Abstract: An optical amplifier with a semiconductor body comprising a layer structure grown on a substrate 1 with an active layer 4 situated between two cladding layers (2, 3) with a strip-shaped amplification region bounded by two end surfaces (7, 8) which form the input and output surfaces of the radiation to be amplified. The active layer comprises a number of quantum well (QW) layers 4A with direct band transition, which are separated by barrier layers 4B of a different semiconductor material. The material, the number, and the thickness of the QW layers 4A and the barrier layers 4B in combination with the length 1 of the amplification region are chosen in such a way that two optical transitions can take place in the active layer 4, maximum amplification occurring at a certain current density through the PN junction for the radiation wavelengths corresponding to these optical transitions, while the end surfaces (7, 8) are provided with an anti-reflection layer 9 which has a reflection coefficient R of at most 0.