Summary form only given. Despite their tremendous success optical fibers of today are limited by the laws of total internal reflection. During the past five years, however, it has become increasingly evident, that a new operational principle of optical fibers is possible, namely guidance due to photonic bandgap (PBG) effect. Photonic bandgaps are forbidden photon energy intervals, which may be displayed by periodic dielectric structures (photonic crystals), and correspond to the electronic bandgaps of semiconductor crystals. Such PBGs may exist in periodic silica/air structures. While it soon became evident that it is possible to guide light in low index core regions using these fibers, it was only recently proven that it is possible to guide light almost entirely within an air-core using PEG fibers. We discuss the theory of such optical fibers and their unique properties. The transverse design of the analyzed fiber is depicted. The central air hole corresponds to the core of the fiber, while the surrounding periodic air/silica region is the cladding structure. The fiber is assumed invariant in the longitudinal direction.
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