High-efficiency wideband SiN_x-on-SOI grating coupler with low fabrication complexity

The chip-fiber grating coupler is a fundamental building block in integrated photonics, providing convenient on-wafer testing and packaging. Couplers based on a silicon nitride (SiN_x) material platform can achieve wider bandwidths than silicon-based couplers, but suffer from lower efficiency due to the relative low material refractive index. The efficiency of the SiN_x grating coupler can be improved by using high-reflectivity silicon grating reflectors underneath. However, such a silicon grating reflector requires several fabrication steps, including lithography, etching, high precision alignment (HPA), and chemical mechanical polishing (CMP). In this Letter, we demonstrate an easy-to-fabricate SiN_x-on-SOI transverse-electric mode grating coupler requiring only one patterning step (grating alone), and without the need for HPA and CMP. A coupling coefficient of -2.5  dB and 1-dB-bandwidth of 65 nm has been experimentally measured.