In recent years, there has been an astounding surge in the volume of data being transferred and processed per second. Emerging technologies like high-dimensional quantum communications, large-scale neural networks, and high-capacity networks demand extensive bandwidth and rapid data transfer rates. One promising avenue to achieve these goals involves replacing traditional metallic wires connecting electronic system components with optical interconnections, harnessing light instead of electricity to establish data transfer pathways.
Optical interconnections have the potential to offer exceptionally high speeds through a method known as mode-division multiplexing (MDM). Through precisely designed structures called waveguides, light can travel in distinct patterns known as “modes.” Since multiple modes can coexist in the same medium without interference, they effectively function as separate data channels, significantly enhancing the overall data transfer capacity of the system.
However, the reported speeds of MDM systems have thus far been limited primarily due to imperfections in device fabrication, leading to variations in the refractive indices of the waveguides. One approach to mitigate these imperfections involves meticulously engineering the refractive indices of the waveguides by optimizing their structure and composition. Regrettably, current methods are constrained by either material choices or the resulting large circuit footprint.
Against this backdrop, a research team, led by Professor Yikai Su from Shanghai Jiao Tong University in China, embarked on a quest to devise a novel method for coupling different light modes. Their findings, published in Advanced Photonics, describe the successful application of this technique in an MDM system, achieving unprecedented data rates.
The key highlight of their research is an ingenious design for a light-mode coupler, a structure capable of manipulating a specific light mode within a neighboring bus waveguide, such as a nanowire carrying the multi-mode signal. This coupler can either inject a desired light mode into the bus waveguide or extract one from it, redirecting it along a different path.
The researchers tailored the refractive index of the coupler to enable robust interaction with the desired light mode across a wide coupling region, even in the presence of fabrication errors, resulting in a high coupling coefficient. They achieved this feat by employing a gradient-index metamaterial (GIM) waveguide.
In contrast to conventional materials, the GIM exhibited a continuously varying refractive index along the direction of light propagation. This characteristic facilitated a smooth and efficient transition of individual light modes to and from the nanowire bus, mitigating parameter variations in the waveguides.
By cascading multiple couplers, the research team created a 16-channel MDM communication system that concurrently supported 16 different light modes, ranging from TE0 to TE15. In a data transmission experiment, this system achieved a remarkable data transfer rate of 2.162 terabits per second (Tbit/s), marking the highest ever reported value for an on-chip device operating at a single wavelength.
Furthermore, the system’s fabrication relied on techniques compatible with semiconductor device manufacturing, including electron beam lithography, plasma etching, and chemical vapor deposition. This ensured that the design could be readily scaled up and harmonized with existing fabrication technology.
In conclusion, the innovative coupling strategy utilizing a GIM structure has the potential to substantially elevate data transfer rates, particularly in fields where large-scale parallel data transmission and computations are commonplace. This advancement could set new benchmarks in hardware acceleration, large-scale neural networks, and quantum communications.