An optical amplifier for accepting input light and outputting amplified light, comprising: A) an optical waveguide for accepting input light and outputting amplified light; B) a gain medium (10;17) comprising a plurality of adjoining semiconductor layers to provide optical gain: C) electrodes (1,8;12,19) to pump electric current through the gain medium; wherein the plurality of adjoining semiconductor layers define one or more quantum wells for electrons and are configured to provide both direct and indirect electron-hole transitions in the gain medium whereby; i) a first quantised electron energy level in the conduction band is located in a first layer; ii) a first quantised hole energy level in the valence band is located in said first layer; iii) a further first quantised hole energy level in the valence band is located in an adjacent second layer; and the second layer has a different material composition to the first layer; characterized in that: the second layer is in contact with the first layer; the first and second layers are configured such that the direct electron-hole transitions are between the conduction band in the first layer and the first quantized hole energy level of the first layer and the indirect electron-hole transitions are between the conduction band in the first layer and the first quantized hole energy level of the second layer, and the first quantised hole energy level in the first layer is a light hole state and the further first quantised hole energy level in the second layer is a heavy hole state. A) an optical waveguide for accepting input light and outputting amplified light; B) a gain medium (10;17) comprising a plurality of adjoining semiconductor layers to provide optical gain: C) electrodes (1,8;12,19) to pump electric current through the gain medium; i) a first quantised electron energy level in the conduction band is located in a first layer; ii) a first quantised hole energy level in the valence band is located in said first layer; iii) a further first quantised hole energy level in the valence band is located in an adjacent second layer; and the second layer has a different material composition to the first layer; characterized in that: the second layer is in contact with the first layer; the first and second layers are configured such that the direct electron-hole transitions are between the conduction band in the first layer and the first quantized hole energy level of the first layer and the indirect electron-hole transitions are between the conduction band in the first layer and the first quantized hole energy level of the second layer, and the first quantised hole energy level in the first layer is a light hole state and the further first quantised hole energy level in the second layer is a heavy hole state. the second layer is in contact with the first layer; the first and second layers are configured such that the direct electron-hole transitions are between the conduction band in the first layer and the first quantized hole energy level of the first layer and the indirect electron-hole transitions are between the conduction band in the first layer and the first quantized hole energy level of the second layer, and the first quantised hole energy level in the first layer is a light hole state and the further first quantised hole energy level in the second layer is a heavy hole state.