Transmission of light in hollow-core photonic crystal fiber (HC-PCF) is traditionally based on the out-of-plane photonic bandgap of the crystal cladding that usually consists of periodically arranged air holes in silica glass. A new kind of wave guidance relying on a special point (Dirac point) of eigenfrequency dispersion plot in high-index glass-based HC-PCF has been reported recently. Pure silica-based HC-PCF is not suitable for this kind of guidance. The existence of the Dirac point in a triangular lattice of air holes surrounded by a germania (GeO2)-doped high-index ring in pure silica host has been demonstrated. The addition of the high-index layer provides a control over the density of states around the Dirac point. The operating wavelength can be tuned precisely by controlling the doping concentration and thickness of the doped glass rings. The structure is very simple and can be fabricated through a conventional stack and draw method. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
