Theory of directional couplers including transition region phase shifts

An analysis is made of the operation of the parallel waveguide directional coupler by incorporating eigenmode phase shifts in the input and output transition regions. The set of modes used in the analysis consists of the orthogonal symmetric and antisymmetric modes of the unperturbed parallel waveguide. It is demonstrated how the switching diagram changes as the transitional phase shifts increase. There is no indication that these built-in phase shifts degrade coupler crosstalk performance for the states normally used. Conditions for symmetric switching are derived.<
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