Ji Junhua, Lin Huaiqing, Sidharthan Raghuraman, Ho Daryl, Zhou Yanyan, Nilsson Johan, Yoo Seongwoo
Appl Opt. 2018 Aug 1;57(22):6388-6395. doi: 10.1364/AO.57.006388.
We investigate mode-area-scaling and bending performances of a Yb-doped large-mode-area fiber with an elongated non-circular core. Such fiber can be bent in the plane of its short axis to suppress bending effects, such as mode area reduction and mode profile distortion. Meanwhile, the other orthogonal axis can be stretched for mode area scaling. Calculations show that for fibers with the same mode area, the higher the aspect ratio between the long axis and short axis, the less sensitive the fiber will be to bending effects. However, mode area scaling is limited by the increased beat length (BL) between the fundamental mode (FM) and the second-order mode, leading to mode degeneracy at higher aspect ratios. Within the 100 mm BL, the FM area is scalable to 3000 μm in a bent fiber. To facilitate FM operation, we study mode-selective gain through confined doping. Thanks to the small bending distortions, the confined-doping approach works well in the bent large-mode-area fiber. In addition, the advantage of tandem pumping is also discussed in terms of preferential modal gain. A non-circular core fiber with a 41 μm short axis and 120 μm long axis was fabricated in-house. We evaluated the fiber in a linear laser cavity pumped by a 975 nm laser diode. The maximum output power obtained was 191 W, with slope efficiency of approximately 67% with respect to launched pump power. The output signal has good beam qualities with M of ∼1.5 and ∼3.1, respectively, along the short and long axis.
我们研究了一种具有细长非圆形纤芯的掺镱大模场光纤的模场面积缩放和弯曲性能。这种光纤可以在其短轴平面内弯曲以抑制弯曲效应，如模场面积减小和模式轮廓畸变。同时，另一个正交轴可以拉伸以进行模场面积缩放。计算表明，对于具有相同模场面积的光纤，长轴与短轴的纵横比越高，光纤对弯曲效应的敏感度就越低。然而，模场面积缩放受到基模（FM）和二阶模之间拍长（BL）增加的限制，导致在较高纵横比时出现模式简并。在100mm的拍长范围内，弯曲光纤中的基模面积可扩展至3000μm²。为便于基模运行，我们通过受限掺杂研究了模式选择性增益。由于弯曲畸变较小，受限掺杂方法在弯曲的大模场光纤中效果良好。此外，还从优先模式增益的角度讨论了串联泵浦的优势。我们自行制作了一种短轴为41μm、长轴为120μm的非圆形纤芯光纤。我们在由975nm激光二极管泵浦的线性激光腔中对该光纤进行了评估。获得的最大输出功率为191W，相对于泵浦光注入功率的斜率效率约为67%。输出信号沿短轴和长轴的光束质量分别约为M²1.5和M²3.1，质量良好。
