The time reversibility of optical propagation impedes the definite performance of many optical devices, such as couplers, polarization converters, etc. We suggest a novel concept in which we use media with loss and gain, thus breaking the time-reversal characteristics, to achieve a unidirectional optical mode interference and coupling, which is a desirable feature for light-wave circuits. Using a matched periodic modulation of both the index of refraction and loss (gain) of the medium, we implement a spatially single sideband perturbation, which breaks the symmetry to allow only a unidirectional energy transfer from mode to mode of the optical structure. We elaborate on this phenomenon in coupling between two modes of a multimode optical waveguide.
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