Single-level phase-compensation for ultra-compact on-chip mode cross-connector

The development of integrated optical communication requires efficient cross-connections among multiplexed mode channels to establish comprehensive communication networks. Conventional mode cross-connections often rely on cascaded directional-coupler stages, which incur extended interaction lengths and multi-level conversions, imposing fundamental limits on device miniaturization and multi-mode adiabatic conversion. Herein, we propose a single-level phase-compensation mechanism based on an inverse-designed subwavelength pixelated lattice to realize ultra-compact on-chip mode cross-connections. The tailored phase delays are achieved via cumulative scattering effects, enabling direct and efficient single-step energy conversions while avoiding inter-level phase mismatch accumulation. The mode conversion length is significantly shortened due to the enhanced optical-matter interaction, ensuring compatibility with system simplicity and compactness. Benefitting from this mechanism, we demonstrate six distinct cross-connections covering TE₀ to TE₂ modes within the C-band, each with a footprint of only 5.4 μm × 5.4 μm, over two orders of magnitude smaller than conventional designs. Furthermore, 5.265 Tbit/s QPSK-OFDM signals are successfully cross-connected among mode channels, with bit error rates below 10⁻⁵, confirming the high-speed performance of the proposed devices. These results highlight the potential of single-level phase-compensation for overcoming the trade-off between device footprint and mode compatibility, improving the dense integration. Leveraging the single-level phase compensation approach, trunk-branch connection also be compatible, paving the way for constructing high-density and multifunctional integrated on-chip communication networks.