MDM is a promising solution to scale optical bandwidth by employing the spatial modes of on-chip waveguides [
50]. Mode (de)multiplexers are key devices in the MDM schemes. Several schemes have been proposed to realize on-chip mode multiplexers, such as adiabatic mode-evolution coupler [
51], asymmetric Y-junction [
52], and asymmetric directional coupler [
53]. The 3D view of the proposed SWG-based three- mode (de)multiplexer is sketched in Fig. 10(a). It consists of a bus waveguide and two SWG access waveguides. The TE
0 mode is injected from the input port of the bus waveguide. The multiplexing of the higher-order modes (TE
1, TE
2 modes) are achieved by using the SWG-based directional coupler, as shown in Fig. 10(b). The simulated power distributions in the proposed SWG-based directional couplers are illustrated in Figs. 10(c) and 10(d). When the TE
0 modes are injected from the input ports of the SWG access waveguides, high efficient TE
1 or TE
2 modes are obtained at the output ports of the bus waveguides with different widths.