Numerical analysis and optimization of high-efficiency 2.8 μm erbium-doped fluoride fiber laser based on dual-wavelength pump

Er³⁺-doped ZBLAN fiber lasers have been widely investigated for generating high-power high-efficiency 2.8 μm mid-infrared lasers. High-power multimode 980 nm semiconductors are generally used as convenient pump sources in Er³⁺-doped ZBLAN fiber lasers. However, the longer lifetime of the lower laser level (⁴I_13/2, 9.9 ms) than that of the upper laser level (⁴I_11/2, 6.9 ms) results in severe self-terminating transition. Although highly Er-doped fibers with improved energy transfer upconversion rates can alleviate this problem to some extent, there are still significant limitations in heat load management. On the other hand, the 1.6–1.7 μm laser is used as another pumpscheme due to the partial spectral overlap between ground state absorption (GSA) and excited state absorption (ESA) for population inversion. This pump scheme demonstrates a slope efficiency of up to 50%. However, due to the weak GSA process, a 10 m-long active fiber is required. To address this issue, we propose a dual-wavelength (1.5 μm and 1.7 μm) pumping technique to achieve high-efficiency 2.8 μm laser output by using an Er³⁺-doped ZBLAN fiber with meter-level length. A simulation model is established for the dual-wavelength pumping scheme. This scheme combines the strong GSA process in the 1.5 μm band and the strong ESA process in the 1.7 μm band to accelerate the population accumulation on the lower laser level, promote the absorption of the 1.7 μm pump, and thereafter enable the conversion to 2.8 μm laser over much shorter gain fiber. By considering the intensities of ground state absorption and emission of the ⁴I_15/2→⁴I_13/2 transition, the pump at 1470 nm is selected to efficiently populate the Er³⁺ to the lower laser level. Then the second pump is optimized to a wavelength of 1680 nm to achieve rapid particle extraction from the lower laser level, thereby realizing population inversion for efficient 2.8 μm laser generation over a meter-long gain fiber. Using the optimized pump wavelengths, the simulation of a 2.8 μm fiber laser based on a 0.5 m-long 0.015 mol/mol erbium-doped fluoride fiber shows that when a 20 W 1680 nm laser is used as the main pump source, only a 1.2 W 1470 nm auxiliary pump is required to achieve a 12.2 W 2.8 μm laser output, with an optical efficiency as high as 58.2%. Furthermore, the fiber laser simulation indicates that when the powers of the two pumps satisfy the relationship of <i>P</i>_<i>λ</i>2 = 20<i>P</i>_<i>λ</i>1 – 4, the output power of the laser system can reach its maximum value. The dual-wavelength pumping technique proposed in this work enables high-efficiency 2.8 μm mid-infrared laser generation by using meter-long Er³⁺-doped fluoride fiber, which significantly improves the laser system integration and economic benefits.