Mid-infrared generation via Raman soliton self-frequency shift in fluoride fibers: a comparative study

We report mid-infrared generation up to 4 μm achieved through Raman soliton self-frequency shift (SSFS) in fluoride fibers pumped by a mode-locked Cr:ZnS laser emitting at 2.3 μm. Moreover, we present a comparative study on six different fluoride fibers (ZBLAN and InF₃) from all manufacturers available on the market. This enabled us to determine key fiber parameters responsible for spectral broadening and identify the unit allowing for optimal frequency conversion up to 4 μm, which, to the best of our knowledge, represents the broadest demonstrated spectral coverage of the SSFS achieved with Cr:ZnS/Se lasers. The relative intensity noise of the generated mid-IR radiation is also investigated with the 0.2% integrated noise (limited by the noise of available detectors), confirming the high stability of the source. Finally, through numerical modelling we determine the limitations for further extension of the mid-IR spectral coverage beyond 4 μm with this technology. Our results provide simple guidance for experiments on SSFS-based frequency conversion of Cr:ZnS/Se laser output for mid-IR generation.