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Optical physics

Fibres control the speed of light

23 Aug 2005

Physicists in Switzerland have found a way to control the speed at which laser pulses travel along an optical fibre based on off-the-shelf technology. The work could have applications in optical delay lines, optical memories and, ultimately, the development of an all-optical router (App. Phys. Lett. 87 081113).

Over the past decade physicists have used exotic media such as ultracold atomic gases and various crystals to make “slow” or “fast” light. Some groups have managed to stop and store light, while others have demonstrated group velocities greater than the speed of light in vacuum.

Although some of these techniques have worked at room temperature, they have not been suitable for deployment in a fibre-optic network. Now, however, Miguel González-Herráez, Kwang-Yong Song and Luc Thévenaz of the École Polytechnique Fédérale de Lausanne (EPFL) have shown that slow and fast light can be made by exploiting an effect known as stimulated Brillouin scattering (SBS) in an ordinary optical fibre.

“Using this simple and flexible approach we have achieved nearly all the results obtained [previously by others] using atomic transitions”, says Thévenaz. “This experiment can be realized on a tabletop in normal environmental conditions so it could be a platform for the development of a wide range of applications in the real world.”

In addition to being more practical, says González-Herráez, who is based at the University of Alcalá in Spain, the fibre-based approach also has a bandwidth that is 10 times greater than that of the other approaches.

In stimulated Brillouin scattering (SBS) a “pump” laser can be used to amplify a “probe” laser at a slightly longer wavelength, with the difference in the energy of the pump and probe photons being equal to the energy of acoustic waves or vibrations called phonons in the fibre.

However, by adjusting the pump power and the wavelength of the probe beam, Thévenaz and co-workers found that the SBS effect could also increase or decrease the speed of the probe pulses. They managed to reduce the speed of light by a factor of 4.3 and, under different conditions, increase it by a factor of 1.4.

One problem with the current approach is that the strength of the probe pulse varies with its speed. The team also wants to increase the bandwidth even further.

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