Non-evanescent adiabatic directional coupler

A directional coupling mechanism based on an adiabatic coupling between thr ee optical modes is suggested. The optical power transfer between two waveg uides which are far apart is mediated by adiabatic coupling between zero-or der optical modes of the individual waveguides and a high-order intermediat e mode. The analytical model for an adiabatic three-mode coupling based on a scalar wave equation is presented. The directional coupling via the adiab atic mode coupling between copropagating modes is described and compared wi th a nonadiabatic directional coupling assisted by periodic perturbation. I t is shown that adiabatic directional coupling has much less sensitivity to the mode parameters and to the wavelength.