High-efficiency Si3N4 grating couplers with hybrid Al2O3/metal-mirror structure for back-reflection suppression

We present a compact high-performance one-dimensional grating coupler based on silicon nitride, which is optimized via superior mode matching method with chirping and engineering on the period and anti-reflection of the grating. A tailored multilayer dielectric stack is optimized above the grating coupler to suppress the back-reflection and ameliorate the refractive index matching between the grating coupler and optical fiber. Meanwhile, a metal reflector in the substrate is used to recycle the leakage light to promote the coupling efficiency of the grating coupler. The three-dimensional finite-difference time-domain method is used to verify the performance of the grating coupler. The simulation results show that the peak coupling efficiency of the proposed grating coupler can reach 95% with a 1-dB bandwidth of 57 nm, which can cover the whole C-band. The effective area of the device is ∼80×15 μm2.