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Optical fiber overcomes diffraction

13 Jan 2003

Australian researchers design an optical fiber that focuses light in the far field without using lenses.

Researchers in Australia have designed an air-silica optical fiber that overcomes free-space diffraction. The team from Sydney University's Optical Fibre Technology Centre says its Fresnel fiber acts as an efficient waveguide for high intensity light and focuses light in the far field for the first time.

Normally, light emerging from a fiber diffracts out and expands. But thanks to the new design, the Fresnel fiber focuses light without the need for lenses at the end of the fiber. John Canning, who leads the research, says the fiber is an alternative to photonic crystal fibres and transmits at the 1550 nm telecom wavelength.

The Fresnel fiber is made up of central hole surrounded by a series of concentric rings of holes. The concentric holes have an effective higher refractive index than the central one. If a laser is fired at this structure, superposition occurs and the beam is concentrated in the central hole. The output from the fiber has a Gaussian profile.

"We can get peak intensity of light guiding by scattering off one ring of holes to generate a mode with a peak intensity in the center of the central hole," explained Canning. "This principle of superposition also means we can get focusing in the far field, the first time this has been demonstrated." This is what Canning describes as a supermode.

The results will be reported in Optics Letters in February.

Phillip Hill is a contributing editor at optics.org and editor of Displays Europe magazine.

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