High-efficiency grating couplers for vertical coupling in thin-film silicon nitride technology.

Silicon nitride has become increasingly prominent in the integrated photonics research community during the last two decades, with an ever-growing number of demonstrations reported for different applications. In particular, thin-film silicon nitride platforms are attracting increasing interest thanks to the possibility of achieving ultra-low propagation loss due to their weak mode-field confinement. However, the realization of efficient grating couplers in the thin-film silicon nitride platforms is extremely challenging due to the poor grating scattering strength. In this work, we propose and experimentally demonstrate a grating coupler for perfectly-vertical fiber-to-chip efficient coupling to thin-film silicon nitride waveguides. The device consists of a silicon-rich silicon nitride apodized grating on top of a 150 nm-thick silicon nitride waveguide. Unlike most previously demonstrated bi-layer grating couplers, which typically employ an amorphous silicon layer to enhance the grating efficiency, the use of a top silicon-rich silicon nitride grating offers improved fabrication tolerances and gives an additional degree of freedom in the design of efficient grating couplers. A record coupling efficiency of -1.78 dB at 1550 nm was experimentally measured without employing any back-reflector or index-matching fluid, demonstrating the device's potential for efficient and fabrication-tolerant fiber-to-chip coupling on thin-film silicon nitride platforms.