  In this paper we numerically study supercontinuum generation by pumping a
silicon nitride waveguide, with two zero-dispersion wavelengths, with
femtosecond pulses. The waveguide dispersion is designed so that the pump pulse
is in the normal-dispersion regime. We show that because of self-phase
modulation, the initial pulse broadens into the anomalous-dispersion regime,
which is sandwiched between the two normal-dispersion regimes, and here a
soliton is formed. The interaction of the soliton and the broadened pulse in
the normal-dispersion regime causes additional spectral broadening through
formation of dispersive waves by non-degenerate four-wave mixing and
cross-phase modulation. This broadening occurs mainly towards the second
normal-dispersion regime. We show that pumping in either normal-dispersion
regime allows broadening towards the other normal-dispersion regime. This
ability to steer the continuum extension towards the direction of the other
normal-dispersion regime beyond the sandwiched anomalous-dispersion regime
underlies the directional supercontinuum notation. We numerically confirm the
approach in a standard silica microstructured fiber geometry with two
zero-dispersion wavelengths.
