55 W kilohertz-linewidth core- and in-band-pumped linearly polarized single-frequency fiber laser at 1950 nm.

Based on core- and in-band-pumped polarization-maintaining ${{\rm Tm}^{3 + }}$Tm-doped single-cladding fiber (PM-TSF, the core diameter is 9 µm) by a 1610 nm fiber laser and a distributed Bragg reflector seed laser, a linearly polarized single-frequency fiber laser (LP-SFFL) at 1950 nm with an output power of 55.3 W and a laser linewidth of 6.95 kHz is demonstrated. The output beam qualities of ${M}_x^2$Mx2 and ${ M}_y^2$My2 are measured to be 1.01 and 1.03, respectively. The slope efficiency with respect to the launched pump power is 71.0%, in comparison with a theoretical quantum efficiency of 82.6%. A polarization-extinction ratio of 19 dB and an optical signal-to-noise ratio of 58 dB are obtained from the 1950 nm LP-SFFL. To the best of our knowledge, to date, this is the highest power of 2.0 µm SFFL output directly from a strict single-mode active fiber. Our experiment offers a promising solution to the current limitations of the high-performance fiber lasers at 2.0 µm, which is particularly essential for coherent detection.