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Data exchange among different mode channels is indispensable for optical communication system adopting modedivision multiplexing. Traditional mode exchange device is complex in procedure and large in footprint, which makes it not suitable for dense and large-scale photonic integration. Utilizing the degree of freedom of silicon meta-structure, we design an ultracompact and optically broadband mode exchange device between TE 0 and TE 1 modes by the step-by-step inverse-design method considering the axisymmetric constraint. Simulation result shows that it is robust to a temperature variation of 100 K and a fabrication error of ±20 nm. The fabricated device is 4 × 1.6 μm 2 in footprint. The simulated conversion efficiencies are over 73% and 71% for TE 0 to TE 1 and TE 1 to TE 0 within the whole C-band, and the experimental results are about 10% lower than the simulation. 40 Gbps OOK and 25 GBaud PAM-4 data transmission through the device are carried out, which shows good signal quality. We envision that the device will offer a flexible mode manipulation in optical interconnect system.

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We propose a 2 × 2 multimode optical switch, which is composed of two mode de-multiplexers, n 2 × 2 single-mode optical switches where n is the number of the supported spatial modes, and two mode multiplexers. As a proof of concept, asymmetric directional couplers are employed to construct the mode multiplexers and de-multiplexers, balanced Mach-Zehnder interferometer is utilized to construct the 2 × 2 single-mode optical switches. The fabricated silicon 2 × 2 multimode optical switch has a broad optical bandwidth and can support four spatial modes. The link-crosstalk for all four modes is smaller than -18.8 dB. The inter-mode crosstalk for the same optical link is less than -22.1 dB. 40 Gbps data transmission is performed for all spatial modes and all optical links. The power penalties for the error-free switching (BER<10) at 25 Gbps are less than 1.8 dB for all channels at the wavelength of 1550 nm. The power consumption of the device is 117.9 mW in the "cross" state and 116.2 mW in the "bar" state. The switching time is about 21 μs. This work enables large-capacity multimode photonic networks-on-chip.

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The development of optical interconnect techniques greatly expands the communication bandwidth and decreases the power consumption at the same time. It provides a prospective solution for both intra-chip and inter-chip links. Herein reported is an integrated wavelength-division multiplexing (WDM)-compatible multimode optical switching system-on-chip (SoC) for large-capacity optical switching among processors. The interfaces for the input and output of the processor signals are electrical, and the on-chip data transmission and switching process are optical. It includes silicon-based microring optical modulator arrays, mode multiplexers/de-multiplexers, optical switches, microring wavelength de-multiplexers and germanium-silicon high-speed photodetectors. By introducing external multi-wavelength laser sources, the SoC achieved the function of on-chip WDM and mode-division multiplexing (MDM) hybrid-signal data transmission and switching on a standard silicon photonics platform. As a proof of concept, signals with a 25 Gbps data rate are implemented on each microring modulator of the fabricated SoC. We illustrated 25 × 3 × 2 Gbps on-chip data throughput with two-by-two multimode switching functionality through implementing three wavelength-channels and two mode-channel hybrid-multiplexed signals for each multimode transmission waveguide. The architecture of the SoC is flexible to scale, both for the number of supported processors and the data throughput. The demonstration paves the way to a large-capacity multimode optical switching SoC.

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