An acoustic- and gain-tailored Yb-doped polarization-maintaining photonic crystal fiber is used to demonstrate 811 W single-frequency output power with near diffraction-limited beam quality. The fiber core is composed of 7 individually doped segments arranged to create three distinct transverse acoustic regions; including one region that is Yb-free. The utility of the Yb-free region is to reduce coupling between the LP01 and LP11 modes to mitigate the modal instability. The application of thermal gradients is utilized in conjunction with the transverse acoustic tailoring to suppress stimulated Brillouin scattering. To the best of our knowledge, the 811 W output represents the highest power ever reported from a near diffraction-limited single-frequency fiber laser.

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We demonstrate coherent beam combining using a two-dimensionally patterned diffractive optic combining element. Fifteen Yb-doped fiber amplifier beams arranged in a 3×5 array were combined into a single 600 W, M²=1.1 output beam with 68% combining efficiency. Combining losses under thermally stable conditions at 485 W were found to be dominated by spatial mode-mismatch between the free space input beams, in quantitative agreement with calculations using the measured amplitude and phase profiles of the input beams.

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