Optical semiconductor device

An optical semiconductor device comprising:

• a semiconductor substrate having a (100) plane orientation and being of a first conductivity type;
• a mesa that is provided on a surface in a <011> direction of the semiconductor substrate, and includes a first cladding layer of the first conductivity type, an active layer, and a second cladding layer of a second conductivity type;
• a semi-insulating buried layer that buries both sides of the mesa, is provided on the semiconductor substrate, and includes a first region and a second region that is arranged farther from the mesa than the first region, the buried layer having a surface including a surface of the first region and a surface of the second region;
• an insulation film placed in physical contact with the surface of the first region and the surface of the second region of the buried layer;
• and
• an electrode placed in physical contact with a surface of the mesa and the surface of the buried layer in the first region,
• wherein the surface of the first region is arranged at a height lower than a height of the surface of the mesa, and the surface of the second region is arranged at a height higher than a height of the surface of the first region.