Coupled mode theory is an approximate, but insightful and often accurate mathematical description of power transfer among two or more waveguides in proximity. Its applications relate to the design and understanding of'modulators, switches, dividers, semiconductor lasers, phased arrays, and fiber optic devices. In this paper we review the theory as it has been employed over the past decade and point out contradictions between the formulations. These result from the neglect of interaction terms, which we show should be retained, especially for the cases of coupling among non-identical waveguides and waveguides with either gain or loss. A newer more accurate and physically meaningful formulation is described and we demonstrate that even in the case of very weak coupling significant differences result.
"A Critical Overview Of Coupled Mode Theory", Proc. SPIE 0835, Integrated Optical Circuit Engineering V, (10 March 1988); doi: 10.1117/12.942352; https://doi.org/10.1117/12.942352