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The first optical communication networks were implemented in the mid-nineteen-eighties having been developed from two technologies, a transmission source, and the laser, invented in 1960 and the discovery six years later that silica glass fibers could carry light waves with no significant attenuation and very little loss of signal.
The development of a room temperature continual operation diode laser in 1970 made it feasible to use optical cables with light transmitters and receptors in telephony and data communication systems.
The backbone of virtually all of our current communication systems is provided by optical networks. Those first optical fiber networks operated at a rate of 2.5 Gb/s over a single data stream. By 2015 an increase to 400 Gb/s using multimode fibers that supported several light sources dramatically improved the available bandwidth. This totaled a massive 160 fold increase over a 30 year period.
Present-Day Optical Networks
Optical networks are still built with and reliant on silicon as the primary material not only in the cables but also in the light sources, light detectors and optical modulators. Given the need to continue the rapid growth in bandwidth to satisfy the needs of businesses for faster connectivity, data traffic from mobile devices and to deliver the demand for higher resolution streaming services, researchers have been looking at integrating graphene into silicon photonics as a solution to building capacity.
Advantages of Graphene in Optical Networks
The unique properties of graphene mean that not only can it offer speed and cost advantages when integrated into silicon photonics systems, but it can also present new functionalities. As graphene demonstrates both electro-absorption and electro-refraction it can be used for ultrafast light modulation and photo-detection.
Graphene-based modulators have already delivered ultra-high transmission rates and because graphene has no bandgap, graphene based photodetectors can, using just this single material, detect light from a much wider spectrum from UV to ultra-violet.
Silicon is limited by an indirect bandgap that limits its ability to release and absorb light. The challenge is to replace the silicon photonics with graphene to leverage all of its advantages while minimizing changes to existing optical technologies. A team at the University of Delaware has been working on an approach than can realize this goal.
Led by Tingyi Gu they recently engineered a silicon -graphene device capable of transmitting radio-frequency waves in less than a picosecond at a sub-terahertz bandwidth. Their key to integrating the graphene is its specific location within the device that optimizes the structure in a way that improves responsivity and speed. This approach of developing graphene photonic devices using standard CMOS manufacturing techniques, rather than building an entirely new infrastructure seems to be the key to bringing the benefits of graphene to the communications industry as quickly as possible.
Importance of Graphene Flagship in Progressing Optical Networks
The Graphene Flagship is a Future and Emerging Technology Flagship by the European Commission. Launched in 2013 with a budget of €1 billion it represents a new form of coordinated joint research that aims to bring together academic and industrial researchers to take graphene from the laboratory on a ten-year trajectory into commercial use. One of their core areas of interest is around developing the use of graphene in photonics and optoelectronics.
Researchers have already shown that a data rate of 50 Gb/s is achievable with graphene photonic components. On the transmitter side, they used a graphene based modulator that encoded the optical signal and on the receiver side, a graphene based receptor converted the modulated output into an electronic signal.
They have also suggested a significantly more cost-effective alternative to the silicon on insulator wafer technology currently used in silicon photonics. Their proposed configuration is to use a pair of single-layer graphene (SLG) layers, a capacitor consisting of an SLG-insulator-SLG stack on top of a passive waveguide. This also has the advantage of further cost savings by replacing the need for using expensive germanium in photodetectors.
"Graphene never ceases to surprise us when it comes to optics and photonics." adding that "the Graphene Flagship has put significant investment to study and exploit the optical properties of graphene. This collaborative work could lead to optical devices working on a range of frequencies broader than ever before, thus enabling a larger volume of information to be processed or transmitted."
Professor Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship
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