4.5 W Er-doped ZBLAN fiber laser at 3.78 μm

In this work, we demonstrate an efficient long-wavelength operation of an Er-doped ZBLAN fiber laser in the 3.5 μm band with ⁴F_9/2→⁴I_9/2 transition. A high-transmission bandpass filter (BPF) is employed in the laser cavity to select a specific lasing wavelength of around 3.78 μm. The fiber laser yields a maximum output power of 4.5 W at 3781.6 nm, with a slope efficiency (with respect to the absorbed 1990 nm pump power) of 17.1% and a spectral linewidth of 0.17 nm. This work represents the highest, to the best of our knowledge, output power ever achieved in fiber lasers beyond 3.7 μm. Wavelength tuning from 3770 nm to 3782 nm, without laser power degradation, is also achieved by rotating the filter.