Efficient grating couplers for optical fibers to ultra-low-loss waveguides

Grating couplers are one of the building blocks in silicon (Si) photonics. A lithium niobate-on-insulator (LNOI) platform has a wide transparency window, an exceptional electro-optic performance, and a favorable mechanical property. Here we propose a grating coupler on the LNOI platform for an optical fiber to an ultra-low-loss silicon nitride (Si3N4) waveguide. Titanium dioxide (TiO2) has a negative thermo-optic coefficient and can be used to compensate the temperature drift of cores with positive thermo-optic coefficients. Here, a TiO2 layer on the high-aspect-ratio Si3N4 core is used for the athermal operation. The fundamental TE mode at a wavelength of 1550 nm is used as an input from the ultra-low-loss waveguide. We achieve a high directionality of 68% at the fixed wavelength. A coupling efficiency of 48% can be obtained. A 3-dB bandwidth of 60 nm from 1520 to 1580 nm is presented by the proposed grating coupler.