Patent ID: 7532798

Claim:
An elongate waveguide for guiding light comprising: a core comprising an elongate region of relatively low refractive index; a microstructured region around the core comprising elongate regions of relatively low refractive index interspersed with elongate regions of relatively high refractive index; and a boundary at the interface between the core and the microstructured region, the boundary comprising, in the transverse cross-section, a boundary region of relatively high refractive index, which is connected to the microstructured region at a plurality of nodes, and at least one relatively enlarged region around the boundary, said enlarged region having a major dimension and a minor dimension, wherein the length of the major dimension divided by the length of the minor dimension is more than 3.0; wherein said microstructured region comprises a photonic band-gap structure and wherein said relatively low refractive index regions make up more than 90% by volume of the photonic bandgap structure.