Methods of manufacturing hollow-core photonic bandgap fibers

A hollow-core photonic-bandgap fibre is fabricated from a stack of capillaries. The stack has a hollow core 405 and the capillaries at a core boundary include first, corner core capillaries 403 and second, intermediate capillaries 403. A differential pressure is applied between the first, corner core and second, intermediate capillaries to control their relative sizes. The stack is reduced to a fibre with a hollow core and a cladding comprising a network of struts and interstitial nodes which define an array of cavities. The pressure differential may result in a fibre with a ratio between a difference in a length of a longest and shortest pitch spacing of the nodes at the core boundary to an average nodal core boundary pitch spacing of less than 0.3. The pressure may be applied separately to the first and second capillaries or first, second and cladding capillaries or first, second and outer capillaries and the core. A first pressure source may be applied to the first capillaries 406, a second pressure source 407 to the second and outer cladding capillaries and a third pressure source 409 to the core. A pre-form stack may be used in fabricating the hollow-core photonic-bandgap fiber.