多模时域反射技术研究

Bounded by the nonlinear Shannon limit, the capacity of a single-mode optical fiber communication system will soon be exhausted. Space-division multiplexing, which is mainly based on mode-division multiplexing (MDM), has experienced exponential growth in recent years. The concept of MDM is straightforward. Its eventual implementation hinges upon a practical solution to mode coupling and crosstalk, which in theory can be compensated using MIMO processing routinely used in wireless communication. However, because of the long modal group delay (relative to symbol duration), the computational complexity of MIMO processing for MDM is orders-of-magnitude higher than that for wireless communication. In the weakly coupled regime, modal group delay scales linearly with the propagation distance; but in the strongly-coupled regime, it scales linearly with the square root of the propagation distance. Hence, mode coupling may not be an obstacle for MDM but on the contrary may reduce complexity for MIMO processing. The emphasis of the proposed effort is to investigate multimode optical time-domain reflectometry (MOTDR). Specific goals include: 1) design and fabricate a lossless mode-division (de)multiplexer with low crosstalk; 2) build an incoherent and a coherent multimode optical time-domain reflectometor; and 3) measure the power and phase MOTDR characteristics of a multimode fiber and subsequently obtain its distance-resolved mode-coupling characteristics.

单模光纤通信系统的容量受非线性香农极限的限制，在不久的将来会无法满足社会需求。以模分复用为主的空分复用光纤通信研究近年来呈指数性增长。模分复用实际应用的核心问题是模式耦合和串扰。虽然模式耦合和串扰可以通过MIMO算法来均衡补偿，其计算量与信号通道间的相对延时成正比、比无线通信MIMO高好几个数量级，成为模分复用系统发展的主要障碍。在弱耦合时，模式时延随传输距离线性增长；但在强耦合时，模式时延随着传输距离的平方根增长。因此模式耦合不是模分复用的障碍反而有助于减少信号处理的复杂度。本项目针对模式耦合和串扰这个核心问题，重点研究多模光纤时域反射技术。主要目标包括：(1) 研制（理论上）无损耗，低串扰模式复用/解复用器,(2) 搭建完整的非相干以及相干多模时域反射装置；（3）首次测量多模光纤功率和相位时域反射性能、得到"距离分辨"模式耦合特性，为多模光纤在将来通信传感系统中的应用打下坚固的基础。

随着光通讯容量需求的指数增长，近年来空分复用光通信成为研究热点。本项目围绕空分复用光纤通信系统中的关键科学技术问题——光时域反射技术——展开研究工作并取得如下成果：第一，建立了少模光线中瑞利散射的标量模型和矢量模型并对模型进行了实验验证；第二，利用光时域反射技术，测量了少模光纤中的模式串扰；第三，利用光时域反射技术，测量了保偏少模光纤中的偏振模式串扰。这些研究工作对进一步研究基于少模光纤的有源无源器件、进一步研究降低空分复用光纤通信系统中的数字信号处理复杂度有帮助。在本项目的支持下，在包括Nature Photonics, Light: Science and Applications, Scientific Reports, Optics Letters等期刊上发表论文17篇，在包括CLEO、OFC、ECOC等学术会议上发表论文27篇（含4次大会特邀报告），培养博士研究生2名和硕士研究生6名。