Coupled waves on optical fibers by power conservation

Using only power conservation, the authors derive the coupling coefficients of coupled-mode theory and a new power conservation law. The derivation provides physical insight into the mechanism of wave coupling on optical fibers. It shows explicitly that the coupled-mode approximation is not generally applicable to Maxwell´s equations, since the polarizability of perturbations is omitted. Examples include nonlinear fiber couplers and fiber nonuniformities, with emphasis on their polarization characteristics. An important class of unsolved problems is revealed