单端泵浦光纤放大器获得4 kW单模窄谱激光输出

窄谱光纤激光器在光束合成等领域有着广泛的应用，然而模式不稳定效应的出现严重限制着窄谱光纤激光器的功率提升。提出并验证了采用新型981 nm稳波长泵浦方案，能够应用于窄谱激光放大并提升模式不稳定效应阈值，通过采用单端后向泵浦结构，将单模窄谱光纤放大器功率提至4 kW以上。实验中采用白噪声相位调制展宽单频激光作为窄谱种子，主放大级分别采用稳波长976 nm和981 nm两种泵浦源单端后向泵浦。在采用976 nm泵浦源泵浦时，窄谱激光最高放大至3.4 kW，出现典型的模式不稳定效应特征，功率提升受到限制。在采用981 nm泵浦源泵浦时，窄谱激光最高放大至4.05 kW，且并未出现模式不稳定效应，输出光束质量M2因子为1.3，进一步功率提升仅受限于泵浦功率。通过优化激光器设计、结合双向泵浦结构，有望实现更高功率的窄谱光纤激光输出。

4 kW single mode narrow linewidth fiber laser achieved in single-end pumped fiber amplifier

Narrow linewidth fiber lasers (NLFLs) are widely employed in application of beam combining. However, the power scaling of the NLFL are severely limited by the transverse mode instability (TMI) effect. In this work, we have verified that the TMI threshold of narrow linewidth fiber amplifier can be enhanced by employing a 981 nm pump scheme. By constructing a 981 nm counter-pumped fiber amplifier, the output power of single mode NLFL is scaled to over 4 kW. In the experiment, the narrow linewidth seed laser is white noise signal phase modulated single frequency laser, and the main amplifier has two types of laser diodes at wavelengths of 976 nm and 981 nm as counter-pump. When the 976 nm laser diodes are employed, the NLFL is scaled to a maximum of 3.4 kW, which is limited by the occurrence of TMI effect. When the 981nm laser diodes are employed, the NLFL is scaled to 4.05 kW without any sign of TMI. The M2 factor of the output laser is 1.3. Further power scaling of the fiber amplifier is only limited by the available pump power. By optimizing the fiber laser design and employing bidirectional-pump, NLFL with higher power is achievable.