Supermodes of grating-coupled multimode waveguides and application to modeconversion between copropagating modes mediated by backward Bragg scattering

An analysis of multimode waveguides where several modes are coupled via qua siperiodic perturbations is presented, The supermodes (or eigenmodes) of th e structure are derived and orthonormality considerations are discussed. In addition, a new type of mode converter between copropagating modes is prop osed, where mode conversion is mediated by a backward-propagating mode. Adi abatic and nonadiabatic coupling coefficients are considered and the superm ode formalism is used to conveniently describe the mode of operation of the device.