3.5 µm watt-level all-fiber Er³⁺-doped laser based on high-damage-resistant multi-color dielectric fiber mirrors

We report ∼3.5 µm watt-level continuous-wave (CW) and gain-switched all-fiber Er³⁺: ZBLAN lasers using multi-color dielectric fiber mirrors (DFMs). A section of 1 mol.% Er³⁺: ZBLAN double-clad fiber with the 976/1976 nm dual-wavelength pumping provides ∼3.5 µm optical gain, and two home-made mid-IR DFMs as the input/output cavity mirrors are used to construct ∼3.5 µm all-fiber resonant cavity by self-made FC/PC fiber connectors. The key is to optimize the film structures of the DFMs and the standing-wave field in the film stacks for 0.98/1.98/2.8/3.5 µm multi-color anti- or high-reflection operation and high damage density of >2.5 MW/cm² (CW) and >74 J/cm² (pulse) at 3.5 µm which is crucial for mid-IR high-power laser generation. We perform the three following experiments: (1) stable 3.5 µm CW laser operation generates a maximum output power of 1.11 W and the power fluctuation is less than 1.23%; (2) 3.5 µm gain-switching operation with the tunable repetition rate of 10-20 kHz achieves the maximum pulse energy of 32.7 µJ and pulse duration of 1.21 µs; (3) comparison study on the 1976 nm core-pumping or clad-pumping scheme (976 nm clad-pumping always) shows that the 976/1976 nm clad-pumping scheme readily generates 2.8/3.5 µm dual-wavelength lasing with a lower conversion efficiency. This work may offer an alternative solution for low-cost, high-performance, all-fiber mid-IR CW and pulsed laser beyond 3 µm.