We develop an algorithm using two coupled parabolic
equations for numerical simulation of wave propagation
over long distances. The coupled parabolic equations
are derived from a two mode wave decomposition. An
iterative procedure is used in our numerical algorithm.
The coupling between the two parabolic equations allow
us to deal with fine scale inhomogeneities in the medium and capture
the back scattering that are usually neglected in a one
way parabolic approximation. We study stability issues
of our numerical algorithm and present numerical examples.