Three-wave interaction grating coupler with the sub-decibel insertion loss at normal incidence

High-efficiency optical coupling between fibers or free-space beams and silicon photonic circuits is critical for reducing system-level losses and enabling scalable packaging. Conventional grating couplers typically require illumination off-normal incidence, limiting their compatibility with vertically oriented sources and fibers. We report the design, fabrication in a commercial foundry, and experimental results of high-efficiency, normal incidence grating couplers for silicon photonics. We observe a maximum coupling efficiency of 85.4% (−0.69dB) with a 1 dB bandwidth of 20 nm at a central wavelength of 1546 nm. These experimental results verify earlier theoretical and simulation results and pave the way for the use of perfectly vertical grating couplers, as an alternative to edge coupling, in silicon photonics applications that are sensitive to input coupling loss. Further, these results enable the use of grating couplers for vertically oriented elements, such as multicore fibers and VCSELs, and address challenges associated with coupling to free space beams.