All-Normal Dispersion Chalcogenide PCF for Ultraflat Mid-Infrared Supercontinuum Generation

We numerically study the dispersion-engineered chalcogenide photonic crystal fiber (PCF), which allows us to generate ultraflat broadband supercontinuum (SC) spectra in all-normal dispersion regime. A 1-cm-long chalcogenide hexagonal PCF made using Ge11.5As24Se64.5 glass pumped at 1.55 mu m produced an SC bandwidth 700 nm at a peak power of 1 kW. By shifting pump at 2 mu m, SC spectra can be extended with a bandwidth of 1900 nm at the same peak power level. In both cases, nonuniform spectral power distribution observes over the entire output bandwidth owing to the lower dispersion slop on the long wavelength side of the dispersion curve. To spanning SC further in the mid-infrared as well as to reduce the spectral asymmetry, we optimize another design for pumping at 3.1 mu m in such a way that the pump source can be employed vicinity to the peak of the dispersion curve. Employing the largest pump peak power up to 5 kW, SC can be extended up to 6 mu m (1.5 octaves), and the power distribution among the spectral components over the entire SC bandwidth can be improved significantly by this design. To enhance the spectral flatness, we optimize a second PCF geometry by reducing its pitch length, and it is possible to obtain ultraflat coherent SC spanning from 2 to 5.5 mu m (> 1 octave) by this structure maintaining nearly symmetric power distribution between both side of its spectral components.