Symmetry-Engineered Subwavelength Metasurfaces for Scalable Mode Order Conversion and Multiplexing

Mode-division multiplexing (MDM) leverages the orthogonality of multimode waveguide eigenmodes to increase optical communication capacity. Mode order converters (MOCs), as a crucial component in MDM systems, play an essential role in enabling controlled mode excitation and conversion. However, existing MOCs encounter limitations in generating and detecting high-order modes, which hinder the scalability of MDM systems. We address these limitations by employing subwavelength metasurfaces to precisely manipulate higher-order mode. This approach demonstrates a unified architecture capable of achieving longitudinal, horizontal, parallel, polarized parallel, and partial pass-through transformations using identical fundamental cell structures. It not only enhances the diversity of single-mode manipulation but also achieves scalability for multiple spatial mode conversions. The symmetric MOC design reduces the original operating requirements by nearly 70% through symmetric boundary conditions. Meanwhile, simulations have shown the spaced cascaded asymmetric directional coupler (ADC) achieves more than twice the bandwidth characterization of the original results (the bandwidth expands from 160 nm to 370 nm), effectively reducing errors in traditional testing systems. Experiments on five fabricated MOCs validates excellent bandwidth (almost all >100 nm), compact footprints (5.695-12.07 μm), and superior scalability. Numerical simulations further verify extended operation to high-order MOCs (TE<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> to TE<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub>, TE<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> to TE<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sub>, TE<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> to TE<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub>), with maintained performance (Length < 4.53 μm, BW<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10dB</sub> >265 nm). The system's viability is demonstrated through clear 64 Gbps eye diagrams, establishing the capacity for high-speed optical communication.