High absorption large-mode area step-index fiber for tandem-pumped high-brightness high-power lasers

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 (similar to 575 mu m(2)) 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 similar to 14 m of the YDF. The amplifier delivered a near-Gaussian beam (M-2 similar to 1.27) at 836 W output power (pump power limited) with a high slope efficiency of similar to 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. (C) 2020 Chinese Laser Press