300-watt single-frequency Yb3+-doped phosphate fiber amplifier

We report a 320-W single-frequency, diffraction-limited laser output (M² < 1.15) achieved using a 20-cm large-mode-area (LMA) phosphate fiber amplifier doped with 8 wt.% Yb^3+. The gain fiber, featuring a 40-μm core diameter and a numerical aperture (NA) of ⁓0.02, exhibits a record-low propagation loss of 0.32 dB/m. The amplifier delivered a >40% electrical-to-optical efficiency, an optical signal-to-noise ratio exceeding 50 dB, and a polarization extinction ratio greater than 17 dB.