Metamaterial-based strategy to achieve high data transfer speeds

Rapidly emerging technologies, such as high-dimensional quantum communications, large-scale neural networks and high-capacity networks, require large bandwidths and high data transfer speeds. One way to achieve them is by replacing the conventional metallic wires between the components in an electronic system with optical interconnectors, using light instead of electricity to establish channels for data transfer.

Optical interconnections can provide high speeds through mode-division multiplexing (MDM). Due to precisely designed structures called waveguides, light can propagate in specific patterns called ‘modes’. Since multiple modes can propagate in the same medium simultaneously without interfering with each other, they act as separate data channels, increasing the overall data transfer rate of the system.

However, the speed of MDM systems is limited due to the imperfections in the device fabrication that cause the refractive index variations of the waveguides. One way to mitigate the imperfections is to engineer the refractive indices of the waveguides by optimising the structure and composition. However, currently available methods are limited by the choice of materials or the resulting large circuit footprint.

Now, a research team from Shanghai Jiao Tong University in China has developed a technique for coupling (or combining) different light modes. As reported in Advanced Photonics, the researchers employed this technique in an MDM system, achieving high data rates. The researchers developed an innovative design for a light-mode coupler, a structure that can manipulate a specific light mode travelling in a nearby bus waveguide, such as a nanowire carrying the total multi-mode signal. The coupler can inject a desired light mode into the bus waveguide or extract one from it, sending it towards a different path.

The refractive index was tailored so that it interacted strongly with the desired light mode in a range of coupling regions in the presence of fabrication errors, thus realising a high coupling coefficient. This was achieved by leveraging a gradient-index metamaterial (GIM) along the direction of propagation of light. This facilitated the efficient transition of individual light modes to and from the nanowire bus by mitigating the parameter variations of the waveguides.

By cascading multiple couplers, the researchers created a 16-channel MDM communication system that supported 16 different light modes simultaneously. A data transfer experiment saw the researchers achieve a data transfer rate of 2.162 Tbit/s. The system was also fabricated using methods that are compatible in semiconductor device fabrication, such as electron beam lithography, plasma etching and chemical vapour disposition.

The proposed coupling strategy using a GIM structure could provide a boost in data rates, especially in fields where large-scale parallel data transmissions and computations are common. This could lead to developments in hardware acceleration, large-scale neural networks and quantum communications.

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