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Brillouin effect controls speed of light

23 Aug 2005

A nonlinear effect in silica fiber could bring big benefits to the world of optical networking and computing.

Scientists in Switzerland have found a way to tune the speed at which light pulses travel along an optical fiber. The development could lead to a elegant solution for constructing optical delay lines, optical memory and ultimately an all-optical router (Applied Physics Letters 87 081113).

Although the speed of light has been slowed down and even brought to a halt by other research teams previously it has relied on complicated atomic effects in gases, crystals and semiconductors which do not suit deployment in an optical fiber network (see related stories).

Now, scientists from the École Polytechnique Fédérale de Lausanne (EPFL) have shown that Stimulated Brillouin Scattering (SBS) in ordinary singlemode silica fiber could be an attractive alternative.

“Using this simple and flexible approach we have achieved nearly all the results obtained [previously by others] using atomic transitions, from delays widely exceeding the pulse duration to superluminal propagation and even negative group velocity,” Luc Thévenaz from EFPL’s Nanophotonics and Metrology Laboratory told Optics.org.

“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.”

The EPFL approach uses SBS -- a nonlinear optical effect that causes a strong pump beam to amplify a slightly longer wavelength probe beam and generate an acoustic wave simultaneously.

Although SBS has been used in the past to make optical amplifiers, the Swiss team found that by adjusting the pump power and the wavelength of the probe beam, the SBS effect could also delay or speed up the propagation of probe pulses.

Experiments with 100 ns probe pulses at 1552 nm showed that about 1 ns of delay/advancement is introduced for every decibel of Brillouin gain/loss. To date, using a pump power of several watts the team achieved an adjustable delay of between -14.4 and +18.6 ns in a 2 m long fiber -- equivalent to an effective fiber length from -3 m to +3.8 m.

And Thévenaz says that larger delays can be achieved by cascading separate lengths of fiber and using attenuators to remove unwanted amplified noise. He believes that a 1m long fiber containing 10 equally-spaced attenuators could produce a delay of 300 ns.

However, before the technique moves out of the lab, the team is working to solve two problems. The first is that the amount of delay depends on the amount of amplification and this results in a data stream of varying strength which is not desirable. The second is that the narrow bandwidth of the Brillouin effect limits its use to signals with a bandwidth of no more than 25 MHz.

Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.

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