弯曲限模对大模场光纤横模不稳定效应的影响

Influence of bending on transverse mode instability of large mode area fiber

横模不稳定效应已经逐渐成为引起高功率光纤激光光束质量急剧恶化并限制其输出功率进一步提升的首要瓶颈问题。基于全光纤化正向泵浦的窄线宽高功率放大平台,对大模场光纤激光器中的横模不稳定效应进行了一系列的探索研究。根据耦合模方程的计算结果,所用大模场光纤25/400μm中LP01、LP11模之间的非线性耦合强度最大,这也直接诱导了横模不稳定效应的发生。为了抑制LP11模在主放大级的产生和放大,通过弯曲限模这种可操作性强的模式滤波技术,将主放增益光纤的弯曲半径从6 cm缩小至5 cm的过程中,高功率光纤激光系统的横模不稳定阈值从1000 W量级提高到了1600 W量级,而且激光器的其他输出性能几乎没有受到影响。这为构建实际的窄线宽高功率全光纤化的激光系统提供了强有力的实验参照。

The effect of transverse mode instability has gradually become the primary problem that causes laser beam quality degradation and limits power scaling of high-power fiber lasers.This paper conducts a series of study on the transverse mode instability(TMI)in large mode area(LMA)fiber based on a co-pumped allfiberized narrow linewidth high power amplification platform.According to the calculation results of the coupled mode equations,the nonlinear coupling strength between the LP01 and LP11 modes of the LMA fiber 25/400μm is the largest,which directly induces the TMI.In order to suppress the generation and amplification of the LP11 mode at the main amplifier,the fiber coiling method was used as an operational mode filtering technique to achieve mode control.The threshold of TMI increased from 1000 W to 1600 W while reducing the bending radius of the main amplifier gain fiber from 6 cm to 5 cm,and the other output performance of the fiber laser was hardly affected.This provides a powerful experimental reference for us to build an actual narrow linewidth,high power,all-fiberized laser system.