Design of subwavelength-engineered surface grating couplers on a hybrid -Si/SiN photonic platform

The design of a high-efficiency grating coupler for silicon nitride (SiN) photonic integrated circuits is presented. Our devices use a high-index amorphous silicon (-Si) overlay and subwavelength metamaterial apodization to achieve superior coupling efficiency compared to SiN-only gratings. The high index contrast of the -Si layer results in increased grating strength, making it possible to radiate more power off-chip within the mode field diameter (MFD) of a standard single mode optical fiber. Simultaneously, through the use of subwavelength grating (SWG) apodization, we shape the radiated field to optimize the overlap with the Gaussian-like profile of the fiber mode at an operating wavelength of 1.31 m. The overlap is further improved by considering a focalizing scheme, wherein the grating is designed to couple to a fiber placed a certain distance away from the chip. In this configuration, the constraint imposed on the length of the grating by the MFD of the fiber mode is relaxed, allowing for a longer structure that will diffract more power off-chip. By combining the use of an -Si overlay with SWG apodization and beam focalization, we achieve a coupling efficiency of - 1.3 dB.