Light speed computing

Photonics investigates how optical technology could replace electrical signals sent through copper cables, which are used in today's computers. Not only would it mean faster computers it would also enable power usage to be cut by up to 40%.

Imagine a computer that can run thousands of times faster than today's state of the art hardware. This is the goal of photonics, where data is transferred literally at the speed of light.

Copper is one of the fundamental building blocks in computer systems. The cables that enable data transfer between servers, the cables inside the box, and the wires imprinted on the circuit boards are all made from copper. It is considered a great conductor of electricity. But copper wires have some significant disadvantages. Researchers say copper has unstable electrical properties, high noise levels and high power consumption, which limit how fast a copper circuit can run and how far a copper cable can stretch.

Today's computer system design has evolved from more than four decades of engineering and development in electronics and microelectronics. Although the size may have shrunk and the performance increased exponentially in this time, all computers are made up of the same fundamental elements - electrically optimised silicon in electrically optimised packages, attached to electrically optimised printed circuit boards that plug into electrically optimised backplanes.

While the level of integration and density has continued to evolve, physical system implementations have remained relatively constant. Terry Morris, who works at HP's Enterprise Storage and Servers Group, says computers have had to be designed to take into account the fundamental limitations of electrical signalling using copper wires, including loss, delay and signal/noise ratio.

What's more, whilst the simplest computer architecture only needs a data bus to connect the processor to memory and peripheral devices, user demand for higher performance has changed this architecture radically. The physical bus architectures in today's computers are supplanted by collections of switched point-to-point circuits. These require additional power and introduce latency into the circuit design.

Optical fibre is being used to overcome the limitation of copper. It is already used in networking and storage area networks to support faster interconnects between datacentres. The next threshold in performance is the connectivity between chips and processors. Researchers believe an optical link between systems could lift the performance of future generations of computers significantly.

Photonics is the area of research that investigates how optical interconnect technology can replace electrical binary "on" and "off" signals in copper cables with a bitstream of light supplied by a bank of minute lasers.

In a recent presentation at HP Labs, HP senior fellow Stan Williams predicted that light transmitted through optical fibre would eventually replace electrical signals sent through copper cables. "The future of communication inside a datacentre, inside a computer rack, inside a blade and eventually even between and on chips belongs to the photon."

While HP has been researching a variety of potential technologies to enable photonic interconnects, from component-level connections to interconnects that are metres in length, to succeed, photonics needs to be adopted industry-wide. "It is clear that creating a multi-scale optical fabric for computation will be a community effort," says Williams.

Unlike industry photonics initiatives of the past, which were dependent upon eventual trickle-down from low-volume, high-cost proprietary systems, Morris says HP is trying to bring the fundamental advantages of photonic signalling technology to the mass market.

Intel sees the importance of mass producing photonics devices and made a breakthrough last year, announcing it had built the world's first 40gbps Silicon Laser Modulator. Given that Intel and other companies in the semiconductor industry are fully tooled up in the fabrication of silicon chips, this device from Intel shows it may be possible one day to mass produce silicon-based optical devices.

The microprocessor giant has a strategy as part of its multi-core roadmap to provide end users with tera-scale computing, where thousands of processor cores combine to boost desktop and server computing. Intel knows how to put multiple processor cores on silicon chips. And over time its fabrication process will be fine-turned to enable it to embed greater numbers of processor cores on a single chip. But to achieve tera-scale computing, Intel needs an electrically efficient mechanism to link thousands of processors together. Copper is unsuitable, and photonics may be its only option.

Justin Rattner, Intel chief technology officer, says, "We see silicon photonics at the heart of future, low cost optical interconnects for tera-scale computing."

Tera-scale computing may seem a long way off, but photonics has some short-term benefits that could improve datacentre computing. One of the most exciting is in the area of green IT. HP is investigating how photonics could be used to replace the copper connections in blade servers. Copper is not energy-efficient it is increasingly scarce and expensive and mining it can create environmental problems. According to HP, photonic interconnects can improve performance, solve bandwidth problems, and operate at much lower power than conventional electrical switches in copper cicuits. Morris says building servers using optical connections could cut power use annually by 40% worldwide, so the potential benefits of photonics are significant.

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This was first published in July 2008

 

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