不同泵浦波长光纤激光器模式不稳定效应对比

一般认为，光纤激光器模式不稳定效应主要来自于泵浦源量子亏损和增益光纤泵浦吸收所产生的热效应。理论分析了光纤中的热源，发现诱发模式不稳定的热效应主要来源于泵浦吸收、其次是量子亏损；利用课题组开发的仿真软件SeeFiberLaser对该结论进行仿真。仿真结果表明:泵浦吸收系数越低，光纤中的最高温度和温度梯度越低，越有利于抑制热致折射率光栅的形成，提高模式不稳定阈值。搭建了纤芯/包层直径为30/400 μm的前向泵浦掺镱光纤激光振荡器，对比研究了中心波长为976 nm 、915 nm和940 nm的泵浦源泵浦时激光器的模式不稳定阈值特性。结果表明，分别采用中心波长为976 nm 、915 nm和940 nm 的半导体激光器作为泵浦源时，激光器模式不稳定阈值分别为279 W、502 W和697 W，光光转化效率分别为67.7%、61%和63%。由此可以发现，泵浦吸收系数对模式不稳定阈值的影响大于量子亏损对模式不稳定阈值的影响，通过改变泵浦波长降低泵浦吸收系数可以有效提升模式不稳定阈值。优化泵浦波长，兼顾量子效率和泵浦吸收系数，是光纤激光器实现高光束质量高模式不稳定阈值的重要技术路线之一。

Transverse mode instability effect of fiber lasers with different pump wavelengths

It is generally believed that the transverse mode instability (TMI) of fiber laser mainly comes from the thermal effect caused by the quantum defect of the pump source and the pump absorption of gain fiber. Theoretically, based on the analysis of the heat source in optical fiber, it was found that the thermal effect of induced mode instability mainly comes from pump absorption, followed by the quantum defect. And the simulation software SeeFiberLaser developed by the research group was used to verify the conclusion. The simulation results showed that the lower the pump absorption coefficient, the lower the maximum temperature and temperature gradient in the fiber, the more beneficial it was to restrain the formation of thermally induced refractive index grating and increase the TMI threshold. Experimentally, a co-pumped ytterbium-doped fiber laser oscillator with a core/inner cladding diameter of 30/400 μm was built and comparatively studied on TMI threshold characteristics of lasers pumped by laser diodes (LDs) with central wavelengths of 976 nm, 915 nm and 940 nm. The results showed that when 976 nm LD, 915 nm LD and 940 nm LD were used as pump sources, the laser TMI threshold was 279 W, 502 W and 697 W respectively, and the laser optical-to-optical (O-O) conversion efficiency was 67.7%, 61% and 63%, respectively. It was be found that the influence of the pump absorption coefficient on the TMI threshold was greater than that of the quantum defect on the TMI threshold, and the TMI threshold could be effectively increased by changing the pump wavelength and reducing the pump absorption coefficient. Optimizing pump wavelength, taking into account both quantum efficiency and pump absorption coefficient, was one of the important technical routes for fiber lasers to achieve high beam quality and high mode instability threshold.