Optical communication formed the backbone of the internet age and is expected to be equally pivotal for the developing 5G networks. Modern communications rely on optical links that fly information at the speed of light, and on circuitry such as photodetectors and modulators which is able to encode a wealth of information onto these light beams. Although silicon is the material of choice for photonic waveguides on optical chips, photodetectors are made from other semiconductors such as GaAs, InP, or GaN, because silicon is transparent at standard telecomm wavelengths. Integrating these other semiconductors with silicon is difficult, complicating fabrication processes and raising expenses. Also, thermal management is becoming a problem as photonic devices keep shrinking while using more power.
Graphene is a promising material for telecomm photodetectors, because it absorbs light over a large bandwidth, including standard telecomm wavelengths. It is also compatible with CMOS technology, which means it can be technologically integrated with silicon photonics. Furthermore, graphene is an excellent heat conductor, promising a reduction in heat consumption of graphene-based photonic devices. For these reasons, graphene for optical communications has been an intense field of research, which is now gaining fruition in full working prototypes.
The first graphene photodetectors were developed in a research lab at IBM already in 2009. These transistor-based photodetectors had bandwidths exceeding 25 GHz and were subsequently used to transfer data over a 10 Gbit s-1 optical data link. The efficiency of detection in those devices was improved by employing an asymmetric metal-graphene-metal transistor configuration. Analysis suggests that the bandwidth of such graphene photodetectors may ultimately exceed 500 GHz.
Figure: Graphene photodetectors speed up
2013 was a productive year for graphene photodetector results. Several teams reported graphene photodetectors of different geometries, utilizing different physical principles, resulting in CMOS-compatible photodetectors that covered all communication bands at bandwidths up to 18 GHz. In all these new realizations, graphene was positioned directly on top of silicon waveguides and light was absorbed as it propagated down the waveguide. These were the first truly CMOS-compatible graphene photodetectors.
In 2016, the bandwidth of graphene photodetectors reached 65 GHz, utilizing graphene/silicon pn junctions with potential bit rates of ~90 Gbit s-1. Already in 2017, graphene photodetectors with a bandwidth exceeding 75 GHz were fabricated in a 6” wafer process line. These record-breaking devices were showcased at the Mobile World Congress in Barcelona in 2018, where visitors could experience the world’s first all-graphene optical communication link operating at a data rate of 25 Gbit s-1 per channel. In this demonstration, all active electro-optic operations were performed on graphene devices. A graphene modulator processed the data on the transmitter side of the network, encoding an electronic data stream to an optical signal. On the receiver side, a graphene photodetector did the opposite, converting the optical modulation into an electronic signal. The devices were made with Graphenea CVD graphene and showcased at the Graphene Pavilion. At the same show, Ericsson showed the first graphene-based optical ultrafast interconnection in mobile access networks, with a graphene-based photonic switch. From the financial aspect, if cost was a barrier to adoption of graphene technology just several years ago, it no longer is.
Graphene-based integrated photonics are seen as a key area of future development, with potential for high-speed optical networks that use less energy than networks based on semiconductor photonics, while keeping the costs low and providing integration with existing technology.
