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The problem of transmitting light through fibers has been synonymous with the identification of low loss materials, since the transparency of a conventional index-guided fiber cannot exceed that of its constituents. To date, precious few materials have been shown to exhibit high transparency at any wavelength and none have been found to exhibit it over wide wavelength ranges. The focus of our research at MIT has been on the elucidation of strategies for creating optical fibers where transparency is determined by the fiber microstructure, the ultimate goal being to create a fiber that is "more transparent than the materials that it is made from." This talk will present the underlying physics, materials selection and processing methodology which have resulted in the design and draw of hollow fibers lined with an interior omnidirectional dielectric mirror. A pair of glassy materials with substantially different indices of refraction but with similar thermo-mechanical properties was used to construct a fiber with multiple layers of alternating high and low refractive indices surrounded by a tough polymer cladding. Confinement of light in the hollow core is provided by the large photonic bandgaps established by the multiple alternating sub-micrometer-thick layers of a high-refractive-index chalcogenide glass and a low-index polymer. The fundamental and higher order transmission windows are determined by the layer thicknesses and can be scaled from 0.75 to 10.6 microns in wavelength. Recent transmission loss measurements were found to be orders of magnitude lower than those of the intrinsic fiber material, thus demonstrating that low attenuation can be achieved by structural design rather than high-transparency material selection. Scaling laws as well as device applications will be surveyed, recent results pertaining to CO2 laser transmission will be presented, and future directions will be discussed.
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