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

A method of fabricating a hollow-core photonic-bandgap fiber, comprising the steps of:

• providing a stack of capillaries, wherein the stack has a hollow core and comprises core capillaries at a boundary of the core which comprise a plurality of first core capillaries at respective corners of the core boundary and a plurality of second core capillaries intermediate respective ones of the first core capillaries at the core boundary;
• applying pressure differentials between the core, the first core capillaries and the second core capillaries, wherein a pressure is applied to the first core capillaries separately of the second core capillaries, whereby a size of the first core capillaries can be controlled in relation to the second core capillaries;
• and
• reducing the stack to a fiber, wherein the fiber has a hollow core and a cladding which surrounds the core at a core boundary and comprises a lattice or network of struts and interstitial nodes which together define an array of cavities;
• wherein the pressure differentials provide that, in the finished fiber, 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 pitch spacing of the nodes at the core boundary is less than 0.3.