Numerical investigation on improvement of energy transfer to longer wavelengths in mid-infrared supercontinuum generation by gradient fiber diameter

We numerically investigate mid-infrared (MIR) supercontinuum (SC) generation in gradient diameter indium fluoride fibers with improved long-wavelength energy transfer and spectral flatness. In a 0.5 m negative-gradient diameter fiber with the core diameter reduced from 9 to 5 µm, the longitudinal increase in nonlinear coefficient and the movement of zero-dispersion wavelengths (ZDWs) sustain high-order soliton propagation in a low-dispersion anomalous-dispersion region, delay soliton fission, and enable phase-matched dispersive wave (DW) emission toward longer wavelengths. Eventually, the simulation demonstrates a broadband SC with a 40 dB bandwidth of 5095 nm covering 1233–6328 nm and a long-wavelength energy transfer efficiency of 85.98%, corresponding to a 23.56% relative improvement over the 9 µm uniform fiber. For spectral flattening, a 7.2–6.6 µm negative-gradient diameter fiber designed around the multi-ZDW region is further simulated, and distributed DW emission between the second and third ZDWs produces a 3 dB bandwidth of 995 nm from 3096 to 4091 nm. These results show that negative-gradient diameter engineering can jointly improve long-wavelength energy transfer and spectral flatness in MIR SC generation with reduced pump requirements.