Fig. 1. Schematic diagram of overall structure of a two-dimensional grating coupler. (a) Overall structure diagram; (b) side view structure diagram; (c) schematic diagram of top triangular silicon layer structure

Fig. 2. Relationship between longitudinal period of silicon grating and coupling efficiency and bandwidth of device. (a) Coupling efficiency and 1 dB bandwidth of silicon grating with different longitudinal periods at wavelength of 1550 nm; (b) coupling efficiency as a function of wavelength at different longitudinal periods

Fig. 3. Effect of LNTF thickness on device coupling efficiency. (a) Coupling efficiency corresponding to LN thin films of different thicknesses; (b) coupling efficiency as a function of wavelength at different LNTF thicknesses

Fig. 4. Coupling efficiency corresponding to different fiber tilt angles in same band and light field diagram of coupler coupling upward when angle of inclination is 8°. (a) Coupling efficiency corresponding to different fiber tilt angles at wavelength 1550 nm; (b) electric field distribution E_y of the grating couplerwhen angle of inclination is 8°

Fig. 5. Coupling efficiency and 1 dB bandwidth corresponding to optimal solution of various parameters

Table 1. Performance comparison of two dimensional grating couplers on lithium niobate waveguide platform