Fig. 1. Mode fields and schematic diagrams of double-layers inverse taper coupler. (a)(b) 1-tip structure; (c)(d) 2-tip structure; (e)(f) 3-tip structure

Fig. 2. Variable sweeping results of end face section. (a) Effect of tip width on mode field matching rate; (b) effect of tip width on effective refractive index; (c) effect of incident spot diameter on mode field matching rate; (d) effect of tip gap on mode field matching rate

Fig. 3. Mode evolution process. (a) 1-tip structure; (b) 2-tip structure; (c) 3-tip structure

Fig. 4. Variable sweeping results of mode field evolution part. (a) Effect of end gap on additional loss of 2-tip structure; (b) effects of end width on additional loss of 3-tip structure; (c) effect of end gap on additional loss of 3-tip structure; (d) effect of upper tip width on additional loss of transition structure

Fig. 5. Overall simulation results. (a)(b) Effects of structure length on coupling loss; (c)(d) coupling losses of different wavelengths in TE and TM modes; (e)(f) alignment tolerances in horizontal and vertical directions

Fig. 6. Manufacture and experimental results. (a)-(c) SEM images of 2-tip, 3-tip and transition structures; (d) effect of tip width on coupling loss of 2-tip structure; (e) transmission spectra of 2-tip and 3-tip structures