Optical semiconductor device

An optical semiconductor device comprising:

• a lower mesa structure;
• an upper mesa structure on the lower mesa structure,
• wherein the upper mesa structure is narrower than the lower mesa structure;
• a multi-quantum well layer included in the lower mesa structure, the multi-quantum well layer including some well layers and some barrier layers alternately overlapping with each other,
• wherein each of the barrier layers is an undoped layer, and
• wherein an outermost layer is one of the barrier layers;
• an optical confinement layer that is included in the lower mesa structure and has a refractive index that is greater than a refractive index of the outermost layer,
• wherein the optical confinement layer has a band gap that is smaller than a band gap of the outermost layer;
• an InGaAlAs guide layer interposed between the multi-quantum well layer and the optical confinement layer,
• wherein the guide layer directly contacts the optical confinement layer,
• wherein the guide layer includes a first adjacent layer in contact with the outermost layer and the guide layer is thinner than the optical confinement layer, and
• wherein the first adjacent layer of the guide layer has a band gap that is larger than a band gap of the optical confinement layer;
• and
• an InP buried layer surrounding the lower mesa structure.

An optical semiconductor device comprising:

• a lower mesa structure;
• an upper mesa structure on the lower mesa structure,
• wherein the upper mesa structure is narrower than the lower mesa structure;
• a multi-quantum well layer included in the lower mesa structure, the multi-quantum well layer including some well layers and some barrier layers alternately overlapping with each other,
• wherein each of the barrier layers is an undoped layer, and
• wherein an outermost layer is one of the barrier layers;
• an optical confinement layer that is included in the lower mesa structure and has a refractive index that is greater than a refractive index of the outermost layer,
• wherein the optical confinement layer has a band gap that is smaller than a band gap of the outermost layer;
• an InGaAlAs guide layer interposed between the multi-quantum well layer and the optical confinement layer,
• wherein the guide layer directly contacts the optical confinement layer,
• wherein the guide layer includes a first adjacent layer in contact with the outermost layer and the guide layer is thinner than the optical confinement layer,
• wherein each of the optical confinement layer and the guide layer is an n-type semiconductor layer,
• wherein the first adjacent layer of the guide layer has a band gap that is larger than a band gap of the optical confinement layer, and
• wherein the optical confinement layer and the guide layer are included in an n-type separate confinement heterostructure layer;
• and
• an InP buried layer surrounding the lower mesa structure.