Kilowatt-level, mode-instability-free, all-fiber and polarization-maintained amplifier with spectral linewidth of 1.8 GHz

Stimulated Brillouin scattering (SBS) and mode instability (MI) effect are two majority limitations in single frequency/narrow linewidth high-power fiber amplifiers. To improve the threshold of SBS, we adopt a piece of doublecladding, large-mode-area (LMA) Yb-doped and polarizationmaintained fiber with a high dopant concentration and short length. In addition, the seed laser is phase modulated for further SBS management. As for mitigating of the MI effect, high injection seed power and 915 nm pumping manner are simultaneously employed. As a result, we obtain kilowatt-level, MI-free output power with the slope efficiency of 67.4% and spectral linewidth of similar to 1.8 GHz. To the best of our knowledge, this is the narrowest linewidth in kilowatt-level, all-fiber and polarization-maintained amplifier without the influence of MI effect.