An optical fibre comprises a core region (20) and a cladding region (30), characterised in that the fibre includes a metallic or semiconducting region (40) in the core region (20) or the cladding region (30). The metallic or semiconducting regions may be embedded in an insulating matrix material (50). For example, the core region may comprise a semiconductor region that provides gain or a semiconductor region that is a saturable absorber. The fibre may be a photonic crystal fibre (PCF) (10). For example, inclusion of metallic or semiconducting regions 40 in a cladding region (30) of a PCF (10) may provide a more robust photonic band gap.