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IRG-I Nugget

Wavelength-scalable hollow photonic bandgap fibers

Cross-sectional SEM micrographs at various magnifications of a hollow cylindrical multilayer fiber mounted in epoxy. The hollow core appears black, the polymer layers and cladding grey, and the glass layers bright white.

Conventional solid-core optical fibers require the use of highly transparent materials in order to have low losses. Such materials are limited in number owing to the fundamental limitations associated with the propagation of light through solids, such as absorption, scattering and nonlinear effects. Hollow optical fibers, however, offer the potential to minimize the dependence of light transmission on the fiber material transparency if the light can be made to propagate in the hollow air-core region.

In a recent article [Temelkuran et al., Nature 420, 650 (2002)], members of IRG-I report on the design and drawing of a hollow optical fiber lined with an interior omnidirectional reflecting dielectric mirror. Confinement of light in the hollow core is thus provided by the large photonic bandgaps established by the multiple alternating submicrometer-thick layers of a high-refractive-index glass and a low-refractive-index polymer. The beauty of this design is that the fundamental and high-order transmission windows are determined purely by the layer dimensions and can be scaled from 0.75 to 10.6 microns in wavelength. The MIT group succeeded in drawing tens of meters of hollow photonic bandgap fibers for transmission of carbon dioxide laser light at 10.6 microns wavelength. The transmission losses were found to be less than 1.0 dB/meter, which are orders of magnitude lower than those of the intrinsic fiber material. This is of key importance because it demonstrates, for the first time, that low attenuation can be achieved through structural design rather than high-transparency material selection.

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