High-efficiency, kilowatt 1034 nm all-fiber amplifier operating at 11 pm linewidth.

We present power scaling results of a monolithic Yb-doped 1034 nm fiber amplifier well-suited for beam combining applications. Stimulated Brillouin scattering suppression is achieved through optical linewidth broadening, and results were compared for both white noise source (WNS) and pseudo-random bit sequence (PRBS) phase modulation schemes. Notably, through PRBS modulation at a clock rate of 3.5 GHz, 1 kW of output power with a slope efficiency of 81% was demonstrated. Beam quality measurements indicated near diffraction-limited operation with no sign of modal instability. At a comparable linewidth and fiber length, power scaling via WNS modulation yielded only 470 W. The kilowatt-class output at a linewidth of 11 pm is the highest power reported for a spectrally narrow all-fiber amplifier operating at the short wavelength end of the high gain range in Yb-doped silica.