425-W kilohertz-linewidth single-frequency Tm-fiber MOPA enabled by distributed cladding-pumping.

In this letter, we present a high-power, narrow-linewidth single-frequency Tm-doped all-fiber master oscillator power amplifier (MOPA) based on a distributed cladding-pumping scheme (DCPS). A theoretical investigation was conducted on the dynamic behavior of stimulated Brillouin scattering (SBS) along the cascaded hybrid active fiber in the DCPS-enabled Tm-fiber amplifier. A ring-cavity configuration was employed to construct a single-frequency seed source with a segment of unpumped holmium-doped fiber as an ultranarrow bandwidth filter, producing a low-noise single-longitudinal-mode (SLM) lasing operation at 1998.13 nm with a spectral linewidth of ∼8.5 kHz and an optical signal-to-noise ratio (OSNR) of > 65 dB. Through distributed counter-pumping of the Tm-fiber amplifier, the single-frequency seed was successfully power-scaled to 425 W at a 9 kHz spectral bandwidth with a slope efficiency of 53%. A high OSNR was maintained at >59 dB at the full output power level with a diffraction-limited beam quality factor M of less than 1.20. The measurement of the relative intensity noise (RIN) demonstrated excellent low-noise properties over the entire laser power range. To the best of our knowledge, this work represents the highest power level ever achieved from a monolithic single-frequency Tm-fiber laser in the 2 µm spectral region.