Data trips between light and sound
As you probably are aware, future communications networks will certainly be based on optics. A research team led by Duke University physicists has done an important discovery which might lead to these future super-fast optical communications networks. The team has found a way to store information coming from a beam of light by converting it to sound waves. More importantly, it was able to retrieve it again as light waves. These reversible data transfers from light to sound are today limited to labs. Several years will pass before commercial companies can use this technique because there are still some technical issues to solve. But read more...
The figure above describes how data can flow between light and sound and light again. "Storage of data pulses as an acoustic disturbance in an optical fiber and their subsequent retrieval after a controllable time interval. In the storage process, a short, intense write pulse that is detuned to the low-frequency side of the data-pulse frequency by the Brillouin frequency shift causes the data pulses to become depleted (A) with the information being stored as an acoustic wave in the medium (B). In the retrieval process (C), a short, intense read pulse at the same frequency as the write pulse depletes the acoustic wave and converts the data back to the original optical frequency, thereby producing a replica of the incident data pulses (D). (Credit: Zhaoming Zhu, Daniel Gauthier and Robert Boyd)
This new step towards fast optical communications networks has been the idea of Zhaoming Zhu, a postdoctoral research associate in physics professor Daniel Gauthier's research group at Duke University. They were joined by Robert Boyd, professor of optics and physics at the Institute of Optics at the University of Rochester.
This new method of manipulating data uses a phenomenon called "stimulated Brillouin scattering." For those of you who -- like me -- didn't know what was "Brillouin scattering" before today, here is a link to the Wikipedia page about it. Here is the introduction. "Brillouin scattering occurs when light in a medium (such as water or a crystal) interacts with density variations and changes its path. The density variations may be due to acoustic modes, such as phonons, or temperature gradients. As described in classical physics, when the medium is compressed its index of refraction changes and the light's path necessarily bends."
Here is a short excerpt from the Duke University news release about their experiments. "'To efficiently create such acoustic waves, you have to have two laser beams of slightly different frequencies interacting with each other,' Gauthier said. In a series of experiments at Duke, Zhu found that if he encoded information onto one of those laser beams, the data could be imprinted on newly-created phonons. Such phonon sounds are much too high-pitched for humans to hear, Gauthier said. Zhu documented that phonons could retain the data for as long as 12 billionths of a second. The information could then be successfully re-transferred from sound to light again by shining a third laser beam through the fiber.
In an article published by Nature News, "Storing light with sound," Katharine Sanderson provides additional details. "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."
Sanderson also describes the issues facing this technique. "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.
In the Duke University news release, Gauthier says that there is another problem to solve. "The other issue is that we're only storing the data for about 10 nanoseconds. There may be a few applications where such short storage times would be okay. But, for many applications, you would like to store it for seconds."
Still, this research work is promising. As wwrites Sanderson, "Gauthier’s system works at room temperature, with standard optical fibres that work with a wide range of frequencies."
If you want to know more about this research work, it has been published in Science under the title "Stored Light in an Optical Fiber via Stimulated Brillouin Scattering" (Volume 318, Number 5857, Pages 1748-1750, December 14, 2007). Here are two links to the abstract and a reprint of the full paper (PDF format, 4 pages, 422 KB) from which the above figure has been extracted.
Sources: Duke University news release, December 14, 2007; Katharine Sanderson, Nature News, December 13, 2007; and various websites
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