Abstract: An optical semiconductor device includes a substrate, a semiconductor multilayer which is formed on the substrate, and includes an optical functional layer, an insulating film formed on the semiconductor multilayer, and an electrode formed on a part of the insulating film. The insulating film covers the semiconductor multilayer except for a region in which the semiconductor multilayer and the electrode are electrically connected to each other. At least a part of a region of the insulating film that is overlapped with the electrode is thinner than a region of the insulating film that is not overlapped with the electrode.

Abstract: A semiconductor laser includes: a multi-quantum well layer in a mesa structure; a buried layer comprising a semi-insulating semiconductor, the buried layer being in contact with each of both sides of the mesa structure; a first cladding layer with a first conductivity type, the first cladding layer having a lower refractive index than the multi-quantum well layer; a high refractive index layer configured to not absorb light oscillating in the multi-quantum well layer, the high refractive index layer having a higher refractive index than the first cladding layer; a diffraction grating layer at least partially constituting a diffraction grating capable of diffracting the light oscillating in the multi-quantum well layer, the diffraction grating layer not contacting the high refractive index layer; a substrate with the first conductivity type; and a second cladding layer with a second conductivity type above the multi-quantum well layer.