715 W sub-kHz single-frequency all-fiber amplifier at 1064 nm based on commercial PM-LMA fiber

We report a high-power single-frequency fiber amplifier operating at 1064 nm, delivering 715 W output with a sub-kHz linewidth of 400 Hz, based on commercially available polarization-maintaining large-mode-area fiber. To the best of our knowledge, this represents the highest output power achieved for sub-kHz linewidth systems using commercial fibers. The amplifier maintains near-diffraction-limited beam quality with a beam quality factor M² of 1.16 and a polarization extinction ratio exceeding 18.3 dB. Built on a master oscillator power amplifier architecture, the system incorporates segmented thermal management to suppress thermally induced mode instabilities and employs minimal fiber length to mitigate stimulated Brillouin scattering. The optical signal-to-noise ratio remains above 65.6 dB across the full power range. The demonstrated all-fiber amplifier combines high power, sub-kHz linewidth, and high polarization purity with long-term stability, offering a compact and scalable platform that bridges laboratory demonstrations and industrial applications such as coherent beam combining and optical metrology.