A reflective semiconductor optical amplifier includes a substrate, a waveguide with a buried heterostructure formed by sequentially laminating a lower cladding, an active layer, and an upper cladding on the substrate, the waveguide including, sequentially, respective straight line, curved and tapered waveguide regions. A current blocking layer surrounds the waveguide to prevent electric current from flowing outside the active layer. Selectively etching portions of the current blocking layer and the substrate around the waveguide forms a trench to reduce parasitic capacitance. Further features include a window region on one end of the tapered waveguide region, an anti-reflection surface on one end of the window region, and a high-reflection surface on one end of the straight line waveguide region.