Mode-Division Multiplexing in Silicon Photonics

Mode‐division multiplexing (MDM) in silicon photonics harnesses distinct spatial modes within a single multimode waveguide to carry parallel data streams, offering a pathway to significantly enhance on‐chip bandwidth without increasing footprint. By encoding information onto orthogonal transverse electric (TE) or transverse magnetic (TM) modes, MDM systems multiply channel capacity and exploit the high refractive‐index contrast of silicon‐on‐insulator platforms for tight confinement and compact devices. Recent advances have addressed core challenges such as selective mode excitation, low‐loss multiplexing and demultiplexing, inter‐mode crosstalk suppression and group‐velocity dispersion management. Techniques ranging from gradient‐index metamaterials and subwavelength grating engineering to inverse design algorithms have delivered scalable mode‐selective components, including high‐order mode converters, add‐drop couplers and broadband filters. These developments not only pave the way for terabit‐scale optical interconnects in data centres and high‐performance computing, but also underpin emerging applications in high-dimensional quantum communications and neuromorphic photonic processors. Ongoing efforts focus on seamless integration with wavelength-division multiplexing, thermal and electro-optic reconfigurability, and the mitigation of fabrication variability to realise robust, large-scale MDM networks on silicon.

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Research from all publishers

On‐chip metamaterial-enabled high-order MDM devices have demonstrated the simultaneous support of TE0 through TE15 modes by embedding a gradient-index metamaterial region within a multimode waveguide. This approach achieves precise group-velocity dispersion control, enabling 16-ary quadrature amplitude modulated signals at 40 GBaud across 16 modes for a net data rate exceeding 2.1 Tbit/s on a single wavelength channel. The record performance highlights the potential to dramatically expand mode-channel counts for ultrahigh-capacity interconnects.

A mode-selective manipulation scheme employing subwavelength grating structures redistributes modal fields in a bus waveguide, allowing independent add-drop of fundamental and higher-order modes. By engineering local refractive-index profiles, unwanted coupling is suppressed while desired mode coupling is enhanced, yielding a three-channel add-drop coupler with excess losses below 2 dB and inter-mode crosstalk under −19 dB across a 75 nm bandwidth. This flexible architecture points to scalable on-chip switching for hybrid MDM/WDM networks.

Inverse-designed high-order mode pass filters, realised via topology optimisation, offer ultra-compact footprints and broad spectral operation. Two prototypes—a binary-search-optimised adiabatic coupler and a tapered asymmetric directional coupler—achieve insertion losses near 2–3 dB and crosstalks down to −27 dB over a 130 nm band. The filters underscore the power of computational design in creating compact, high-performance MDM components for dense photonic integration.

Mode-Division Multiplexing in Silicon Photonics publication trend

The graph below shows the total number of articles in mode-division multiplexing in silicon photonics across all publications each year (not limited to Nature Index journals).

Technical terms

Mode‐division multiplexing (MDM): A technique that encodes separate data streams onto distinct spatial modes within a multimode waveguide to increase channel capacity.

Silicon photonics: A platform leveraging silicon‐on‐insulator substrates to fabricate optical components using CMOS-compatible processes for large-scale integration.

Waveguide modes: Discrete field distributions supported by an optical waveguide, each characterised by its electromagnetic field profile and propagation constant.

Insertion loss: The reduction in optical power due to the introduction of a photonic component into a transmission path, typically measured in decibels.

Crosstalk: Unwanted coupling or leakage of signal power between distinct modes or channels, degrading signal integrity.

Subwavelength grating: A periodic nano‐structured region with feature sizes below the operating wavelength, used to tailor effective refractive indices and modal distributions.

Metamaterial: An engineered composite structure designed to exhibit tailored optical properties, such as gradient refractive-index profiles, not found in natural materials.

References

1. On-chip metamaterial-enabled high-order mode-division multiplexing. Advanced Photonics (2023).
2. On-chip mode-selective manipulation based on the modal-field redistribution assisted with subwavelength grating structures. Nanophotonics (2023).
3. Inverse design and demonstration of on-chip silicon high-order mode pass filter. APL Photonics (2024).