A short absorption length ytterbium (Yb)-doped large-mode area (LMA) fiber is presented as a step forward to mitigate the stern problem of nonlinear scatterings in a tandem pumping scheme adopted for high-power fiber laser. The short absorption length was realized by incorporating high Yb concentration in the fiber core. Furthermore, by replacing the inherent silica cladding with a Ge-doped cladding, we were able to obtain low core numerical aperture (NA) and negate the detrimental effect of index-raising by high Yb concentrations. This overcomes the long-standing limitation in step-index Yb-doped fibers (YDFs) where high cladding absorption inevitably results in high NA, thus hampering single-mode operation. We report an LMA (∼575 μm2) YDF with NA of 0.04 and absorption of 27 dB/m at 976 nm—both traits promote power scaling of single-mode tandem pumped fiber lasers. To our knowledge, this is the highest cladding absorption attained in a low-NA step-index fiber to date. An all-fiber tandem-pumped amplifier was built using only ∼14 m of the YDF. The amplifier delivered a near-Gaussian beam (M2∼1.27) at 836 W output power (pump power limited) with a high slope efficiency of ∼83%. Thanks to the short length and the tandem pumping, no indication of limiting factors such as stimulated Raman scattering, photodarkening, and transverse mode instability was observed.

中文翻译：

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用于串联泵浦高亮度高功率激光器的高吸收大模式面积阶跃折射率光纤

提出了一种短吸收长度的掺镱 (Yb) 大模式面积 (LMA) 光纤，以减轻高功率光纤激光器采用的串联泵浦方案中非线性散射的严重问题。短吸收长度是通过在纤芯中加入高 Yb 浓度来实现的。此外，通过用掺 Ge 包层代替固有的二氧化硅包层，我们能够获得低纤芯数值孔径 (NA) 并消除高 Yb 浓度引起的折射率升高的不利影响。这克服了阶跃折射率掺镱光纤 (YDF) 的长期限制，其中高包层吸收不可避免地导致高 NA，从而阻碍单模操作。我们报告了 NA 为 0 的 LMA（~575 μm2）YDF。04 和 976 nm 处 27 dB/m 的吸收——这两种特性都促进了单模串联泵浦光纤激光器的功率缩放。据我们所知，这是迄今为止在低 NA 阶跃折射率光纤中获得的最高包层吸收。仅使用约 14 m 的 YDF 构建了一个全光纤串联泵浦放大器。该放大器以 836 W 的输出功率（泵功率受限）提供近高斯光束（M2∼1.27），斜率效率高达 83%。由于较短的长度和串联泵浦，没有观察到诸如受激拉曼散射、光暗化和横向模式不稳定性等限制因素的迹象。该放大器以 836 W 的输出功率（泵功率受限）提供近高斯光束（M2∼1.27），斜率效率高达 83%。由于较短的长度和串联泵浦，没有观察到诸如受激拉曼散射、光暗化和横向模式不稳定性等限制因素的迹象。该放大器以 836 W 的输出功率（泵功率受限）提供近高斯光束（M2∼1.27），斜率效率高达 83%。由于较短的长度和串联泵浦，没有观察到诸如受激拉曼散射、光暗化和横向模式不稳定性等限制因素的迹象。