  We have numerically studied the effect of free-carrier-induced loss and
dispersion on the modulational instability (MI) gain at low input powers in
silicon-on-insulator (SOI) nanowaveguides with normal and anomalous
second-order dispersion. We have shown that the free carriers affect the gain
spectra even at low input powers. First time we have reported the gain in
normal SOI nanowaveguides even in the absence of higher order dispersion
parameters, which is due to the interaction of free-carrier-induced dispersion
and nonlinearity. The MI gain in an anomalous SOI nanowaveguide vanishes even
at a few milliwatt range of input power due to this interaction. We have shown
that the gain could be achieved in an anomalous nanowaveguides by reducing the
free carrier lifetime.
