Supermodes of Grating-Coupled Multimode Waveguides and Application to Mode Conversion Between Copropagating Modes Mediated by Backward

Abstract—An analysis of multimode waveguides where several modes are coupled via quasiperiodic perturbations is presented. The supermodes (or eigenmodes) of the structure are derived and orthonormality considerations are discussed. In addition, a new type of mode converter between copropagating modes is proposed, where mode conversion is mediated by a backward-propagating mode. Adiabatic and nonadiabatic coupling coeffi-cients are considered and the supermode formalism is used to conveniently describe the mode of operation of the device. Index Terms — Coupled mode analysis, gratings, multimode waveguides, optical fiber communications, optical fiber devices, optical fiber theory. I