Ultra efficient silicon nitride grating coupler with bottom grating reflector

We theoretically propose a silicon nitride (Si3N4) grating coupler (GC) with both ultrahigh efficiency and simplified fabrication processes. Instead of using a bottom distributed Bragg reflector (DBR) or metal reflector, a bottom Si grating reflector (GR) with comparable reflectivity is utilized to improve the coupling efficiency. The fully etched Si GR is designed based on an industrially standard silicon-on-insulator (SOI) wafer with 220 nm top Si layer. By properly adjusting the trench width and period length of the Si GR, a high reflectivity over 90% is obtained. The Si3N4 GC is optimized based on a common 400 nm Si3N4 layer sitting on the Si GR with a SiO2 separation layer. With an appropriate distance between the Si3N4 GC and bottom Si GR, a low coupling loss of -1.47 dB is theoretically obtained using uniform GC structure. A further record ultralow loss of -0.88 dB is predicted by apodizing the Si3N4 GC. The specific fabrication processes and tolerance are also investigated. Compared with DBR, the bottom Si GR can be easily fabricated by single step of patterning and etching, simplifying the fabrication processes. (C)2015 Optical Society of America