Fig. 1. Picture of nonuniform grating obtained by scanning electron microscopy

Fig. 2. Bidirectional vertical grating coupler

Fig. 3. Focusing grating coupler

Fig. 4. Images of Si3N4 grating coupler obtained by scanning electron microscopy; (b) inverse taper tip; (c) grating region

Fig. 5. Focused asymmetric etched grating coupler

Fig. 6. Fiber converter with cantilevered waveguide and index-matched cladding

Fig. 7. (a) Image of cantilevered fiber-to-waveguide converter obtained by scanning electron microscopy (dot line represents Si overlapped tapers buried in SiO2 cantilevered waveguide in the 3rd section); (b) image of non-completely cantilevered structure obtained by optical microscopy; (c) image of completely cantilevered structure obtained by optical microscopy

Fig. 8. Pictures of converters obtained by scanning electron microscopy[43]. (a) Overall view; (b) zoom-in cascaded converters; (c) zoom-in single converter

Fig. 9. Spot size converter with silicon dioxide spacer

Fig. 10. Schematic of 3-D tapered converter

Fig. 11. General production steps of 3-D tapered converter