Hollow-core photonic bandgap fibers and methods of manufacturing the same

A hollow-core photonic bandgap fiber has a hollow core 103 and a cladding 105 which surrounds the core at a core boundary 107. The cladding has a lattice or network 108 of struts 109,109 and interstitial nodes 111,111 which together define an array of cavities 115. A ratio of the effective diameter of the core to the effective diameter of the cladding is at least about 0.33 when five rings of cavities surround the core. This ratio for other fibres is at least about 0.3 when six rings of cavities surround the core or at least about 0.27 when seven rings of cavities surround the core.