Photonic crystal fibers that guide light by virtue of a photonic band gap. Fibers that are formed from materials having higher refractive indices than silica are provided using an optical waveguide including a core with a relatively low refractive index and a photonic band gap structure that can substantially confine light to the core. The structure includes elongate regions of relatively low refractive index interspersed with elongate regions of relatively high refractive index, with the band gap residing above the fifth photonic band of the band gap structure.