Modern optical data and telecommunications employ phase modulators to increase the amount of data relayed and data rate efficiency, i.e. the speed at which information is relayed. Phase modulators traditionally work by grouping several bits of information into fewer symbols, or packets, reducing the overall size, or spectral width. The smaller the spectral width, the higher the data rate efficiency. However, this efficiency is reaching a maximum with silicon based devices, and so a novel solution is needed to bridge the gap between the increase in demand for data and the efficiency in transmitting it.
This novel solution may have come in the form of graphene. Graphene is regarded as ideally suited to integrate with pre-existing silicon photonics, due to its large optical modulation and high-speed operation. The multi-national collaborative team, headed by Marco Ramagnoli of the National Inter-University Consortium for Telecommunications (CNIT), conducted several tests to explore the efficiency of a graphene-based modulator. A single layer of graphene was grown, via CVD, and transferred onto a silicon photonic platform. Romagnoli explains: “A small piece of graphene was placed on top of the silicon like an adhesive tape. This made the resulting phase modulator work at any wavelength and the spectral efficiency was ten times more than that of a state of the art silicon phase modulator.”
This hybrid phase modulator can have lower optical losses, reduced energy consumption and error-free bit operation for up to 50 km transmission distance. Furthermore, by optimizing processes and device geometry the radio frequency bandwidth could be raised to match high-end existing modulators.
“From the point of view of a customer,” continues Romagnoli, “you want to watch films and other things on mobile devices, but don’t want to increase what you pay to the operator. So this means you want to increase the performances, bandwidth, but at the same time you want to reduce the cost per data bit. We have to find the technology that is scalable in performance but is, at the same time, cheaper”. “That is why we believe that graphene is a good candidate…This, as an experiment, is very simple and very inexpensive”.
This technology may even hold the key to reducing the carbon footprint of mobile technology as Daniel Neumaier, leader of Division 3, based at Graphene Flagship partner AMO GmbH, explains, “Optical communication systems form the backbone of the world wide web, which already now contributes significantly to the global CO2 footprint. This work demonstrates that graphene based optical phase-modulators could become key components of optical data links in order to reduce the energy consumption. The reported modulation efficiency, which is one of the decisive key parameters for the overall energy consumption, is already outperforming conventional silicon based modulators. The next crucial step in order to bring this device towards applications is the wafer scale CMOS integration. This challenge is currently addressed by leading European research centres and companies within the Graphene Flagship.”