Coupling of Nearly Degenerate Modes in Parallel Asymmetric Dielectric Waveguides

Coupling of degenerate modes of parallel optical waveguides has been discussed by Kapany1 and, to a greater extent, by Marcuse.2 Such coupling is of particular interest in the field of fiber optics, since it may cause undesirable crosstalk between adjacent optical fibers used for light transmission. Marcuse2 has applied the theory of degenerate mode coupling to the problem of crosstalk between cladded optical fibers embedded in a lossy medium and between cladded dielectric slab waveguides. The fabrication of devices which would actually take advantage of mode coupling, such as for light switching, modula717 tion, or power transferral,3 is fraught with practical difficulties, since the specification of physical parameters must necessarily be stringent. These difficulties require us to view the theory of optical waveguide coupling from a new vantage point. Let us first sketch briefly what is known. If two optical waveguides each have a mode with the same propagation constant then when the two waveguides are placed parallel to each other, the double waveguide system supports two new modes whose propagation constants are / + = /3 + 8 and /3_ = /3 -- 8. These two modes are approximately 3 symmetric and anti-symmetric combinations of the original modes in the isolated waveguides. The shift in propagation constant, 5, is related to the coupling coefficients involved in a description of the modes by means of general coupled line equations. It can also be expressed via a perturbation treatment of Maxwell's equations.