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Optical system stores light as sound

13 Dec 2007

A new technique for storing light pulses as sound waves could improve the performance of optical communications networks.

US researchers have discovered a simple way to "store" light pulses in a material by converting them into sound waves. The technique, which involves just two lasers and a piece of standard optical fiber, could be used to create memory devices that could boost the performance of optical telecommunications networks.

The device is designed to relieve a key bottleneck in optical networks, which transfer vast amounts of data through optical fiber in the form of packets of light pulses. When a network is extremely busy, two packets can arrive at the same node at the same time. This means that one packet needs to be stored for a short period of time so that both packets can be processed in turn.

Existing networks solve that problem by converting the excess pulses into electrical signals, which can be stored in a memory chip before being converted back into light. However, this optical-electronic-optical conversion takes time and generates a lot of heat, which has prompted researchers to find ways to store light pulses without having to convert them to electricity.

Now, Dan Gauthier and colleagues at Duke University and the University of Rochester have discovered a way of storing optical pulses in an optical fiber by converting the pulses into sound waves (Science 318 1748).

Read and write pulses

The researchers achieved this by sending two consecutive 2 ns laser "data" pulses into one end of a glass fiber, while a 1.5 ns "write" laser pulse is sent into the other end. When the data and write pulses collide within the fiber they interfere with each other, which causes the data pulses to transfer nearly all of their energy to the fiber in the form of acoustic waves – a process called stimulated Brillouin scattering.

A few nanoseconds later, a 1.5 ns "read" pulse is fired through the fiber in the same direction as the write pulse. This scatters from the acoustic waves, creating two light pulses that propagate back in the same direction as the original data pulses. Significantly, these pulses have approximately the same width and spacing as the original data pulses.

By varying the delay between the write and read pulses, Gauthier and colleagues were able to "store" data pulses in the fiber for up to 12 ns.

Gauthier told physicsworld.com that unlike other techniques under development for storing light, such as spectral-spatial holography and electromagnetically induced transparency (EIT), their method does not require very cold temperatures and is not limited to specific wavelengths.

However, Gauthier says that further improvements are needed before the technology can be used in a practical device. The team is currently trying to find ways of storing more pulses for longer periods of time, while decreasing the intensities of the read and write pulses required. Gauthier believes that this can be achieved by searching for materials with the right combination of optical and acoustical properties.

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