# Silicon Photonics: The future of High-Speed Data

IBM’s research in brain-inspired computing, quantum computing, and silicon photonics is preparing to take computing in entirely new directions. The neuromorphic chip is getting smarter, the quantum bits are being scaled out, and in the near future, my team’s CMOS Integrated Nano-Photonics Technology will help ease data traffic jams in all sorts of computing and communications systems – pushing cloud computing and Big Data analytics to achieve their full potential.
For the first time, we have designed and tested a fully integrated, wavelength-multiplexed silicon photonics chip capable of optically transmitting and receiving information at data rates up to 25 Gb/s per channel. This will soon make it possible to manufacture optical transceivers capable of transmitting 100 gigabits of data per second.
Silicon photonics technology gives computational systems the ability to use pulses of light to move data at high speeds over optical fibers, instead of using conventional electrical signals over copper wires. Optical interconnects, based on vertical-cavity surface-emitting laser (VCSEL) technology and multi-mode fiber, are already being used in systems today. But their transmission range is limited to a relatively short distance of about 150 meters. Today, large data centers continue to scale in size to support exponentially growing traffic from social media, video streaming, cloud storage, sensor data, and much more. The longest optical links in such systems can be more than a kilometer in length. As a result, new optical interconnect solutions that can meet these requirements at low cost are needed to keep up with future system growth.

How light boosts bandwidth

Our silicon photonics technology is designed to transmit optical signals via single-mode optical fibers, which can support links many tens of kilometers long. Moreover, we have built in the capability to use multiple colors of light, all multiplexed to travel within the same optical fiber, to boost the total data capacity carried. The recently demonstrated silicon photonic chip can combine four wavelengths (all within the telecommunications infrared spectrum), allowing us to transmit four times as much data per fiber. The chip demonstrates transmission and reception of high-speed data at 25 Gb/s over each of these four channels, so within a fully multiplexed design, we’re able to provide 100 Gb/s aggregate bandwidth.
 A cassette carrying several hundred chips intended for 100 Gb/s transceivers, diced from wafers fabricated with IBM CMOS Integrated Nano-Photonics Technology
In addition to the expanded range and bandwidth per fiber, our new photonics technology holds several other advantages over what is available today. Perhaps most importantly, the technology’s manufacturing makes use of conventional silicon microelectronics foundry processes, meaning volume production at low cost. In addition, the entire chip design flow, including simulation, layout, and verification, is enabled by a hardware-verified process design kit, using industry-standard tools. As a result, a high-speed interconnect circuit designer does not require an in-depth knowledge of photonics to build advanced chips with this technology. They can simply follow the standard practices already in place in the CMOS industry.
This unified design environment is mirrored by our integrated platform, which allows us to fabricate both the electronic and photonic circuit components on a single silicon chip. Rather than breaking up the electrical and optical functions, we integrated the optical components side by side with sub-100nm CMOS electrical devices. This results in a smaller number of components required to build a transceiver module, as well as a simplified testing and assembly protocol, factors which further contribute to substantial cost reductions.
 Performance of the fully integrated, wavelength-multiplexed silicon photonics technology demonstrator chip. The eye diagrams illustrate four separate transmitter channels (right) exchanging high-speed data with four receiver channels (left), each running at a rate of 25 Gb/s.
While the primary applications for silicon photonics lie within the data center market, driven by Big Data and cloud applications, this technology is also poised to have a large impact within mobile computing. There’s a need for low-cost optical transceivers to shuttle large volumes of data between wireless cellular antennae and their base stations, often located many kilometers away. As the data bandwidth available to mobile users increases generation after generation, the number of individual cells required to support the traffic does the same. Our technology can deliver faster data transfer in higher volume and across larger areas, in order to support the inevitable growth while controlling costs.
There has been significant discussion around the 50thanniversary of Moore’s Law and about whether it has reached its end. Silicon photonics fits into that “next switch” conversation. On the processor side, there’s still a fairly consistent trajectory in terms of CMOS technology scaling – down to 10nm, 7nm, and even smaller. The role of our CMOS Integrated Nano-Photonics technology will be to reduce communication bottlenecks inside of systems, and to allow expansion of their capacity for processing huge volumes of data in real time.

Kilometer-scale data centers are emerging. Big Data and the Internet of Things are connecting people and information in ways that were unimaginable only a few years ago. IBM’s silicon photonics technology will augment that growth on the ground, into the Cloud, and beyond.

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