OPTICAL SEMICONDUCTOR DEVICE

An optical semiconductor device includes: a mesa stripe structure including an n-type cladding layer, an active layer, and a p-type cladding layer laid one on another; and a buried layer buried on opposite sides of the mesa stripe structure, wherein the active layer is a multiple quantum well structure having well layers and carbon-doped barrier layers, the buried layer includes a p-type semiconductor layer and an Fe-doped or Ru-doped high-resistance semiconductor layer laid one on another, side surfaces of the n-type cladding layer are covered with the p-type semiconductor layer and are not contiguous with the high-resistance semiconductor layer, and side surfaces of the active layer are not contiguous with the p-type semiconductor layer.