Monolithic low-loss short-length 2.113-µm Tm-doped fiber laser oscillator made by phase-mask-assisted femtosecond-laser inscription.

We report a monolithic low-loss short-length gain-switched thulium-doped fiber laser (TDFL) beyond 2.1 µm. Phase-mask-assisted 800-nm femtosecond laser exposure method was used to fabricate the monolithic laser cavity on ∼10-cm long straightened silica Tm-doped fiber (TDF) after single-shot spatial alignment. The low loss (<0.2 dB per FBG) of inscribed fiber Bragg gratings (FBGs) is comparable to the loss of FBGs made by phase-mask-assisted UV-laser exposure method, while multiple auxiliary processes of hydrogen loading and the annealing are involved in the latter fabrication. 2.113-µm lasing with 10 ns width was obtained from the gain-switched TDFL, under inband 1.55-µm core-pumping scheme. The maximum output peak power, pulse energy, and polarization extinction ratio of the TDFL were 360 W, 3.6 µJ, and >22 dB, respectively. The 2.113-µm TDFL shows slope efficiency of 29%, comparable with that in 2.1-µm gain-switched holmium doped fiber laser (HDFL), indicating that such a short-length TDFL is competitive to HDFL for lasing beyond 2.1 µm. In the time domain, it has been observed that intracavity pulsed amplified spontaneous emission in 1.8-1.9 µm is contributed to the efficient generation of the ultralong-wavelength TDFL beyond 2.1 µm, similar to the reabsorption mechanism of the previously reported long-length 2-2.2 µm TDFL. But our fabricated short-length TDFL with high peak power is obviously advantageous over its long-length counterpart for suppressing nonlinear effects occurring in the laser cavity. An additional demonstration has been carried out to generate 2.325-µm stimulated Raman scattering with 10% slope efficiency, 3.6 W peak power, and 50 nJ pulse energy in 40m-long highly nonlinear silica fiber, using the short-length gain-switched TDFL as the pump. At last, our simulation shows the feasibility to extend efficient lasing in cm-long ultrashort TDFL oscillator up to 2.2 µm, when FBGs cavity mirrors with high reflectivity and low loss can be made. It therefore indicates such a short-length TDFL a promising compact high-peak-power mid-IR laser source for applications beyond 2.1 µm.