Fig. 1. Structural diagram of grating coupler ^[15]. (a) In-coupling; (b) out-coupling

Fig. 2. Typical grating couplers^[18]. (a) Whole structural diagram; (b) cross sectional diagram

Fig. 3. In-coupling based on asymmetrical grating

Fig. 4. Simulation of slanted grating and conventional rectangular grating ^[20]

Fig. 5. Realization of tilted etching using Faraday cage to change plasma direction in RIE^[21].（a）Structural diagram; (b) SEM image of obtained slanted grating

Fig. 6. Cross-section of SiO₂ slanted grating ^[22]

Fig. 7. SEM image of slanted grating etched with tilted RIE ^[15]

Fig. 8. Design and fabrication of slanted grating coupler. (a) Incident slanted grating coupler ^[23]; (b) SEM image of slanted grating coupler in SOI waveguide layer based on FIB ^[26]

Fig. 9. Angle relationship of blazed grating^[27]

Fig. 10. SEM image of blazed grating ^[34]

Fig. 11. PMMA template grating ^[36] .FIB cross-sectional SEM images (a) before and (b) after imprinting; (c) imprinted grating in OrmoCore

Fig. 12. Design principle of binary blazed grating^[35]. (a) Blazed grating; (b) equivalent gradient refractive index grating; (c) binary blazed grating

Fig. 13. Shadowing effect of blazed grating^[39]

Fig. 14. Structural diagram of binary blazed grating. (a) 2D structure^[43]; (b) 3D structure^[43]; (c) coupling schematic of binary blazed gratings^[47]; (d) SEM image of binary blazed grating coupler^[48]

Fig. 15. Several less popular asymmetrical gratings. (a) Schematic of tilted rectangular grating coupler^[50] ; (b) double-depth double-groove grating coupler^[51]; (c) angle evaporation schematic of high-refractive-index material^[52]; (d) structure obtained based on Fig. (c) ^[52]