Following optical communication’s pivotal role in the internet age, it is expected to be similarly central for the evolution of 5G networks. Contemporary communications are reliant on optical links, with the ability to transfer information at the speed of light, and circuitry, including photodetectors and modulators, which is capable of encoding vast quantities of data into these light beams.
Despite the popularity of silicon as a material for photonic waveguides on optical chips, due to its transparency at standard telecomm wavelengths, photodetectors are being made from alternative semiconductors such as GaAs, InP or GaN. Attempting to integrate these semiconductors with silicon is tricky, causing complications in fabrication processes and leading to increased costs. In addition to this, as photonic devices are simultaneously shrinking in size while growing in power usage, thermal management is now becoming an issue.
As graphene is able to absorb light over a broad bandwidth, including at standard telecomm wavelengths, it demonstrates great potential for telecomm photodetectors. In addition to this, thanks to its compatibility with CMOS technology, graphene can be integrated with silicon photonics. Finally, as an excellent heat conductor, graphene can guarantee lower heat consumption in photonic devices. These factors have resulted in a high level of research into graphene for optical communications, which is now succeeding in developing full working prototypes.
The earliest graphene photodetectors were transistor-based and were developed at an IBM research lab in 2009. They had bandwidths higher than 25 GHz and consequently, were used to transfer data over a 10 Gbit s-1 optical data link. The use of an asymmetrical metal-graphene-metal transistor configuration helped to improve the efficiency of detection in those instruments. According to analysis, the bandwidth of graphene photodetectors of this type could eventually pass 500 GHz.
Figure: Graphene photodetectors speed up
In 2013, a fruitful year for graphene photodetector results, a number of teams announced graphene photodetectors of a variety of geometries, making use of differing physical principles, and resulting in CMOS-compatible photodetectors that covered all communication bands up to 18 GHz. In each of these developments, graphene had been placed directly on top of silicon waveguides, allowing light to be absorbed as it disseminated down the waveguide. These were the first graphene photodetectors to be truly CMOS-compatible.
In 2016, usinggraphene/silicon pn junctions with potential bit rates of ~90 Gbit s-1, the bandwidth of graphene photodetectors hit 65 GHz. Just a year later, in 2017, graphene photodetectors with a bandwidth exceeding 75 GHz were manufactured in a 6” wafer process line. Visitors were then able to experience the planet’s first all-graphene optical communication link, operating at a data rate of 25 Gbit s-1 per channel, as the record-breaking devices were demonstrated at the 2018 Mobile World Congress in Barcelona.
For this demonstration, the active electro-optic operations were carried out on graphene devices, with a graphene modulator processing the data on the transmitter side of the network and converting the electronic data stream to an optical signal. A graphene photodetector did the reverse on the receiver side, decoding the optical modulation into an electronic signal. These devices, displayed at the Graphene Pavilion, were formulated with Graphenea CD graphene.
Ericsson also used this show to exhibit the first graphene-based optical ultrafast interconnection in mobile access, with a graphene-based photonic switch. If in recent years, high costs have previously been a hurdle in the embrace of graphene technology, this is no longer the case.
With the ability to minimize costs while providing integration with existing technology, and the further potential for fast optical networks that are more energy-efficient than semiconductor photonic-based networks, it is easy to see why graphene-based integrated photonics are seen as a vital area for future growth.
This information has been sourced, reviewed and adapted from materials provided by Graphenea.
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