基于耦合模理论与光束传输算法的多模光波导迭代设计方法

The design of the multimode optical waveguide components whose modes are controllable is one of the key issues to be solved in the research and development of the high power fiber laser. The coupled mode theory (CMT) is an effective simplified theory to describe the mode field evolution in the multimode optical and microwave waveguides. The iterative method based on CMT and the steepest descent principle has been applied in the design of multimode microwave waveguide successfully. However, there is still no effective inverse design method for the multimode optical waveguide. The goal of this project is to develop an iterative design method for the multimode optical waveguide based on CMT. A few kinds of structural constraint functions and merit functions are designed according to specific physical goals such as mode conversion and mode field shaping. For precise description of the optical effects such as the mode distortion and radiation mode leakage in the complex multimode optical structures, the beam propagation method (BPM) which naturally includes these effects is to be applied instead of CMT. And the iterative method based on BPM needs to be rebuilt and improved. This method is to be applied in the design and optimization of the axis-bending, radius-varying and multi-core structures in order to provide a theoretical tool for the design of the components and structures in large mode area high power fiber laser systems.

模式可控的大模场多模光波导器件的设计是高功率光纤激光系统研发中必须解决的关键问题之一。耦合模理论是描述多模光波导和微波波导中的模场演化的有效简化理论。基于耦合模理论和最速下降原理的迭代法已在微波波导结构设计中成功应用，但是在光波导结构中尚无有效的反向设计理论。本申请项目拟构建一套基于耦合模理论的多模光波导迭代设计方法，针对模式变换和模场整形等特定物理目标设计结构约束函数与优化品质函数。对复杂多模光波导结构，为了更精确地描述模式畸变和辐射模泄露等波导光学效应，拟采用自然包含这些效应的光束传输算法替代耦合模理论，并以此为基础改进迭代设计理论。本申请项目拟将该方法应用于轴线弯曲、半径渐变和多芯光波导结构的设计和优化中，为大模场高功率光纤激光系统中的器件和结构设计提供理论工具。

模式可控的大模场多模光波导器件的设计是高功率光纤激光系统研发中必须解决的关键问题之一。耦合模理论是描述多模光波导和微波波导中的模场演化的有效简化理论。基于耦合模理论和最速下降原理的迭代法已在微波波导结构设计中成功应用，但是在光波导结构中尚无有效的反向设计理论。本项目构建了基于耦合模理论的多模光波导迭代设计方法，针对半径渐变结构，设计相应结构约束函数和优化品质函数，在不同光波段实现LP0m的模式转换，效率高达99%。采用Rsoft商用软件验证理论结果，设计的多模光波导结构均能实现>95%的模式转换效率，同时具有>130 nm的1 dB带宽。因此，基于耦合模理论的迭代设计方法具有可靠性、普适性等特点，可实现多模式，多波段，高效率，大带宽和转换可逆等特点的模式转换器，在科研、通讯等领域具有广泛应用前景。本项目还构建了基于光束传输算法的多模光波导迭代设计方法，采用Rsoft-Matlab联合编程，针对轴线弯曲结构进行结构优化，实现LP11-LP01高效率模式转换，优化效果依然有待改善，但有望在高功率光纤激光器应用实现模式不稳定的抑制。