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  • New Holey Fiber Is Easy to Manufacture

Photonics Spectra
Oct 2003
Breck Hitz

Scientists at Virginia Polytechnic Institute and State University in Blacksburg have developed a new form of holey fiber that is easier to fabricate than previous types. They call it "random-hole optical fiber" because the airholes are randomly dispersed through the fiber, rather than periodically spaced as they are in normal holey fiber.

New Holey Fiber Is Easy to ManufactureHoley fibers -- optical fibers with a fine array of airholes running down their length -- can be designed to exhibit optical characteristics not unobtainable with conventional fibers. The dispersion and birefringence of holey fibers, for example, can be much higher or much lower than the values in conventional fibers. And holey fibers can support single-mode operation of a broad range of wavelengths.

But the rigid precision of their structure has made these fibers difficult to fabricate. Generally, a set of many hollow glass tubes must be assembled to create a preform, which is then drawn into a fiber by conventional means.

Normal holey fibers exhibit a photonic bandgap because of the periodicity of their structure, but random-hole optical fiber has no periodicity so does not display this property. Nonetheless, the volume of the airholes is sufficiently large and sufficiently constant over the length of the fiber to provide a consistent lowering of the refractive index of the cladding with respect to the core. And as long as the core of a fiber has a larger refractive index than the cladding, optical power can be confined in the core by conventional total internal reflection.

Moreover, the researchers point to a theoretical analysis that indicates that random-hole optical fiber possesses most of the unique, desirable characteristics of normal holey fiber.

The airholes in random-hole optical fiber do not exist in the preform but are created during the fiber-drawing process. The preform consists of a core of fused silica suspended concentrically in a fused-silica tube. The space between the core and tube is filled with silica powder and a small amount of gas-producing material such as silicon nitride or silicon carbide. When these materials are heated, they oxidize and release N2 and CO2 in the silicon nitride and silicon carbide, respectively.

As the preform is heated and the fiber is drawn, bubbles of N2 or CO2 are formed in the cladding and are drawn into long tubes as the fiber is drawn down to its final diameter. Although the individual tubes do not extend over the length of the fiber, they have very tapered ends, and new tubes begin as others end, so the density of holes remains relatively constant over the length of the fiber. Because the cladding is created from a mixture of powders, virtually any dopant can be added, resulting in another potential degree of freedom in the design of the fiber.

The measured loss of the random-hole optical fiber at 1550 nm was 2.11 dB/m, but the researchers believe that this figure can be lowered by using low-loss synthetic material in the core and by reducing the size of the airholes in the fiber.

fused silica
Glass consisting of almost pure silicon dioxide (SiO2). Also called vitreous silica. Frequently used in optical fibers and windows.
optical fiber
A thin filament of drawn or extruded glass or plastic having a central core and a cladding of lower index material to promote total internal reflection (TIR). It may be used singly to transmit pulsed optical signals (communications fiber) or in bundles to transmit light or images.  
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