  Using the matrix Riemann-Hilbert factorization approach for nonlinear
evolution systems which take the form of Lax-pair isospectral deformations and
whose corresponding Lax operators contain both discrete and continuous spectra,
the leading-order asymptotics as $t \to \pm \infty$ of the solution to the
Cauchy problem for the modified nonlinear Schr\"{o}dinger equation, $i
\partial_{t} u + {1/2} \partial_{x}^{2} u + | u |^{2} u + i s \partial_{x} (| u
|^{2} u) = 0$, $s \in \Bbb R_{>0}$, which is a model for nonlinear pulse
propagation in optical fibers in the subpicosecond time scale, are obtained:
also derived are analogous results for two gauge-equivalent nonlinear evolution
equations; in particular, the derivative nonlinear Schr\"{o}dinger equation, $i
\partial_{t} q + \partial_{x}^{2} q - i \partial_{x} (| q |^{2} q) = 0$. As an
application of these asymptotic results, explicit expressions for position and
phase shifts of solitons in the presence of the continuous spectrum are
calculated.
