Compact and Flexible On-Chip Mode Division Multiplexing Enabled by a Grating Engineered Backward Mode Conversion Engine

Mode-division multiplexing (MDM) greatly enhances the capacity of on-chip photonic interconnects, yet traditional MDM techniques struggle with direct access to specific lower-order modes within multimode bus waveguides, limiting scalability and flexibility. In this paper, we introduce a compact and flexible MDM architecture featuring an engine that combines a grating-engineered backward mode converter (BMC) with a bidirectional asymmetric directional coupler (Bi-ADC). The BMC is precisely engineered to leverage the modal properties of the multimode bus waveguide, enabling efficient switching between two designated modes while minimizing crosstalk with other modes. The Bi-ADC operates analogously to a 2 × 2 coupler and integrates a single-mode access waveguide with the bus waveguide, allowing for independent routing of two modes through the access waveguide's ports. This approach provides a more compact design compared to conventional MDM techniques and facilitates the insertion of lower-order modes at arbitrary positions along the bus waveguide. Furthermore, this architecture can be combined with an optical switch to develop a bidirectional, dual-mode switchable add-drop mode multiplexer. To demonstrate its feasibility, we design two BMC-based engines for TE₀-TE₁ and TE₀-TE₂ mode conversions and assess their performance numerically as 4-mode/5-mode demultiplexers. Experimental results showcase a 4-channel mode demultiplexer (TE₀, TE₁, TE₂, and TM₀) based on the TE₀-TE₁ engine, achieving insertion losses (ILs) of 2.4 dB, 0.3 dB, 1.28 dB, and 0.75 dB at a wavelength of 1550 nm. The modal crosstalk is below −17.5 dB for all channels, with an IL<3 dB bandwidth exceeding 100 nm.