A hollow core photonic bandgap optical fibre comprising: a cladding comprising capillaries in a hexagonal array, the capillaries separated from each other by struts connected at nodes, the cladding having a cladding diameter; and a hollow core formed by excluding a hexagonal group of nineteen capillaries from the centre of the hexagonal array, the core having a core diameter; wherein the core is bounded by a first ring of capillaries comprising corner capillaries disposed adjacent to corners of the excluded group and side capillaries positioned between the corner capillaries; and the hexagonal array has dimensions such that: a core size ratio is between 1.34 and 1.82 inclusive; where the core size ratio is defined as a ratio of the core diameter to the cladding diameter normalised to the ratio of the core diameter to the cladding diameter in an optical fibre formed from the same number and arrangement of capillaries in an undistorted hexagonal array; a first ring ratio is between 0.65 and 2.20 inclusive; where the first ring ratio is defined as a ratio of the mean length of radially aligned struts separating the capillaries of the first ring to the length of a strut in an undistorted hexagonal array with a core size ratio of 1; and a core node spacing is between 0.60 and 1.90 inclusive; where the code node spacing is defined as a ratio of a strut length around the core of a largest corner capillary and a strut length around the core of a smallest side capillary. a cladding comprising capillaries in a hexagonal array, the capillaries separated from each other by struts connected at nodes, the cladding having a cladding diameter; and a hollow core formed by excluding a hexagonal group of nineteen capillaries from the centre of the hexagonal array, the core having a core diameter; wherein the core is bounded by a first ring of capillaries comprising corner capillaries disposed adjacent to corners of the excluded group and side capillaries positioned between the corner capillaries; and the hexagonal array has dimensions such that: a core size ratio is between 1.34 and 1.82 inclusive; where the core size ratio is defined as a ratio of the core diameter to the cladding diameter normalised to the ratio of the core diameter to the cladding diameter in an optical fibre formed from the same number and arrangement of capillaries in an undistorted hexagonal array; a first ring ratio is between 0.65 and 2.20 inclusive; where the first ring ratio is defined as a ratio of the mean length of radially aligned struts separating the capillaries of the first ring to the length of a strut in an undistorted hexagonal array with a core size ratio of 1; and a core node spacing is between 0.60 and 1.90 inclusive; where the code node spacing is defined as a ratio of a strut length around the core of a largest corner capillary and a strut length around the core of a smallest side capillary. a core size ratio is between 1.34 and 1.82 inclusive; where the core size ratio is defined as a ratio of the core diameter to the cladding diameter normalised to the ratio of the core diameter to the cladding diameter in an optical fibre formed from the same number and arrangement of capillaries in an undistorted hexagonal array; a first ring ratio is between 0.65 and 2.20 inclusive; where the first ring ratio is defined as a ratio of the mean length of radially aligned struts separating the capillaries of the first ring to the length of a strut in an undistorted hexagonal array with a core size ratio of 1; and a core node spacing is between 0.60 and 1.90 inclusive; where the code node spacing is defined as a ratio of a strut length around the core of a largest corner capillary and a strut length around the core of a smallest side capillary.