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 patt...