Pump absorption, laser amplification, and effective length in double-clad ytterbium-doped fibers with small area ratio.
Pump absorption, laser amplification, and effective length in double-clad ytterbium-doped fibers with small area ratio.
复制标题
DOI：
10.1364/oe.27.026821
复制
发表时间：
2019-09
期刊：
影响因子：
3.8
通讯作者：
Yutong Feng;Pablo G Rojas Hernández;Sheng Zhu;Ji Wang;Yujun Feng;H. Lin;O. Nilsson;Jiang Sun
中科院分区：
文献类型：
--
作者：
Yutong Feng;Pablo G Rojas Hernández;Sheng Zhu;Ji Wang;Yujun Feng;H. Lin;O. Nilsson;Jiang Sun
我们使用光束传播方法 (BPM) 和速率方程进行数值模拟，研究包层与纤芯面积比较小（范围为 1-3）的双包层掺镱光纤的泵浦吸收和放大特性。与掺杂区域低重叠的模式（或者斜射线）的存在阻碍了圆形几何形状中的泵浦吸收，但我们发现对于~2.5或更小的面积比，这种影响很小。我们推导了基于射线的小信号吸收表达式，显示了类似的结果。然而，即使小信号吸收与面积比的倒数几乎理想地成比例，当大部分 Yb 离子在小面积比光纤中被激发时，运算放大器中的吸收也会低得多，并且对面积比的依赖性也弱得多。我们推导出的方程表明了这一点，与传统的面积比（例如 100）相比，光纤长度对所需增益的依赖程度大于对所需泵浦吸收的依赖程度。然而，远短于 1 m 的光纤仍可实现足够的泵浦吸收和增益。非线性相互作用的有效长度受此影响较小，因为信号功率较高时 Yb 激发较低。尽管我们处理的是单模纤芯，但 BPM 放大器模拟显示，包层引导模式中存在百分之几的信号功率，与掺镱纤芯高度重叠。然而，根据我们的模拟，利用小面积比圆对称双包层光纤可以获得高效率和模式纯度。
We use numerical simulations with the beam propagation method (BPM) and rate equations to investigate the pump absorption and amplification characteristics in double-clad ytterbium-doped fibers with small cladding-to-core area ratios, in the range 1-3. The presence of modes with low overlap with the doped region (or alternatively, skew rays) hampers the pump absorption in a circular geometry, but we find that the effect is small for area ratios of ∼2.5 or less. We derive ray-based expressions for the small-signal absorption which show similar results. However, even when the small-signal absorption scales nearly ideally with the inverse of the area ratio, the absorption in an operating amplifier is much lower, and the dependence on the area ratio much weaker, when a large fraction of the Yb-ions is excited in a small-area-ratio fiber. We derive equations which show this, and that in contrast to conventional area ratios of, e.g., 100, the fiber length depends more strongly on the required gain than on the required pump absorption. However, fibers substantially shorter than 1 m still allow for adequate pump absorption and gain. The effective length for nonlinear interactions is less affected by this, since the Yb-excitation is low where the signal power is high. Although we treat single-mode cores, the BPM amplifier simulations show there are a few percent of the signal power in cladding-guided modes with high overlap with the Yb-doped core. Nevertheless, according to our simulations, it is possible to achieve high efficiency and mode purity with a small-area-ratio circularly symmetric double-clad fiber.