A unidirectional eigenmode expansion method is available via the propagate script command. This command calculates the resulting output mode profile of an arbitrary input mode after it has propagated through a waveguide for some distance. This is done by decomposing the input mode into modes supported by the waveguide (ie. the currently calculated modes) using overlap integrals. Each supported mode is then propagated through the waveguide. The resulting modes are then added coherently to give the final mode profile.
See the propagate script function for details.