Fig. 1. Schematic structure for four-mode (de)multiplexer on silicon platform. Inset is the zoom-in view of the tapered directional coupler for mode conversion.

Fig. 2. (a) Simulated effective indices of four multiplexed eigenmodes of a SiO2-cladding silicon waveguide versus waveguide width w for a waveguide height h=220  nm. (b)–(d) Simulated mode transmission and crosstalk for three high-order modes. Insets are the mode evolution processes.

Fig. 3. Optical microscope image and scanning electron microscope (SEM) pictures of the fabricated four-mode division multiplexing device in the 2 μm waveband. (a) The microphotograph of the whole structure. (b)–(d) The details of three tapered directional couplers for (b) TE1, (c) TE2, and (d) TE3 modes. Insets of (d) show the zoom-in details with marked geometric parameters.

Fig. 4. (a) Measured and simulated transmission spectra of the vertical grating coupler. (b), (c) Measured transmission spectra at two output ports when the TE0 mode is launched at the input ports of the two-mode multiplexing structure.

Fig. 5. Measured transmission spectra at four output ports when the TE0 mode is launched at input ports of (a) CH₁, (b) CH₂, (c) CH₃, and (d) CH₄, corresponding to the TE0, TE1, TE2, and TE3 modes, respectively.

Fig. 6. (a) Experimental setup for 5 Gbit/s OOK data transmission through the MDM system. AWG, arbitrary waveform generator; DML, directly modulated laser; TDFA, thulium-doped fiber amplifier; SMF, single-mode fiber; TF, tunable filter; VOA, variable optical attenuator; PD, photodetector. (b) Measured optical spectra before modulation (red) and after modulation (blue).

Fig. 7. Measured BER curves versus received OSNR and eye diagrams for (a) two-channel and (b) four-channel data transmission through MDM system. B to B, back-to-back.