Optical semiconductor device

An optical semiconductor device comprising:

• an n-type semiconductor substrate;
• an n-type cladding layer provided on the n-type semiconductor substrate;
• an active layer of a semiconductor laser provided on the n-type cladding layer, the active layer having an uppermost side;
• a waveguide layer of a waveguide provided on the n-type cladding layer, the waveguide layer having a side facing a side of the active layer and having an uppermost side aligned with the uppermost side of the active layer;
• a p-type cladding layer provided on the active layer and the waveguide layer;
• and
• a middle layer provided between the side of the active layer and the side of the waveguide layer, provided between the n-type cladding layer and the waveguide layer, not provided on the active layer, and having a band gap greater than a band gap of the waveguide layer, the middle layer including a first side and a second side opposing the first side, the first and second sides facing the n-type cladding layer and the p-type cladding layer, respectively, and the second side aligned with the uppermost side of the active layer and aligned with the uppermost side of the waveguide layer.