Light without logic

Light without logic

�GOD is light�, says the Bible. Light is also a source of inspiration in computing. Ever since the first optical transistors were developed in the late 1980s, researchers have dreamed of building a light-powered computer, radiating with knowledge. Yet this breakthrough has proved elusive. Now, however, new developments mean that optical technologies are starting to appear inside computers. The all-optical computer remains a dream, but selected components that can work with light will make their way into computers ever more deeply.

It is easy to see the attraction of replacing electrons, which travel along copper wires and make todays computers tick, with photons. These particles of light are the fastest things in the universe, so an optical computer could theoretically process information at speeds that make even a supercomputer look glacial. So far, however, optical technology has been confined mostly to telecoms networks and some of the cabling in data centres. Photons are ideal for piping information over long distances. They whizz through optical fibres, rarely getting lost or interfering with one another (which is why different coloured signals can be sent down a single fibre, to multiply its capacity).

But at each end of the fibre, optical signals must be converted to and from the electrical signals that computers use to process information. The components that do such conversion are expensive. This does not matter in a network, where costs can be spread among many users. But this expense has kept optical data-links from being used inside personal computers and servers. That is now changing because computer systems are outrunning their electrical wiring. Peripheral devices like printers, hard drives and screens are getting more demanding; networks are running faster and, most importantly, the power of processors continues to increase exponentially. The so-called �interconnects� between all these components are struggling to keep up. It is in this area where a number of new optical alternatives are emerging from some of the biggest firms in the business.

One of these new interconnects, called Light Peak, has been developed by Intel. It is being used to give ordinary PCs the ability to connect with other devices using high-speed optical cables at ten gigabits per second�20 times faster than a standard USB cable. This means the cable could drive a high-definition display or transfer a movie in seconds. Light Peak, predicts Mario Paniccia, the head of Intels photonics lab, will make optical connections as pervasive as wireless ones�and drive demand for more powerful processors, which explains Intels interest.

Data in a flash :

Intel did not have to invent anything new, but it did have to work out how to make small, cheap versions of the converters that turn electrical signals into light and vice versa. Having developed a simplified, low-cost chip to do the job, Intel also devised ways to assemble and test the components quickly, and signed up a group of suppliers to churn them out by the million, starting next year.

Hewlett-Packards concern is keeping its servers competitive: their cabling is getting bulkier, and data centres are becoming much harder to cool and increasingly energy hungry. So it is developing an optical replacement for the interconnects in server �racks�. Instead of optical fibre, HP is using waveguides�small strips of plastic with grooves on their highly reflective metallic walls. Again, using this technology to transmit light is not a new thing, but HPs researchers have managed to cut costs by making waveguides with an injection-moulding system similar to that used to mass-produce CDs.

Over at IBM, researchers are using optical interconnects to make supercomputers run faster. To speed up the flow of data, electrons need to be turned into photons �as close as possible to where the signal is processed�, explains Bert Offrein of IBM Research. For this reason, it is mounting fibre-optic cables straight onto the chips that direct the traffic between a supercomputers multiple processors.

The idea of using similar optical interconnects between a computers various components is, based on existing technology, not something that is about to appear in humble home or office PCs any time soon. It is hard to make such components small and cheap enough to compete with copper wiring. But one technology that does show promise in making such connections is called �silicon photonics�. It uses similar methods to those employed to manufacture processors and other types of integrated circuits.

Conveniently, silicon is not a bad material for making optical devices. Researchers at HP Labs recently managed to etch a pattern into a flat piece of silicon so that it could focus light �like a spoon�, says Raymond Beausoleil of HP Labs. This effect, he says, could be used to improve lasers and replace expensive lenses in DVD players and other consumer products.

For its part, IBM has used silicon to develop a fast and extremely thin photodetector to convert optical signals into electrical ones. And Intel has come up with an entire kit of tiny optical devices made of silicon, which it hopes one day to combine on optical chips, such as waveguides and lasers. But one vital building block is missing from Intels kit: an optical equivalent of the transistors that perform the logical operations at the heart of a computer.

This missing bit does not surprise David Miller of the Photonics Research Centre at Stanford University. Optical transistors, he says, will have a hard time competing with electrical ones, not least because there is no agreement over the best way to build them. Various techniques for making optical transistors regularly appear in laboratories. But using light to process information is tricky, requires exotic materials and lasers that demand more power than conventional transistors. Moreover, miniaturisation is not straightforward, not least because lasers cannot be made as small as transistors. So mass-produced optical processors remain far off. But at least the other bits are on the way.

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