Wavefront shaping enables high-power multimode fiber amplifier with output focus.

High-power fiber lasers are powerful tools used in science, industry, and defense. A major roadblock for further power scaling of single-frequency fiber laser amplifiers is stimulated Brillouin scattering. Efforts have been made to mitigate this nonlinear process, but these were mostly limited to single-mode or few-mode fiber amplifiers, which have good beam quality. Here, we explored a highly multimode fiber amplifier in which stimulated Brillouin scattering was greatly suppressed due to a reduction of light intensity in a large fiber core and a broadening of the Brillouin scattering spectrum by multimode excitation. By applying a spatial wavefront shaping technique to the input light of a nonlinear amplifier, the output beam was focused to a diffraction-limited spot. Our multimode fiber amplifier can operate at high power with high efficiency and narrow linewidth, which ensures high coherence. Optical wavefront shaping enables coherent control of multimode laser amplification, with potential applications in coherent beam combining, large-scale interferometry and directed energy delivery. Editor's summary: High-power fiber lasers are used in a range of scientific fields in addition to their standard use for technology. However, increases in laser output power are limited by nonlinear effects that can damage the optical components and reduce the beam quality. Rothe et al. used a spatial wavefront-shaping technique for multimode fiber lasers that mitigates the detrimental processes, thus enabling output power to be increased appreciably while maintaining beam quality. —Ian S. Osborne [ABSTRACT FROM AUTHOR]