Birefringent photonic bandgap optical waveguide

A birefringent elongate waveguide for guiding light, comprises: a core region, comprising an elongate region of relatively low refractive index; and a cladding region, comprising elongate regions of relatively low refractive index interspersed with elongate regions of relatively high refractive index. In a transverse cross-section of the waveguide, a (5) relatively high refractive index boundary region is provided that surrounds the core region and has either (1) at most two-fold rotational symmetry or (2) a rotational symmetry that reduces the rotational symmetry of the waveguide to at most two-fold rotational symmetry. The symmetry of the boundary region results at least in part from azimuthal variations therein, which are substantially uncharacteristic of the cladding region.