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Inverse scattering in multimode structures

We consider the inverse scattering problem associated with any number of interacting modes in one-dimensional structures. The coupling between the modes is contradirectional in addition to codirectional, and may be distributed continuously or in discrete points. The local coupling as a function of position is obtained from reflection data using a layer-stripping type method, and the separate identification of the contradirectional and codirectional coupling is obtained using matrix factorization. Ambiguities are discussed in detail, and different {\it a priori} information that can resolve the ambiguities is suggested. The method is exemplified by applications to multimode optical waveguides with quasi-periodical perturbations.