A semiconductor optical amplifier for amplifying input optical signals is disclosed. The optical amplifier includes a substrate; a first active layer laminated on the substrate for generating pumping lights; a second active layer laminated on the substrate being gain-clamped by the pumping light and amplifying the input optical signals; and a grating formed on an upper portion of the substrate, adjacent to a boundary between the first active layer and the second active layer, for partially allowing the transmission of the pumping lights to the second active layer and partially reflecting the pumping lights.