High absorption large-mode area step-index fiber for tandem-pumped high-brightness high-power lasers
Kang-Jie Lim, Samuel Kai-Wen Seah, Joash Yong’En Ye, Wendy Weiying Lim, C. P. Seah, Yunn-Boon Tan, Suting Tan, Huiting Lim, Raghuraman Sidharthan, Arumugam Rajendra Prasadh, Chen-Jian Chang, Seongwoo Yoo, Song-Liang Chua
Abstract
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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m1"> <mml:mrow> <mml:mo form="prefix">∼</mml:mo> <mml:mn>575</mml:mn> <mml:mtext> </mml:mtext> <mml:msup> <mml:mrow> <mml:mi>μm</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> ) 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m2"> <mml:mrow> <mml:mo form="prefix">∼</mml:mo> <mml:mn>14</mml:mn> <mml:mtext> </mml:mtext> <mml:mi mathvariant="normal">m</mml:mi> </mml:mrow> </mml:math> of the YDF. The amplifier delivered a near-Gaussian beam ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m3"> <mml:mrow> <mml:msup> <mml:mi>M</mml:mi> <mml:mn>2</mml:mn> </mml:msup> <mml:mo>∼</mml:mo> <mml:mn>1.27</mml:mn> </mml:mrow> </mml:math> ) at 836 W output power (pump power limited) with a high slope efficiency of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m4"> <mml:mrow> <mml:mo form="prefix">∼</mml:mo> <mml:mn>83</mml:mn> <mml:mi>%</mml:mi> </mml:mrow> </mml:math> . 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.