Fig. 1. Structure diagram of non-uniform MMI

Fig. 2. Optical field patterms of the first four guided modes in uniform and non-uniform MMIs. The first guided mode in (a) the uniform MMI and (b) the non-uniform MMI; the second guided mode in (c) the uniform MMI and (d) the non-uniform MMI; the third guided mode in (e) the uniform MMI and (f) the non-uniform MMI; the fourth guided mode in (g) the uniform MMI and (h) the non-uniform MMI

Fig. 3. Distribution of the optical field intensity at the end of the uniform MMI

Fig. 4. Distribution of the optical field intensity at the end of the non-uniform MMI

Fig. 5. Distribution of the optical field intensity at the end of the non-uniform MMI after tuning the refractive index

Fig. 6. Schematic diagram of the mode splitter. (a) 3D structure diagram; (b) top view; (c) cross-section diagram

Fig. 7. Transmission spectra of the output ports. (a) Output2; (b) output1; (c) output3

Fig. 8. Transmission spectra of the output ports after optimization again. (a) Output2; (b) output1; (c) output3

Fig. 9. Light field diagrams of each mode. (a) TE0; (b) TE1; (c) TE2

Fig. 10. Schematic diagram of the chip process (EBL: electron beam lithography, ICP: inductively coupled plasma etching, PECVD: plasma enhanced chemical vapor deposition)

Fig. 11. Variation ranges of insertion loss and crosstalk of mode splitter under different temperature deviation range. (a) Insertion loss; (b) mode crosstalk