高功率宽带光纤激光器中模式不稳定性的理论及抑制方法

The mode instability (MI) effect is one of the most important limiting factors for the power scalability of the single mode fiber laser to 10kW and beyond.Since the report of the MI effect in the large mode area photonic crystal fiber laser in 2010, experimental and theoretical studies on MI effect have become the hotspot in high power fiber laser technology area. However, the theoretical models in literatures have not take the influences of the laser spectral width, the mode distortion and loss into account systematically. Few researches have been carried out according to the broadband multi-kW class fiber lasers and few solutions have been proposed to suppress the MI effect which have the engineering potential. In this proposed project, a more comprehensive high dimensional multiphysics coupling model is to be developed which integrates three physical models, i.e. the wave equation of the complex amplitude field, the laser rate equation of the upper level population distribution field and the heat equation of the temperature field. The MI threshold, the transient fluctuation of mode field and cladding power introduced by mode loss are to be studied based on the parameters of the broadband multi-kW class fiber laser. Suppression methods of MI effect are to be explored. A novel design of the fiber structure is to be proposed and the model biult is to be applied to evaluate and optimize the suppression of MI effect.

模式不稳定性（MI）是限制单纤单模激光系统功率向10kW量级提升的重要障碍之一。自2010年在超大模场光子晶体光纤激光器中的MI现象报道以来，相关的实验和理论研究成为近年来高功率光纤激光技术研究领域的热点。但是，文献报道的理论没有系统地考虑激光谱宽以及模式畸变和损耗的影响，也缺少针对宽带数kW级高功率光纤激光器的研究。本申请项目拟建立描述MI效应的更为完备的高维度的多物理场耦合模型，集成描述激光复振幅场的波动方程，描述上能级粒子数分布场的激光速率方程以及描述温度场的热传导方程三组模型。针对宽带数kW级光纤激光器参数，对MI阈值功率、模场瞬态抖动特性以及模式损耗引入的包层模功率进行研究。探索抑制MI效应的方法，提出新型的光纤结构设计并基于建立的理论模型对抑制的效果作评估和优化。

高功率宽带光纤激光器在前沿科学研究、工业加工等领域有广泛的应用，同时这些应用也不断推动着宽带光纤激光的功率提升。然而，模式不稳定现象(MI)目前限制了宽带光纤激光的功率提升和在前述领域的应用，亟待开展相关的理论和实验研究。因此，本项目建立了描述MI效应的完备的高维度多物理场耦合模型，系统研究了光纤参数、激光系统参数等对MI的影响，提出了变芯包比光纤、低热光系数光纤、分区掺杂光纤等方法有效抑制MI；针对宽带数kW级光纤激光器参数，研究了MI阈值功率、模场瞬态抖动特性以及模式损耗引入的包层功率；发现了非线性受激拉曼效应对MI影响的物理现象；探索了MI的抑制方法，利用后向泵浦方案、弯曲选模、抑制受激拉曼散射效应有效提升了MI的阈值。本项目研究成果可以为10 kW单纤单模激光输出提供可靠的理论和实验指导，进一步推动光纤激光在各领域的应用。