Abstract: A semiconductor optical amplifier has an optical path from its input to its output through an optical amplification region, the path having a length that exceeds 1 mm. In use, the amplifier is fed at its input with a modulated optical signal that is modulated at a given bit rate, and a target wave, so that the amplifier produces modulation of the target wave according to the modulation of the optical signal. The path length is selected to be longer than that at which optical saturation occurs. The gain of the amplifier exhibits a −3 db bandwidth that extends beyond 20 GHz in respect of the modulation bit rate. The amplifier may also be used for four wave mixing.