Coupling coefficient in antiguided coupling: magnitude and sign control

The coupling coefficient between two passive antiguided waveguides is investigated. A novel approach for identifying the in-phase and out-of-phase coupled modes in an antiguided system through the dispersion curves is described. The coupling coefficient in these modes is found to be real, and its sign and magnitude are influenced by the thickness and refractive index of the inter-element region between the two waveguide cores. The change of sign of the coupling coefficient is verified by the power-transfer direction reversal in numerical simulations of a two-section antiguided waveguide. In addition, design points have been identified where the coupling coefficient is relatively invariant against the perturbation of both inter-element separation and inter-element refractive index, benefiting the fabrication tolerance.