Double-parabola Yb ³⁺ -doped fiber for 5.58 kW high-brightness linearly polarized laser output

A 5.58 kW narrow-linewidth linearly polarized single-mode fiber amplifier is demonstrated using a double-parabola active fiber. The fiber design ensures strong fundamental-mode selectivity under bending through its double-parabola refractive index and Yb³⁺-doped profiles, while stress zone modulation is applied to approach the cutoff condition of the fast-axis mode. The system ultimately delivers a 5.58 kW output with a broadened spectrum of 88.5 GHz for SBS suppression, while maintaining a polarization extinction ratio above 19.6 dB throughout the power scaling process and M² ∼ 1.1 at 5.2 kW, representing the highest power-beam quality-polarization combination at this level.