高功率大模场光纤激光模式不稳定性的理论与实验研究

Based on large-mode-area fiber the diffraction limited output with the average and peak power up to kW and GW have been obtained in the high power fiber laser. However, the mode instabilities that the beams degenerate from the fundamental to high order modes has been observed when the power level exceed a certain threshold, which seriously influence the beam quality and power scaling. The argument about the physical origin of such phenomenon has not been settled, and there are few experiments to verify these theories and few effective suppressing methods have been reported. So it has been a research hotspot recently. .This study aims at the increasing of the mode instabilities threshold, and we will quantify the modal interference,refractive index grating and the phase shift between them in the large-mode-area fibers by a 3D finite element analysis and obtain the complete theoretical explanations. We will experimentally test such theories by studying phase shift in the mode instabilities. A kind of feedback control method for the mode instabilities based on above experiment will also be proposed and tested. This study will promote the understanding of the physics process in the large-mode-area fiber laser, and provide theoretical and experimental support for the promotion of the fiber laser and amplifier performance.

基于大模场光纤的高功率激光器、放大器已获得平均功率kW、峰值功率GW量级的近衍射极限输出，但当功率超过某一特定阈值后输出光表现由基模向高阶模蜕化的模式不稳定现象，严重影响光束质量和功率的进一步提升。其物理根源尚存争论，对其理论的实验研究和有效的抑制方案鲜见报道，因此成为近期研究热点。.本研究围绕提高模式蜕化阈值这一目标，拟从大模场光纤三维有限元分析及实验研究该现象的相移条件出发，量化光纤中模间干涉、折射率光栅及其之间相移等物理过程，获得该现象较完整的理论解释；实验探明相移过程在模式不稳定性现象中的作用以检验此理论；以此为基础探索基于反馈控制原理的实验方案以抑制模式蜕化。本研究将推进大模场光纤中激光物理过程的深入理解，对提高光纤激光器、放大器输出特性具有重要参考价值。

大模场高功率光纤激光器、放大器和高功率NPRO激光器能分别产生平均功率数千瓦和数瓦近衍射极限光束，它们已成为科学研究、工业、国防等领域的重要激光技术。但大模场高功率光纤激光中模式不稳定现象，以及高功率NPRO激光器中的模式蜕化现象，严重影响其光束质量和功率的进一步提升，亟需深入研究。.本研究使用有三维限元方法分析了大模场光纤中模间干涉、离子反转布居分布，温度分布、折射率光栅等物理过程，使用Matlab和Comsol联合仿真，结合Crank-Nicolson方法的差分格式，实现了对1m长、纤芯直径20um、截面节点数量～100左右的光纤模型，计算时间控制在300s以内，获得了较高计算效率，并直观展示了上述物理过程的三维空间形貌，加深对对该现象的理解。同时使用有限元方法分析了NPRO种子激光器，采用规则网格划分，获得了晶体在数瓦功率下的温度、热应变、形变分布；在振荡光束路径上对晶体切分，并采用斯特列尔比方法量化每一片晶体单元内等效热透镜，使用传输矩阵方法获得了晶体内振荡激光的光束包络分布；将晶体的应变分布结合线性弹光效应、琼斯矩阵及传输矩阵方法计算了应变对振荡激光偏振的影响；计算了激光与泵浦光之间的模式失配，获得了高功率NPRO激光模式蜕化现象的理解。.本研究获得的系列结果有助于对高功率大模场光纤激光器模式不稳定性以及高功率NPRO激光器中模式蜕化现象的理解，提供了一种优化设计高功率NPRO激光器的思路。.