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 the capillaries at a boundary of the core comprise a plurality of first, corner core capillaries (403") and a plurality of second, intermediate core capillaries at the boundary of the core intermediate respective ones of the corner core capillaries (403"); characterized by: applying pressure differentials between the core, the corner core capillaries (403") and the intermediate core capillaries, wherein a pressure is applied to the corner core capillaries (403") separately of the intermediate core capillaries, whereby a size of the corner core capillaries (403") can be controlled in relation to the intermediate 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 (109, 109') and interstitial nodes (111, 111') which together define an array of cavities; wherein the pressure differentials are controlled to provide that, in the finished fiber, a ratio between a difference in a length of a longest and shortest pitch spacing (ΔI) of the nodes at the core boundary to an average pitch spacing of the nodes at the core boundary is less than about 0.3. providing a stack of capillaries, wherein the stack has a hollow core and the capillaries at a boundary of the core comprise a plurality of first, corner core capillaries (403") and a plurality of second, intermediate core capillaries at the boundary of the core intermediate respective ones of the corner core capillaries (403"); characterized by: applying pressure differentials between the core, the corner core capillaries (403") and the intermediate core capillaries, wherein a pressure is applied to the corner core capillaries (403") separately of the intermediate core capillaries, whereby a size of the corner core capillaries (403") can be controlled in relation to the intermediate 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 (109, 109') and interstitial nodes (111, 111') which together define an array of cavities; wherein the pressure differentials are controlled to provide that, in the finished fiber, a ratio between a difference in a length of a longest and shortest pitch spacing (ΔI) of the nodes at the core boundary to an average pitch spacing of the nodes at the core boundary is less than about 0.3. applying pressure differentials between the core, the corner core capillaries (403") and the intermediate core capillaries, wherein a pressure is applied to the corner core capillaries (403") separately of the intermediate core capillaries, whereby a size of the corner core capillaries (403") can be controlled in relation to the intermediate 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 (109, 109') and interstitial nodes (111, 111') which together define an array of cavities; wherein the pressure differentials are controlled to provide that, in the finished fiber, a ratio between a difference in a length of a longest and shortest pitch spacing (ΔI) of the nodes at the core boundary to an average pitch spacing of the nodes at the core boundary is less than about 0.3.