环光纤中轨道角动量模式复用关键技术的研究

Orbital Angular Momentum (OAM) based mode division multiplexing is an effective approach to increase the transmission capacity of optical fibers. However, two bottlenecks limit its development: (i) OAM modes in fibers are difficult to be excited; (ii) its transmission distance is relatively short. In this project, we propose a scheme to combine the ring fiber and the micro-ring resonator which can emit beams with well-controlled amounts of OAM, and aim at overcoming the crucial technical challenges in OAM mode division multiplexing. First, we will develop theoretical model for the ring fiber and investigate its transmission principle of OAM modes. On this basis, we will propose novel structures of ring fiber and introduce the effect of mode-reorganization. With the analysis of OAM mode-reorganization rules and OAM modes conversion characteristics, we will try to solve the problems of field distortion and inter-mode crosstalk during transmission and increase the propagation distance of OAM modes. Meanwhile, we will study the excitation and detection of OAM modes in the ring fiber by using mode expansion method. The spatial light modulator which decides the polarization, phase as well as power intensity of OAM beams emitted from the micro-ring resonator will be investigated to match the modal field between the OAM beam and the OAM modes in ring fiber and increase the excitation efficiency. Further, we will focus on the optimization of multiple concentric micro-ring resonators to reduce the inter-mode crosstalk and power loss during excitation and detection, and achieve a high efficiency of OAM mode division multiplexing in the ring fiber.

轨道角动量（OAM）模式复用是提升光纤信道容量的有效途径，目前制约该技术发展的主要瓶颈是光纤OAM模式激励困难和传输距离短。我们提出将可产生空间OAM光束的微环谐振腔与环光纤相结合，通过对两者的理论分析和实验研究，攻克OAM模式复用中的关键技术难题。首先建立环光纤的理论分析模型，掌握现有环光纤的OAM传输机理；在此基础上提出改进的新型环光纤结构，尝试引入模式重组效应，通过对OAM模式重组规律和转换点特性的研究，解决传输过程中的模场畸变和模间串扰问题，实现环OAM模式传输性能的改善。同时，采用模式展开方法研究微环谐振腔对环光纤中OAM模式的激励和检测问题，研究其所产生空间OAM光束的偏振、相位与能量分布的调制方案，解决激励光束和环光纤中OAM模场匹配的问题，实现高效激励；并针对多个同心微环腔结构进行优化设计，解决激励和检测过程中的串扰与损耗问题，实现环光纤中同一波长上的多OAM模式复用。

本课题针对光纤中轨道角动量（OAM）模式激励困难和传输距离短的问题展开了基础研究和应用探索。主要完成了以下工作：首先，采用高阶全矢量频域有限差分算法建立了环光纤的理论分析模型，系统研究了环光纤中OAM模式的特性，掌握了OAM模式的传输规律，给出了支持不同OAM模式数的环光纤最优参数设置方案；在此基础上，提出了改进环光纤结构，通过在环芯层外设置一高折射率层，引入模式重组效应，实现了不同OAM模式间~10-3的有效折射率差，改善了环光纤中OAM模式的传输性能。另一方面，采用角向偏振偶极子代替角向光栅衍射单元，建立了内置角向光栅的微环谐振腔的理论分析模型，仿真分析了所产生的空间辐射光束的偏振、OAM量子数及光场分布等特性，并设计了组合 波片实现了圆柱对称偏振态向圆偏振态的变换，解决了微环腔辐射光场和环光纤中OAM模场的匹配问题。另外，采用自干涉微环腔结构并借助于耗散耦合效应，通过对输出光强的测量实现了对波导臂长变化的检测。采用传输矩阵法推导了该自干涉微环腔输出光强的解析式，完成了仿真分析，实现了的 的检测灵敏度。