Two distant (stable) self-guided beams, that are initially parallel,
travel like rays in an axially uniform waveguide whose
effective index gradient 
is specified by linear
theory [50]. Classical geometric optics then
determines nonlinear beam interactions, e.g. when the beams
are in phase, 
decreases monotonically and the beams
will, in general, spiral about each other helically in a self focusing
material of rather arbitrary dependence on intensity
[6]. Similar concepts apply to dark solitons [51].