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 fibers, 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.

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 Science ¹.

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.

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.”

“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.