A novel optical coupler design with graded-index photonic crystals

Abstract

We explore an efficient method to couple light from a dielectric medium into a two-dimensional photonic-crystal waveguide (PCW). This is achieved by inserting a graded-index photonic crystal (PC) between a dielectric waveguide and a conventional PCW obtained by removing one row of air holes along the Gamma K direction in a triangular-lattice PC. Effective mode transformation is achieved with a spot-size conversion ratio of 12: 1. The low transmission efficiency (30%) that occurs in a direct coupling from dielectric waveguide into the PCW is raised to similar to 80% (0.98 dB) over a broad frequency range. Further improvements for nearly perfect transmission appear to be feasible.