Theoretical Investigation of a Multistage Cascaded Fiber Raman Soliton Frequency Shift System in Mid-Infrared Region

We demonstrate numerically a novel repeater-based three-stage cascaded soliton self-frequency shift (SSFS) structure including a germania-core fiber, an Er ³⁺ ZBLAN fiber amplifier, and an indium fluoride (InF ₃ ) or TeO ₂ -Bi ₂ O ₃ -ZnO-Na ₂ O (TBZN) fiber. Wide wavelength tunability of 2-4.4 μm and 2-5.0 μm was achieved with a 5 m-long InF ₃ and a 0.2 m-long TBZN fiber, respectively. Numerical results show that with the same input pulses, stronger SSFS effect with longer wavelength edge occurred in TBZN fiber with shorter fiber length, while Raman solitons with higher energy, conversion efficiency and shorter pulse duration were generated in InF ₃ fiber. Compared with the commonly used single frequency shift structure, both the energy and conversion efficiency of Raman solitons in the designed repeater-based three-stage cascaded structure were significantly higher when the same tunable range was achieved. Our work could provide an efficient way to simultaneously improve the tunability, output energy, and conversion efficiency of the existing all-fiber laser sources with a lower operation energy.