Ultrahigh-efficiency apodized grating coupler using fully etched photonic crystals

We present an efficient method to design apodized grating couplers with Gaussian output profiles for efficient coupling between standard single mode fibers and silicon chips. An apodized grating coupler using fully etched photonic crystal holes on the silicon-on-insulator platform is designed, and fabricated in a single step of lithography and etching. An ultralow coupling loss of $-1.74\mathrm{dB}$ (67% coupling efficiency) with a 3 dB bandwidth of 60 nm is experimentally measured.