Cryogenic all-fiberized single-frequency linearly-polarized 2-μm Tm-doped distributed feedback fiber laser with near-1-ns pulse generation.

We demonstrate a cryogenic single-frequency, linearly polarized, gain-switched, distributed feedback (DFB) 2-μm thulium doped silica fiber laser. The Tm-doped π-phase-shifted DFB cavity was inscribed in a 24-mm-long single-mode non-polarization-maintaining (non-PM) thulium-doped silica fiber, using a phase-mask-assisted femtosecond-laser inscription technique. The π-phase shift was positioned with an 8.3% offset from the cavity center. An in-band pumping scheme was employed, utilizing a 1550 nm nanosecond-pulsed erbium-doped silica fiber laser with a pulse width of 50 ns and a repetition rate of 50 kHz. Enhanced lasing performance was observed in the cryogenic thulium doped fiber laser (TDFL) at -196 C. The cryogenic Tm-doped silica DFB fiber laser achieved 1.96-μm lasing with a high slope efficiency of 17.2%, representing a more than two-fold increase compared to the slope efficiency at room temperature. Under maximum pump power, the average output power reached 72.1 mW with a pulse width of 1.7 ns, corresponding to a peak power of 850 W. No significant nonlinear effects were observed in the fiber laser at its maximum output power. To the best of our knowledge, this is the first time the slope efficiency of an all-fiberized Tm-doped silica DFB fiber laser has exceeded 15%.