Modeling and optimization of cascaded lasing in a holmium doped fluoride fiber laser with efficient output at 3.92 µm

In this paper, we investigate the output performance of a continuous-wave Ho³⁺-doped fluoride fiber laser operating at 3.92 µm from the ⁵I₅ → ⁵I₆ transition of Ho³⁺ using numerical simulation. A double-clad Ho³⁺-doped fluoroindate fiber with a doping concentration of 10.0 mol.% is assumed, with direct pumping at 888 nm. We propose simultaneous lasing on the ⁵I₆ → ⁵I₇ transition to enhance the slope efficiency while reducing the threshold and heat accumulation. Simulation results indicate that a slope efficiency of 17% and a threshold of 2.5 W can be obtained using a 9 cm-long fiber. Moreover, with the heat accumulation reduced by >40%, watt level laser output can be achieved in this cascade system at room temperature without the gain fiber being damaged by heat accumulation. The theoretical maximum output power of 1.27 W is 6 times higher than the highest reported value (197 mW), which is limited by the fiber damage due to excess heat load.