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

1. 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.

2. The method of claim 1 , wherein the stack comprises cladding capillaries outwardly of the core capillaries, and a pressure is applied to the first core capillaries separately of the second core capillaries and the cladding capillaries.

3. The method of claim 1 , wherein the stack comprises cladding capillaries outwardly of the core capillaries, and pressure is applied to the first core capillaries separately of the second core capillaries, the cladding capillaries and the core.

4. The method of claim 3 , wherein a first pressure source is applied to the first core capillaries, a second pressure source is applied to the second core capillaries and the cladding capillaries, and a third pressure source is applied to the core.

5. The method of claim 1 , wherein a greater pressure is applied to the second core capillaries than to the first core capillaries.

6. The method of claim 1 , wherein a pressure differential of at least 0.1 kPa is applied between the second core capillaries and the first core capillaries.

7. The method of claim 6 , wherein a pressure differential of at least 0.1 kPa and less than 2 kPa is applied between the second core capillaries and the first core capillaries.