Hollow Core Fibers

A hollow-core fiber is an glass fiber that guides light essentially within a hollow region in order that only a minor portion of the optical power propagates within the solid fiber material (typically a glass). According to the standard physical mechanism for guiding light in fiber, this should not be possible: normally, the refractive index of the fiber core has to be higher than that of the encompassing cladding material, and there's no way of obtaining a index of refraction of glass below that of air or vacuum, a minimum of within the optical spectral region. However, another guiding mechanism can be used:

One possibility is to exploit a photonic bandgap, as can be realized in a photonic crystal fiber with a certain structure. Such fibers are also called photonic bandgap fibers. (Note that not all photonic bandgap fibers have a hollow core.)

A particularly simple design (also leading to simplified production) is that of the revolver hollow-core fibers [12, 21] containing a pattern of silica rings (with circular or elliptical cross-section) around the hollow core; those are not using a photonic bandgap and cannot be considered as photonic crystal fibers. The fiber preform can be made relatively simply by arranging a number of silica capillaries, and these result in thin glass membranes after drawing into a fiber. A more refined version contains additional smaller rings nested within the larger rings [20, 29], and can provide further reduced propagation loss. A loss reduction can also be achieved already by slightly separating the tubes, avoiding nodes where they would touch each other. The term negative curvature fibers underline the boundary curvature in a direction opposite that to a ring around the core. Other terms, containing the attribute antiresonant, emphasize the aspect of loss reduction by designing the glass structure for optical anti-resonance, i.e., suitable relative phase changes for reflection at different interfaces.