Nonuniform photonic crystal taper for high-efficiency mode coupling.

A nonuniform-shape photonic crystal taper integrated with a single-mode optical fiber and a photonic crystal waveguide for high-efficiency mode coupling is presented. The curvature of the tapering section is varied by the parameter alpha. For values of alpha set to 2 and 0.5, concave and convex tapers are obtained, respectively. Numerical calculations yield an average coupling efficiency greater than 97% at a short taper length of 20.52 mum for the convex-shape taper. Subsequently, the value of the parameter alpha is varied for investigating the effects of curvature on coupling efficiency and compactness of different types of taper.