输及其光场调控机理；研究并提出BOTDR传感性能提升的新方案；长距离高精度BOTDR温度应变同时测量的研究。针对这些内容，开展如下研究：.（1）研究了用于分析介质光散射特性的传输矩阵测量方法，在实验室搭建了测量光场通过光纤等散射介质传输特性的传输矩阵装置，为提升FMF-BOTDR传感距离和测量精度等性能奠定基础的同时，也拓展并促进了课题组在透过散射介质成像、光纤内窥成像以及散射介质加密等方面研究。.（2）从理论和实验上，研究并提出了FMF-BOTDR传感性能提升的两种新方法:① 基于增益开关调制的BOTDR性能提升方法；②一种基于快速响应对数检波的时域信号包络实时提取方案，在保证传感空间分辨率和测量精度的前提下，降低数据采集端的采样带宽和采样率需求，通过性能优化和参数匹配可以提高解调速度，从而提升传感响应性能。.（3）研究了长距离高精度双模双参量矩阵传感模型，分析了泵浦光功率、光源线宽、FMF长度等因素对阈值的影响，在保证传感距离情况下，完成了光源线宽优化提升BOTDR测量精度的研究。.项目研究成果可为BOTDR的传感性能提升提供借鉴，促进BOTDR传感在国桥梁、大坝、输油、输电等大尺度、高危险、环境恶劣场合的应用。相关成果主要以论文、专利、人才培养等形式输出。发表论文11篇（均受本项目资助，并标注了项目基金号），其中SCI论文10篇，北大中文核心期刊1篇；申请发明专利8项，其中已授权6项；项目组成员参加国内外学术会议16人次；为国家培养毕业研究生7名。