Mode-selective modulation by silicon microring resonators and mode multiplexers for on-chip optical interconnect.

Mode-division multiplexing (MDM) for on-chip interconnect, as a degree of freedom to enable further scaling the communication capacity, has attracted wide attention. However, selective loading information to the multimode light carriers of MDM systems is not as simple as the situation in wavelength-division multiplexing (WDM). In this paper, we demonstrate a scalable mode-selective modulation device for on-chip optical interconnect. It consists of two functional blocks. In one block, we use carrier-depletion add-drop silicon microring resonators to implement the simultaneous mode de-multiplexing from the multimode bus waveguide and high-speed modulation function. In the other block, we use asymmetric directional coupler based mode multiplexers to restore the modulated signals from fundamental mode to original mode sequences. By this structure, each mode channel from input port is separated and can be processed individually. In other words, we can selectively modulate arbitrary mode channels as requirement. The structure could be scaled to numerous mode channels. As a proof of concept, we design and fabricate a device with four microring resonators and a four-channel mode multiplexer. The insertion losses for all modes are less than 2.1 dB, and the inter-mode crosstalk is lower than -19.7 dB. 25 Gbps on-off key (OOK) electrical signals are utilized to drive the microring resonators, the optical eye-diagrams derived from every mode channels are clear and open. The preliminary demonstration of the device with a 50 Gbps OOK signals is also investigated. Our approach can provide more manipulation flexibility to the multimode optical interconnect.