A monolithic integrated semiconductor optical element is disclosed. The semiconductor optical element including a substrate, a first waveguide, formed on the substrate, including a first active layer for generating light, and a semi-insulating, grown on the substrate so as to surround a perimeter of the first waveguide layer, including a window area for diffusing the light outputted from the first waveguide and outputting the diffused light. The semiconductor optical elements also includes at least one second waveguide, formed in the window area close to the first waveguide, including a second active layer for detecting a part of the light diffused by the window area.