Published online 13 December 2007 | Nature | doi:10.1038/news.2007.376

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Storing light with sound

Turning optical-fibre messages into sound could help store the information.

Light fantastic: sometimes you need to slow down information.Corbis

The information in a beam of light can be stored for a while by converting it into a sound signal, then reading it back out again as light, researchers have found. The process, which can be done in commercially-available optical fibres, could be used to help make computer processing more efficient in future.

Data can be sent great distances at speed in beams of light: modern optical networks, like those used to transmit information in the internet, can deliver 10 gigabits of information per second. But the information can't be processed as quickly. To get around this, optical bits are turned into electric signals that can be stored for a short time and then turned back into optical signals to be read. But this process generates heat, and as more bits need to be moved around, more heat is going to be generated. In future, even greater speeds of information delivery mean that electronic processing will no longer be viable.

The sound of light

Researchers wondered whether they could get around this problem by turning the signal into sound instead. Daniel Gauthier, at Duke University in Durham, North Carolina, and his colleagues have demonstrated that this might be possible.

They first send optical data as a stream of light pulses into a short piece of standard optical fibre. Into the other end of the fibre they send a different short pulse: the 'write' pulse. When the two sets of pulses collide, they interfere, and an interference pattern is set up in the fibre with areas of high and low intensity. This interference pattern in turn affects the physical properties of the fibre, setting up an acoustic wave because of a phenomenon called electrostriction.

As the light pulse leaves the fibre, the acoustic wave with all its inherited information lags behind: the speed of light in the fibre, at some 200 million metres per second, far exceeds the more sluggish 5,000 metres per second of sound. “The acoustic wave is essentially stationary over the duration of our experiment,” says Gauthier, whose work is published in Science1.

To get the information from the acoustic wave out again, a third light pulse, the 'read' pulse, is sent in. When it reaches the part of the fibre being affected by the acoustic wave, the light scatters in such a way as to regain the information that was left behind by the initial pulse. The newly-formed data pulse leaves the fibre, resuming the journey in the same direction as the original pulse, taking the same information with it.

In the tests done so far, a 2-nanosecond pulse could be held in the fibre for up to 12 nanoseconds.

“What is so cool about this process is that the original data stream is recreated with reasonable fidelity,” says Gauthier: the initial light pulse and the emerging one have nearly the same shape. In theory it is possible to have perfect fidelity. But this relies on a mass of certain conditions that haven't yet been achieved.

Still work to do

A big problem for Gauthier’s system at the moment lies in the power of the read and write pulses. At the moment, this needs to be about 100W. “This is beyond the power levels of most of current optical components — some might simply evaporate,” says Ortwin Hess, at the University of Surrey in Guildford, UK, who recently proposed a different, theoretical way to store light (see How to trap a rainbow). But Hess is sure that the problems can be worked on. “It’s a first proof of principal,” he says. “It underpins the strong need for achieving and finding ways to have optical storage.”

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“I think there is a very realistic chance we can achieve at least a 100-fold reduction [in the peak power] using materials that are currently available,” says Gauthier, “It will just take time and a little money.”

Other light-storage systems, apart from the ones proposed by Gauthier and Hess, involve very cold gases, or only work on a single frequency of light. But Gauthier’s system works at room temperature, with standard optical fibres that work with a wide range of frequencies. "Our method could easily integrate with existing technologies," says Gauthier. 

  • References

    1. Zhu, Z., Gauthier, D. J. & Boyd, R. W. Science 318, 1748-1750 (2007). | Article |
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