Elongate photonic bandgap waveguide with low refractive index core

An elongate waveguide 400 for guiding light comprises a core 410, comprising an elongate region of relatively low refractive index; and a photonic band-gap (PBG) structure 420 that surrounds the core 410. The PBG structure 420 is arranged to provide a photonic band-gap H that can substantially confine light to the core 410. The PBG structure 420 comprises elongate regions of relatively low refractive index interspersed with elongate regions of relatively high refractive index. The band-gap H may reside above the fifth photonic band of the band-gap structure 420 eg. between bands 8 and 9 as shown.