Coupled Mode Theory is a method to analyze the light propagation in perturbed or
weakly coupled waveguides. The basic idea of the Coupled Mode Theory method is
that the modes of the unperturbed or uncoupled structures are defined and solved
first. Then, a linear combination of these modes is used as a trial solution to Maxwell’s
equations for complicated perturbed or coupled structures. After that, the derived
coupled mode equations can be solved analytically or by numerical methods. The
theory assumes that the field of the coupled structures may be sufficiently
represented by a linear superposition of the modes of the unperturbed structures. In
many practical cases, this assumption is valid and does give an insightful and often
accurate mathematical description of electromagnetic wave propagation. The
Coupled Mode Theory is most useful in the analysis of the interaction between
several near resonance guided modes. It has been widely used and proven to be one
of the most productive and efficient methods for the analysis of waveguide devices.