Intermediate-mode-assisted optical directional couplers via embedded periodic structure

The exchange of power between two identical coupled waveguides with embedded periodic structure along the direction is studied in the framework of Floquet-Bloch theory. A one-period propagator is calculated and a spectrum of its eigenvalues (eigenphases) is studied. We show that due to special symmetry properties of the Floquet-Bloch operator, a strong enhancement or a total suppression of power exchange can he obtained. The power exchange control is realized by the interaction of lowest order waveguide modes with high-order modes. A numerical example is presented in which the beat length is shortened by more than five orders of magnitude.