EU's Graphene Flagship describes gigahertz-bandwidth graphene–silicon phase modulator

Based on a graphene-insulator–silicon capacitor, the high-speed device is more energy-efficient than conventional modulators.

Researchers from Graphene Flagship partners at the National Inter-University Consortium for Telecommunications (CNIT) in Italy, IMEC in Belgium, and the University of Cambridge in UK have created and tested a graphene-silicon phase modulator that outperforms existing silicon-based ones.1

Used in telecom, phase modulators can group several bits of information into fewer packets, reducing the spectral width; the smaller the spectral width, the higher the data-rate efficiency. Due to a natural trade-off, however, this efficiency is reaching a maximum with silicon-based modulators.

A novel solution has arrived in the form of a graphene-insulator–silicon capacitor. Graphene is ideally suited to integrate with pre-existing silicon photonics, due to its large optical modulation and high-speed operation. The device, which works at a 1550 nm wavelength, has a phase-shifting length of 300 μm, an extinction ratio of 35 dB, and a 5 GHz electro-optical bandwith. The modulator operates at 10 Gbit/s.

A single layer of graphene was grown via chemical vapor deposition and transferred onto a silicon photonic platform. "A small piece of graphene was placed on top of the silicon like an adhesive tape," explains Marco Ramagnoli of the National Inter-University Consortium for Telecommunications (CNIT), a Graphene Flagship partner. "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."

The hybrid phase modulator can have lower optical losses, reduced energy consumption, and error-free bit operation at transmission distances up to 50 km. By optimizing processes and device geometry, the radio-frequency bandwidth could be raised to match high-end existing modulators.

More energy-efficient than silicon modulators
In addition to helping usher in 5G mobile technology, this modulator may also aid in 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 worldwide web, which already now contributes significantly to the global CO2 footprint," says Neumaier. "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 centers and companies within the Graphene Flagship."

"Photonics and optoelectronic applications have been identified as having great application potential since the very start of the Flagship," says Professor Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship. "This work demonstrates that this technology is competitive with and can surpass the state of the art. The work already underpins a spearhead project targeting a~400Gbit/s data link for 2020, ready to be integrated in the business units of telecom and datacom companies."



1. V. Sorianello et al., Nature Photonics (2018); doi: 10.1038/s41566-017-0071-6.

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