大功率光纤激光器功率不稳定和模式不稳定机理及其抑制方法研究

Power and beam quality are the key performance indicators in high power fiber lasers, which have been the main obstacle to further develop the fiber lasers. The main factors to influence the two indicators include photo-darkening, traditional nonlinear effects and mode instability. The project proposes to study the formation mechanism of photodarkening related color centers from rare-earth impurities, charge transfer, optical basicity and coordination environment, then reduce the photodarkening effect by decreasing the overlap factor of pump and signal light, changing the matrix material’s optical basicity and adjusting the ytterbium ions’ coordination environment. Optimization of index profile, doping distribution and mode area will be applied to reduce the nonlinear effects. Based on the Maxwell’s equations, the theoretical model for dynamic and quasi-static mode instability will be built to investigate its mechanism, in which, fiber gain, temperature field, fiber bending loss, heat induced Rayleigh scattering, photodarkening and nonlinear effects are all included. The reduction methods will be studied from aspects of fiber design, index and doping distribution, and laser system optimization. Through the physical mechanism investigation on interaction between photodarkening, traditional nonlinear effects and mode instability, as well as reduction methods based on fiber design, this project aims to solve the power and mode instability in high power fiber lasers.

功率和光束质量是大功率光纤激光器最核心的两个性能指标，正成为大功率光纤激光器进一步发展的瓶颈，影响的主要因素有光纤中的光子暗化、传统非线性和模式不稳定效应。项目提出从稀土离子杂质、电荷转移、基质的光学碱度以及镱离子的配位环境等角度研究与光子暗化相关的色心形成机理；从降低泵浦光与信号光重叠因子、改变基质光学碱度以及调整镱离子配位环境等方面探究抑制方法；通过改善折射率剖面、离子掺杂分布和模场来抑制非线性效应；从麦克斯韦波动方程出发，综合考虑增益、温度场、光纤弯曲、热致瑞利散射、光子暗化，建立与光子暗化横向分布相关的动态和准静态模式不稳定理论模型，研究模式不稳定机理；从波导结构设计、折射率和掺杂分布、基质材料成分等方面探究模式不稳定的抑制方法；通过对光子暗化效应和传统非线性效应与模式不稳定效应相互作用的物理机制探索，从光纤设计寻找共同抑制的方法，为解决功率不稳定和模式不稳定提供有效的解决方法。

光子暗化效应导致的光纤损耗增加影响光纤的效率导致输出功率的下降且热积累增加、输出激光功率超过一定值时模式急剧恶化即出现模式不稳定现象以及在大功率激光输出时模式与非线性效应相关影响制约功率攀升问题，以上三方面的问题限制了光纤激光进一步提升功率及应用范围，成为当下光纤激光发展瓶颈问题。本项目针对万瓦级的高功率光纤激光器用掺镱光纤展开研究，为大功率光纤激光器提供能够长期稳定运行的大模场面积单模运转的光纤材料。. 主要开展的研究内容含盖三方面光子暗化效应、模式不稳定效应、非线性效应之间的影响规律及物理关系、功率不稳和模式不稳定的有效抑制方法、高模式不稳定阈值及弱光子暗化效应的大模场增益光纤设计及制备。通过项目研究工作的开展，理论分析并试验验证了光子暗化、模式不稳定效应及非线性效应之间存在的内在物理联系，建立了模式不稳定理论演化模型；基于低泵浦吸收系数光纤的热光效应控制技术实现了模式不稳定的抑制，模式不稳定阈值提升超2倍。基于单锥形光纤实现了模式不稳定阈值超过5kW，实现6.11kW的激光功率，拉曼抑制比大于42dB。基于纺锤形光纤并结合热光效应控制技术抑制模式不稳定和非线性效应实现了6kW量级激光输出。成果方面研制的低热光系数有源光纤性能比肩进口产品，模式不稳定阈值和非线性效应阈值改善技术相关成果实现产业化落地，累计产值超2亿元。