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Mid-infrared supercontinuum generation in chalcogenide multi-step index fibers with normal chromatic dispersion

We experimentally demonstrate mid-infrared supercontinuum (SC) generation in chalcogenide multi-step index fibers (MSIF) pumped by a femtosecond laser. The fabricated chalcogenide MSIF is composed of a high refractive index core (C1) in the center, which is enclosed by a lower refractive index core layer (C2) and an outer cladding. This fiber structure is advantageous to tailor the chromatic dispersion with higher freedom and to keep the effective mode area small at long wavelengths. The high refractive index core, low refractive index core, and the outer cladding materials are As2Se3, AsSe2 and As2S5, respectively. When the diameter of C1 and C2 are 7.8 and 30 μm, respectively, the zerodispersion wavelength (ZDW) of the fiber is 12.5 μm. The chromatic dispersion profile is near-zero and flattened within the range of ±20 ps/km/nm in the wavelength range from 4 to 17 μm and a broad normal dispersion region is obtained in the wavelength range shorter than the ZDW. In practice, a 2.8 cm long fiber is pumped at 10 μm by using a femtosecond laser whose pulse width is ~200 fs. The SC generation extending from 2 to 14 μm is obtained. Most of its spectrum is in the normal dispersion region of the fiber. These results are promising for the highly coherent mid-infrared SC generation.