Tunable Raman solitons beyond 4 μm in fluorotellurite fiber driven by a high-peak-power femtosecond source

The generation of mid-infrared femtosecond pulses based on the soliton self-frequency shift in fluorotellurite fibers has attracted widespread attention. However, direct generation of soliton pulses beyond 4 μm in fluorotellurite fibers remains challenging due to the limitations of the pump source and the intrinsic absorption of the Raman gain medium. Here, we report the generation of Raman solitons beyond 4 μm using fluorotellurite fiber. The mid-infrared pump source is a high-peak-power Raman soliton laser operating in the range of 2.15-2.38 μm. By employing fluorotellurite fiber as the nonlinear medium, we show tunable Raman solitons spanning 3.6-4.2 μm, marking the first extension of Raman solitons beyond 4 μm in fluorotellurite fiber. Additionally, tunable Raman solitons covering the range of 3.25-3.8 μm were achieved using a 9-cm-long fluorotellurite fiber, with a maximum pulse energy of 48.7 nJ and a peak power reaching 640.8 kW.