Coupling silica waveguides to photonic crystal waveguides through a multilayered Luneburg lens

We present a detailed analysis of a coupler based on the Luneburg lens to couple a silica waveguide to a photonic crystal (PhC) waveguide. The dependence of coupling efficiency on the lens’s truncation, cut position of the PhC structure, coupler tip width, and misalignment are investigated with a two-dimensional finite element method. We implement the lens with a concentric ring-based multilayer structure. We also present a method to replace layers with very narrow widths by layers of predetermined minimum widths in the structure of the lens. The coupling loss of the designed 2.7-µm-long coupler, connecting a 2.79-µm-wide silica waveguide to a PhC structure with a rod-type square lattice, is lower than 0.49 dB in the C-band. The average coupling loss in the entire S, C, L, and U bands of optical communications is 0.70 dB.