Spectral broadening in high-power Yb-doped fiber lasers employing narrow-linewidth multilongitudinal-mode oscillators

High-power continuous wave fiber lasers with narrow linewidths are required for spectral beam combining systems. A 2.9 kW Yb-doped fiber laser with an output linewidth as narrow as 0.31 nm employing a narrow-linewidth multilongitudinal-mode oscillator is experimentally demonstrated. The numerical simulations reveal that the spectral broadening of the bandwidth grows almost linearly with injected mode number, which provides a potential way to further suppress the nonlinear spectral broadening in high-power fiber lasers.