A semiconductor optical amplifier comprises a substrate (1) and an active layer (4) constructed as a quantum well structure. The active layer includes a plurality of well layers (14,15) and a plurality of barrier layers (13a,13c) which are alternately layered. The composition of each well layer (14,15) is such that the well layers have different quantum levels. The barrier layers are subjected to biaxial strain to shift the energy band structures so as to eliminate the polarization dependency of optical gain in the active region.