Fig. 1. Schematic diagram of bidirectional coupling vertical grating coupler structure

Fig. 2. Relationship between PSO iteration and grating coupling efficiency

Fig. 3. Effect of initial parameters on coupling efficiency. (a) Waterfall plot of coupling efficiency with different duty cycles; (b) waterfall plot of coupling efficiency with different grating periods

Fig. 4. Loss analysis results with or without Al reflector. (a) Normalized optical power without reflector; (b) normalized optical power with Al reflector

Fig. 5. Effect of different thicknesses of Si₃N₄ on the upper reflectivity

Fig. 6. Comparison of grating coupling efficiency and upper reflectivity before and after adding Si₃N₄ coating

Fig. 7. Influence of changes in parameters H₁ and D₁ on coupling efficiency

Fig. 8. Optimization of double-layer Si₃N₄ parameters. (a) Gap H₂ between upper and lower Si₃N₄; (b) thickness D₂ of upper Si₃N₄

Fig. 9. Bandwidth performance simulation results of vertical grating coupler under optimal parameters

Fig. 10. Cross section of light field distribution before and after structural optimization. (a) Before optimization; (b) after optimization

Fig. 11. Alignment of incident light center with uniform grating. (a) Case 1; (b) case 2

Fig. 12. Analysis of device processing tolerance. (a) Alignment tolerance; (b) angle tolerance

Fig. 13. Device processing process

Fig. 14. Schematic diagram of unidirectional coupled vertical grating coupler structure

Fig. 15. Cross section of light field distribution for unidirectional coupled vertical grating coupler