A semiconductor optical amplifier according to the present invention comprises a lower cladding layer of a first conductive type, an upper cladding layer of a second conductive type, an active layer, and a diffraction grating layer. The lower cladding layer includes first and second regions. The first and second regions are arranged in a predetermined direction, and the upper cladding layer is supported by the first region. The active layer is provided between the first region of the lower cladding layer and the upper cladding layer, and the diffraction grating layer has a diffraction grating. The diffraction grating extends in the predetermined direction. The diffraction grating layer is supported by the second region, and the diffraction grating layer is optically coupled to the active layer.