Fig. 1. (a) Conceptual schematic of the on-chip multi-band MDM interconnect scheme based on the inverse-designed mode MUX/DEMUX. (b) Structural schematic of the broadband three-mode MUX. (c) Simulated effective indices of TE0 to TE2 modes at 1550 nm and 2000 nm under different waveguide widths. Inset is the cross-section view of the silicon waveguide with SiO2 cladding and buried oxide layers.

Fig. 2. (a) The workflow of the inverse design method, including the topology optimization (TO), edge-guided analog-to-digital conversion, and digital optimization (DO). (b) The permittivity profiles of the design region at the 10th, 100th, and 338th iterations during the topology optimization. The pattern becomes more binarized as the iteration number increases. (c) The feature map, decision map, and the converted pattern obtained during the edge-guided conversion process. (d) Evolution of FOM and proportion of edge pixels during the optimization.

Fig. 3. (a)–(c) Simulated transmission spectra of the broadband three-mode MUX in the wavelength range from 1500 nm to 2100 nm, when the light is incident from (a) upper, (b) middle, and (c) bottom ports of the device. The target mode is TE0, TE1, and TE2, respectively. (d)–(f) Simulated light propagation process of the MUX at the wavelengths of 1.55 μm and 2 μm for (d) TE0, (e) TE1, and (f) TE2.

Fig. 4. (a) Microscope image of the fabricated MDM circuit. (b) SEM images of the MDM circuit and the mode MUX. (c)–(e) Measured transmission spectra for (c) Ch. TE0, (d) Ch. TE1, and (e) Ch. TE2.

Fig. 5. (a) Experimental setup and DSP flows of the on-chip multi-band MDM interconnect. TL, tunable laser; OA, optical amplifier; EA, electrical amplifier. (b) The end-to-end system frequency response for the two bands. (c) BER performance of 30 GBaud PAM-8 signal transmission under different MZM bias voltages for the two bands.

Fig. 6. Experimental results of the 1.55-μm-waveband signal transmission. (a) Measured BER performance under different ROPs across three mode channels for 80 GBaud PAM4 signal transmission. (b)–(c) Measured BER performance under different data rates of (b) PAM4 and (c) PAM8 signal transmission. Inset (i): eye diagrams of 90 GBaud PAM4 signals of TE0, TE1, and TE2 channels. Inset (ii): eye diagrams of 60 GBaud PAM8 signals of TE0, TE1, and TE2 channels.

Fig. 7. Experimental results of the 2-μm-waveband signal transmission. (a) Measured BER performance under different ROPs across three mode channels for 40 GBaud PAM4 signal transmission. (b)–(c) Measured BER performance under different data rates of (b) PAM4 and (c) PAM8 signal transmission. Inset (i): eye diagrams of 48 GBaud PAM4 signals of TE0, TE1, and TE2 channels. Inset (ii): eye diagrams of 38 GBaud PAM8 signals of TE0, TE1, and TE2 channels.

Fig. 8. Comparison of mode density of ultra-broadband mode MUXs on SOI platform. The color of each point denotes the absolute value of inter-mode crosstalk.

Fig. 9. The simulated transmission spectra of the broadband three-mode MUX under −10 to +20nm pixel size variations. Simulated insertion losses and inter-mode crosstalk when the light is incident from (a) the upper port, (b) the middle port, and (c) the bottom port of the device.

Fig. 10. The simulated transmission spectra of the broadband three-mode MUXs optimized with pixel size of 140 nm (yellow), 120 nm (red), 100 nm (blue), and 80 nm (green). Simulated insertion losses and inter-mode crosstalk when the light is incident from (a) the upper port, (b) the middle port, and (c) the bottom port of the device.