Demonstration of monolithic ytterbium-doped fiber amplifier with record power of 5.1 kW emitting at 1100 nm

Wavelength extension of high-power fiber lasers holds critical importance for spectral combining systems. In this work, we demonstrate a 5.1 kW all-fiber amplifier operating at 1100 nm based on master oscillator power amplifier (MOPA). When designing fiber lasers operating at wavelengths above 1080 nm, effective suppression of stimulated Raman scattering (SRS) and transverse mode instability (TMI) must be prioritized. The length of the ytterbium-doped fiber (YDF) was optimized to mitigate SRS while maintaining high power conversion efficiency. Wavelength-stabilized laser diodes (LDs) at 981 nm were employed to reduce quantum defect and enhance the TMI threshold. By optimizing the trade-off between total pump absorption and SRS intensity, a slope efficiency of 75.7 % and an optical signal-to-noise ratio (OSNR) of 23.5 dB were achieved. At 5100 W, the high beam quality was maintained with M²_x/M²_y = 2.39/2.33. Experimental results demonstrate that SRS significantly degrades beam quality through spectral broadening and power transfer to Stokes waves. This work establishes an important reference for extending monolithic fiber amplifier wavelengths beyond conventional bands while maintaining multi-kilowatt performance.