High-Efficiency focusing grating coupler with distributed Bragg reflector

We propose a high-efficiency silicon nitride focusing grating coupler with a bottom reflector. The bottom reflector consists of a distributed Bragg reflector (DBR) with a stack of Si3N4 and SiO2 layers. Moreover, the taperless structure of focusing grating couplers is utilized to further increase the density of integration. The result shows that the reflectivity of the grating coupler with 8-layer DBR reaches 98% at 1550nm, the peak coupling efficiency (CE) is -2.1dB and the 1dB bandwidth is 66nm. Compared to the focusing grating without DBR, the CE is improved by 1.87 dB with slight change in 1dB bandwidth, and the footprint is only 27.1×37.1μm^2.