COUPLED-MODE THEORY

The principal features of coupled mode theory are reviewed, in particular as applied to passive structures, such as coupled resonators and coupled waveguides. The former are examples of coupling of modes in time, the latter are examples of coupling of modes in space. Active structures are considered briefly insofar as they obey conservation laws in terms of positive and negative energies. The complications caused by nonorthogonality of the mode energies, or powers, are brought up. These were the topic of recent publications. The coupling of modes formalism has intuitive appeal in that the coupling equations in their simplest form can be written down by inspection. When "fine points" as, for instance, cross talk are at issue, then more formal derivations are required. The one presented here is based on a variational principle.