  We propose and optimize theoretically a supercontinuum (SC) laser source in
the mid-IR based on using As-Se-Te/As-S core/clad step-index fibers and a
femtosecond all-fiber laser system at 2 {\mu}m. Numerically simulated spectra
extending from ~1 {\mu}m to more than 8 {\mu}m are demonstrated for pump energy
of order 100 pJ in a fiber with a core diameter of 2 {\mu}m. To the best of our
knowledge, the possibility of such long-wavelength spectral conversion of pump
pulses at the wavelength of 2 {\mu}m in optical fibers is demonstrated for the
first time. The theoretical calculations are performed on the base of real low
loss step-index As-Se-Te/As-S glass fibers with various core-clad diameter
ratios.
