Identifying hollow waveguide guidance in air-cored microstructured optical fibres

An analysis of leaky modes in real microstructured optical fibres fabricated specifically for photonic band gap guidance in an air core has been used to identify alternative guiding mechanisms. The supported leaky modes exhibit properties closely matching a simple hollow waveguide, uninfluenced by the surrounding microstructure. The analysis gives a quantitative determination of the wavelength dependent loss of these modes and illustrates a mechanism not photonic band gap in origin by which colouration can be observed in such fibres. These findings are demonstrated experimentally.