We experimentally demonstrate a single-frequency Yb-doped multimode fiber amplifier (~76 modes) with a focused Gaussian output beam and enhanced stimulated-Brillouin-scattering (SBS) threshold. With few-mode excitation, the SBS threshold output power (57 W) is already an order of magnitude higher than that for a single-mode fiber amplifier of the same length. The output beam, however, is slightly speckled. To achieve high beam quality, we optimize the input wavefront to focus output light to a single diffraction-limited spot and simultaneously achieve much higher SBS threshold (up to 105 W), because forming a tight output focus requires coherent amplification of many modes.
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