High-efficiency dual-level grating coupler with heterogeneous integration on lithium niobate

In this article, a high-efficiency grating coupler based on a lithium niobate platform is presented. To enhance the coupling efficiency, a silicon nitride strip waveguide grating is introduced beneath the lithium niobate grating. Through the optimization of grating period, duty cycle, and other parameters using a particle swarm optimization (PSO) algorithm, a TE-polarized grating coupler with high coupling efficiency is achieved, resulting in a simulated peak coupling efficiency of approximately -2.19 dB. To further improve the performance of the grating coupler, a chirped design of the grating etch slots is implemented. A double-layer design with inversely linear variation in duty cycle is adopted, and the variation coefficients are optimized using the PSO algorithm, achieving a simulated peak coupling efficiency of approximately -1.72 dB with a 1 dB bandwidth of approximately 42 nm. Process tolerance simulations are also performed on the key parameters of the double-layer grating coupler, demonstrating good process tolerance of this design.