Fig. 1. (a) Schematic illustration of the dual-mode waveguide crossing. (b) and (c) Self-image beat lengths of both modes for the conventional waveguide-based MMI coupler and SW-structure-assisted one as a function of W1, respectively.

Fig. 2. (a) Schematic of the dual-mode crossings based on SW-structure MMI couplers. (b)–(e) Manually set and random initial patterns. (f)–(i) Optimized patterns for the manually set and random initials. (j) Calculated FOMs after every iteration for the manual and random initial patterns. (k) and (l) Transmission spectra for TE0 and TE1 of the four optimized devices with different initial patterns, respectively.

Fig. 3. (a) and (b) Simulated magnetic field distributions of Hz for TE0 and TE1 at the wavelength of 1560 nm, respectively. (c) and (d) Simulated transmission spectra for TE0 and TE1, respectively.

Fig. 4. (a) SEM image for the fabricated device composed of a dual-mode waveguide crossing and four (DE)MUXs. (b) SEM picture for the reference MDM system. (c) and (d) Detailed SEM images for dual-mode waveguide crossing and MUX. (e) and (f) Normalized measured transmission spectra for the fabricated reference MDM link and the entire fabricated device consisting of our proposed crossing and a pair of (DE)MUXs, respectively.

Fig. 5. (a) and (b) Measured transmission spectra with different numbers of cascaded crossings for TE0 and TE1, respectively. (c) and (d) Measured ILs at 1560 nm as a function of cascaded crossing numbers for TE0 and TE1, respectively.