INTERMEDIATE-MODE-ASSISTED OPTICAL DIRECTIONAL-COUPLERS VIA EMBEDDED PERIODIC STRUCTURE

The exchange of power between two identical coupled waveguides with em bedded periodic structure along the z direction is studied in the fram ework of Floquet-Bloch theory. A one-period propagator is calculated a nd a spectrum of its eigenvalues (eigenphases) is studied. We show tha t due to special symmetry properties of the Floquet-Bloch operator, a strong enhancement or a total suppression of power exchange can be obt ained. The power exchange control is realized by the interaction of lo west order waveguide modes with high-order modes. A numerical example is presented in which the beat length is shortened by more than five o rders of magnitude.