全光纤化的空间模式自适应控制与相干合束机理研究

The work study adaptive spatial mode control and coherent beam combination (CBC) in double-clad large mode area fiber by using an all fiber-based structure with a photonic-lantern-based beam combining. This approach shows promise and has implications toward mode-selective amplification in large mode area active rare earth doped fiber amplifiers, combating the onset of transverse-mode instability (TMI) of large mode area (LMA) fibers, and new type beam combination. We will discuss the three recognized causes of loss specific to N×1 (N≥6) photonic lanterns (that can be reduced or eliminated by careful design and fabrication), that are mode number matching, mode-order changes due to symmetry, and adiabaticity of the transition. The relationship of three causes is be researched. We study the principle of light from the input fibers propagates into the waist of photonic lantern. The necessary and sufficient conditions will be researched if an unpredictably or incoherently excited input set of modes is to map reversibly onto an output set of modes. The effects of the input polarization, amplitude, and phase of the inputs to the out puts mode of photonic lantern will be studied. The differences between photonic lantern and fiber combiner are analysed. The method of adaptive spatial mode control based photonic lantern will be studied, and technique of control TMI in even larger mode area and even higher power fiber amplifiers are researched by the method. To comparative analysis the divergence of the transformation between the different waveguide systems of photonic lantern and fiber combiner, mechanization and requirements for output fundamental mode by using adaptive spatial mode control in an all fiber-based photonic lantern are be studied. We will perform the analysis of the photonic-lantern-based coherent beam combination for high power high efficiency and high bean quality. It may be studied to allow continued scaling of the photonic-lantern-based coherent beam combination by scaling the individual fiber number and so the modal area.

提出基于光子灯笼的全光纤结构的空间模式自适应控制和相干合束机理研究，为实现大模场增益光纤模式选择性放大、抑制高功率光纤激光模式不稳定等非线性效应、以及新型光纤相干合束提供新思路与新方法。项目主要研究N×1（N≥6）光子灯笼绝热拉锥、模式匹配、几何排布等条件的内在关联，探索单模光纤组束的输入单模数与多模端模式数间对应性的物理基础与满足的充分必要条件，研究光子灯笼中单模光纤各参量影响输出模式特征的规律，进而阐述光子灯笼与合束器模式演化规律差异。分析基于光子灯笼空间模式自适应控制的方法，研究基于光子灯笼的更大模场增益光纤与高功率条件下的模式自适应控制方法与模式不稳定性抑制机理。对比研究N×1光子灯笼与合束器模式演化过程，分析基于光子灯笼自适应控制实现稳定基模的物理机理以及实现条件，进行基于光子灯笼结构的高功率、高效率、高光束质量相干合束机理以及基于光子灯笼的高光束质量合束的可扩展性的探索研究。

在大模场光纤中实现可控模式输出，是实现高功率同时抑制非线性效应的一种重要技术方案。本项目“全光纤化的空间模式自适应控制与相干合束机理研究”基于光子灯笼的全光纤结构，开展了大模场光纤的空间模式自适应控制和相干合束机理研究，为实现大模场增益光纤模式选择性放大、抑制高功率光纤激光模式不稳定等非线性效应以及新型光纤相干合束提供新思路与新方法。. 项目完善了模式控制用光子灯笼的结构设计准则，探索了N×1（N=3,5,6,7,8）光子灯笼单模光纤端输入基模光场各参量影响多模端输出模式特征的规律，给出了光子灯笼作为线性光学器件的传输矩阵。探索了N×1（N=3,5,6,7,8）光子灯笼的制备工艺，制备出了性能优良的光子灯笼。研究了基于光子灯笼的更大模场增益光纤放大器与高功率条件下的模式自适应控制方法与模式不稳定性抑制机理。搭建了实验系统，实现了在30 μm, 42 μm, 48 μm芯径多模光纤中的模式控制和功率放大。在大芯径多模光纤（30 μm, 42 μm, 48 μm）中，实现了高功率、高纯度的、稳定的基模光束和涡旋光束输出，验证了该系统能够有效抑制大模场光纤放大器中的模式不稳定性。本项目扩展了模式不稳定性的抑制方法和光束合成方式，为高效率、高光束质量和更高功率的光纤激光系统提供新的发展思路。