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.