Enhancement of power transfer in periodic array of optical waveguides via intermediate Bloch states

A directional coupling mechanism between different waveguides in a periodic array of waveguides is suggested. The optical power transfer between two different waveguides is mediated by the coupling between zero-order modes of two of the waveguides and the second band of the periodic structure. Analytical solutions for the no-detuning (narrow band) and far-from-resonance cases are presented. The far-from-resonance case is shown to resemble a simple two-mode system with complete optical power transfer between the two waveguides, coupled by localized gratings. The transfer is mediated by the second band of the periodic structure. The transition length depends strongly on the shape of the perturbation, and depends exponentially on the distance between the waveguides, yet it allows us to transfer power from one waveguide to another at such distances, for which the transition via conventional directional tunneling mechanism is impossible. Our analytical results are supported by numerical calculations carried out for a model problem with realistic parameters.