We present a rigorous approach for designing a highly efficient coupling between single mode optical fibers and silicon nanophotonic waveguides based on diffractive gratings. The structures are fabricated on standard SOI wafers in a cost-effective CMOS process flow. The measured coupling efficiency reaches -1.08 dB and a record value of -0.62 dB in the 1550 nm telecommunication window using a uniform and a nonuniform grating, respectively, with a 1 dB-bandwidth larger than 40 nm.

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A highly efficient grating structure for the coupling between standard optical fibers and single-mode waveguides in the silicon-on-insulator platform realized in a CMOS fabrication process is presented. The cost-effective method introduces a backside metal mirror to the grating coupler without need of an extensive wafer-to-wafer bonding. A coupling efficiency of -1.6 dB (around 69%) near the telecommunication wavelength 1550 nm and a large 1 dB-bandwidth of 48 nm are achieved.

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