Metastructure-enabled scalable multiple mode-order converters: conceptual design and demonstration in direct-access add/drop multiplexing systems.

The rapid advancement of multimode photonic technologies, optical computing, and quantum circuits, leveraging higher-order modes, necessitates the development of on-chip multiple mode-order converters (MMOCs). However, existing schemes face limitations in traffic capacity, polarization-dependence, and scalability. Herein, we propose a novel highly scalable MMOC design framework enabled by subwavelength grating (SWG) metastructures. By integrating SWG arrays into a taper-tailored multimode waveguide, the design synergizes coherent scattering and beam shaping to achieve efficient target-supermode excitations and precise phase controls, simultaneously. In this way, the target MMOC can be realized according to the functional requirements of mode manipulations by optimizing the metastructures. Experimentally fabricated devices exhibit ILs < 1.85 dB and CTs < -12.5 dB across (22 or 50) nm bandwidths, with a polarization-independent quad-mode operation. Notably, the dual-pair mode exchanging MMOC pioneers simultaneous TE0↔TE2 and TE1↔TE3, doubling exchange efficiency over conventional single-pair solutions. Integrated into a direct-access mode add/drop system (DAMAD), TE0/TE1 dual-mode add/drop operations achieve ILs < 4.5 dB and CTs < -15.5 dB across 41 nm bandwidth. Thereupon, clear eye diagrams at 32/64 Gbps operations demonstrate the capability for the high-speed optical communication. The proposed concept offers a novel strategy for on-chip multiple mode manipulations, with transformative potential in higher-order modes based optical communications.