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Holey fiber delivers industrial laser

18 Dec 2002

Photonic bandgap fiber can be tuned to transmit anywhere between 0.75 and 10.6 micron.

Hollow optical fiber fabricated by US scientists can deliver carbon dioxide laser beams with a loss of just 1 dB/m.

The fiber, which is based on a photonic bandgap structure, can be designed to transmit wavelengths anywhere between 0.75 and 10.6 micron. This is because the transmission windows are determined by the thickness of the layers used in the fiber structure, rather than the absorption characteristics of the material used (Nature 420 650).

Yoel Fink and colleagues at the Massachusetts Institute of Technology (MIT) combined two materials to construct the fiber: arsenic triselenide - a glass with a refractive index of 2.8; and a thermoplastic polymer with a refractive index of 1.55.

This high refractive-index contrast leads to large photonic bandgaps and omnidirectional reflectivity, say the scientists. "The large photonic bandgaps result in very short electromagnetic penetration depths within the layer structure, significantly reducing radiation and absorption losses while increasing robustness," they report.

Despite the widespread commercial use of carbon dioxide lasers, waveguides operating at the normal 10.6 micron emission wavelength typically suffer from high losses or cannot be manufactured in sufficient length to be useful industrially.

The MIT team fabricated tens of metres of its fiber, and measured a loss of 0.95 dB/m when they fired a 25 kW carbon dioxide laser built by Coherent subsidiary DEOS, US, through it. In this test, the fiber was fixed at both ends and held straight. Bending losses are estimated at below 1.5 dB for right angles.

According to the team, no damage to the fiber was seen after the experiment (maximum laser power density coupled into the fiber was 300 W/cm2). The fiber could also be constructed to transmit other wavelengths efficiently, such as the 3 micron emission of an erbium laser.

"These results indicate the feasibility of using hollow multilayer photonic bandgap fibers as a low-loss wavelength-scalable transmission medium for high-power laser light," concluded the team in the paper.

Michael Hatcher is technology editor of Opto and Laser Europe magazine.

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