Stimulated Brillouin scattering (SBS) and mode instability (MI) effects are two major limitations in single frequency/narrow linewidth high-power fiber amplifiers. To improve the threshold of SBS, we adopt a piece of double cladding, large-mode-area Yb-doped and polarization-maintained fiber with a high dopant concentration and a short length. In addition, the seed laser is phase modulated for further SBS management. As for mitigating the MI effect, a high injection seed power and 915 nm of pumping manner are simultaneously employed. As a result, we obtain 827 W MI-free output power with a spectral linewidth of ~1.8 GHz. To the best of our knowledge, this is the narrowest linewidth in a kilowatt-level allfiber polarization-maintained amplifier, without the influence of an MI effect.
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