Coupled-Mode Theory of Nonparallel Optical Waveguides

A coupled-mode theory for treating the coupling between nonparallel optical waveguides is developed using the coupled-mode assumption. Both coupling equations and coupling coefficients are obtained by using approximations that are equivalent to those in the coupled-mode theory of parallel optical waveguides. In particular, coupling characteristics of a case, which are obtained using this theory, are different from that obtained using another theory. Not only does the present theory avoid the awkwardness of having to specify, in a somewhat arbitrary manner, the separation between the two nonparallel waveguides, but also, it considers the effect of both optical amplitude and phase on the coupling equations and the coupling coefficients fully. Therefore, the present theory is more accurate in describing the coupling between the nonparallel optical waveguides.