High-efficiency dual-level heterogenous grating coupler on CMOS-compatible silicon-lithium niobate platform

We present the design and experimental demonstration of multilayer etched CMOS-compatible grating couplers with high efficiency on a heterogeneous silicon-lithium niobate platform. The dual-level grating coupler comprises 90 nm-thick Si waveguides and 220 nm-thick Si grating with a linear chirped structure without etching LN. The design changes the grating diffraction properties, which not only reduce back reflection but also improve directionality and fiber-to-chip mode match. In comparison with existing studies, this work achieves high coupling efficiency solely through CMOS-compatible etching without additional bottom reflectors or high-index overlays. Theoretical calculations predict a fiber-to-chip coupling efficiency of -1.76 dB and an off-chip diffraction efficiency of -1.1 dB for the TE mode. The experimental measurement of the peak coupling efficiency is -2.84 dB with the back reflection as low as -26 dB. The grating coupler paves the way for monolithic integration of Si and LN.