A device comprises a tapered semiconductor optical waveguide which tapers, at an angle of taper with respect to a longitudinal axis of the waveguide, from a relatively wide region to a relatively narrow region, wherein the angle of taper varies along the length of the tapered waveguide such that at all points along the taper it is no greater than, and within a substantially constant angular separation of, the adiabatic limit of the fundamental mode of the waveguide at that point along the taper.