Mode dynamics in high-power Yb-Raman fiber amplifier

Yb-Raman fiber amplifier (YRFA) is a compact setup that can be applied to achieve high-power narrow linewidth or special wavelength lasers. In this Letter, we realized a high-power YRFA with seed wavelengths of 1090 nm and 1150 nm, tandem-pumped by a 1018 nm fiber laser. The dynamic of mode interaction has been carefully studied. The beam cleanup effect in the large mode area, step-index fiber has been observed for the first time, to the best of our knowledge, when the pump power ranges from 800 W to 1700 W. A model taking into account the Raman mode interaction is proposed to explain this phenomenon, which agrees well with the experiments. The mode instability (MI) effect is also observed in the amplifier, and the threshold is about 2 kW, which is lower than the conventional Yb-doped fiber amplifier. Stimulated Raman scattering is attributed to the onset of MI. Finally, the 1338 W 1150 nm laser is achieved by this YRFA, which we believe to be the highest power reported at this wavelength.