Modeling of a Cascaded Mid-Infrared Fiber Laser at 2.78 μm and 3.44 μm Dual-Wavelength Pumped at 0.98 μm and 1.15 μm

A novel 0.98 μm and 1.15 μm dual-wavelength pumping scheme for cascaded mid-infrared Er-doped ZBLAN fiber laser at 3.44 μm and 2.78 μm is proposed and analyzed by a detailed theoretical model. In our pumping scheme, the ⁴ I _13/2 level (the lower level of 2.78 μm laser), which is taken as a virtual ground state, is first populated by 976 nm pump and 2.78 μm transition. The 1150 nm virtual ground state absorption (VGSA), corresponding to ⁴ I _13/2 → ⁴ F _9/2 transition, excites the Er ions accumulated in ⁴ I _13/2 level to ⁴ F _9/2 level (the upper level of 3.44 μm laser), emptying the ⁴ I _13/2 level as well as providing positive gain for 3.44 μm laser. Followed by the 3.44 μm laser transition, the Er ions would rapidly decay to ⁴ I _11/2 level (the upper level of 2.78 μm laser) via nonradiative transition, realizing the re-population of ⁴ I _11/2 level and enhancing the 2.78 μm laser gain. Hence, a cycling of Er ions between ⁴ I _13/2 level and ⁴ F _9/2 level, which includes both 2.78 μm and 3.44 μm laser transitions, is realized via the ⁴ I _13/2 → ⁴ F _9/2 VGSA. An interesting point in our proposed pumping scheme is that both the 2.78 μm and 3.44 μm laser power are dominantly contributed by the VGSA pump, while the main role of 976 nm pump is to provide initial population to sustain the ions cycling. By optimizing the wavelength of VGSA pump, dual-wavelength pump power ratio and laser cavity parameters, the overall optical efficiency of the two laser output power with respect to pump power could be improved to over 55%. This work provides a new insight on the cascaded mid-infrared fiber laser and also a promising pumping scheme for efficient 2.78 μm and 3.44 μm laser generation simultaneously from single Er-doped ZBLAN fiber.