Fig. 1. Schematic diagram of designed coupler. (a) Three-dimensional structure; (b) top view; (c) side view

Fig. 2. Coupling loss caused by port mode field mismatch when three ports of Si₃N₄ waveguide are directly coupled to fiber. (a) Port coupling loss varying with W_d under different W_t1; (b) mode field distribution diagrams of three-port branch port when W_t1=80 nm, W_d=0.7 μm

Fig. 3. Port coupling loss of fiber and SU-8 tapered structure varying with W_s1

Fig. 4. Factors affecting transmission loss of coupler. (a) Effect of output width W_s2 of SU-8 structure on transmission loss of coupler under different W_d; (b) mode field distribution diagrams of three-port branch port when W_t1=180 nm,W_d=0.57 μm; (c) effect of thickness H_t2 of SiO₂ substrate under three-port branch waveguide on transmission loss of coupler under different output thicknesses H_s2 of SU-8 structure; (d) effect of branch length L_t1 on transmission loss of coupler; (e) effective refractive indexes of different modes under different Si₃N₄ waveguide widths; (f) effect of output taper length L_t2 on transmission loss of coupler; (g) effect of L_s1 of SU-8 structure on transmission loss of coupler

Fig. 5. Overall coupling performance analysis. (a) Horizontal alignment tolerance of coupler; (b) vertical alignment tolerance of coupler; (c) effect of mode field diameter on coupling loss; (b) effect of wavelength on coupling loss