Theoretical study on cascade energy-transfer pumping heavily holmium-doped fluoroindate fiber laser at-4 μm with high-efficiency

In this article, we propose a cascade pumping scheme that employs 1.945 mu m and 1.66 mu m fiber laser as the pump sources to realize efficient laser emission at 3.92 mu m in the commercially available heavily holmium-doped fluoroindate fibers. Compare to the conventional 888 nm laser diode, longer wavelength pump sources provide higher quantum efficiency, which is critical for the acquisition of high output power at a relatively low input pump power considering the low damage threshold of InF3 glass. Output performance is optimized by conducting detailed investigation on fiber length, output coupler mirror reflectivity and launched pump power. Simulation results show that when employing cladding pump at both 1.945 mu m and 1.66 mu m, a slope efficiency of 25 % can be achieved with a threshold of 1.6 W for the 1.66 mu m pump source. This is attained using a 15 cm-long fiber under a pump power of 5 W at 1945 nm. Furthermore, the maximum optical-to-optical efficiency reaches 11.17 % when considering the total pump power. To our knowledge, the efficiency achieved in this study appears to surpass that of previous works. This research provides a novel approach and valuable guidance for efficient laser output at the important - 3.9 mu m wavelength region, for applications in various fields such as free-space communications, remote sensing and medical diagnostics.