Provided is an optical semiconductor device which has long-term reliability since a threshold current is small, and a relaxation oscillation frequency is high. An optical semiconductor device includes an InP semiconductor substrate, a lower mesa structure that is disposed above the InP semiconductor substrate, and includes a multiple quantum well layer, an upper mesa structure that is disposed on the lower mesa structure, and includes a cladding layer, a buried semiconductor layer that buries both side surfaces of the lower mesa structure, and an insulating film that covers both side surfaces of the upper mesa structure by being in contact with both side surfaces of the upper mesa structure, in which the lower mesa structure includes a first semiconductor layer, above the multiple quantum well layer, and the upper mesa structure includes a second semiconductor layer which is different from the cladding layer in composition, below the cladding layer.

A quantum cascade laser is configured with a semiconductor substrate, and an active layer having a multistage lamination of emission layers and injection layers. The active layer is configured to be capable of generating first pump light of a frequency .sub.1 and second pump light of a frequency .sub.2, and to generate output light of a difference frequency by difference frequency generation. An external diffraction grating is provided for generating the first pump light, outside an element structure portion including the active layer, and an internal diffraction grating is provided for generating the second pump light, inside the element structure portion. The frequency .sub.2 is set to be fixed to a frequency not coincident with a gain peak, and the frequency .sub.1 is set to be variable to a frequency different from the frequency .sub.2.

A quantum cascade laser is configured with a semiconductor substrate, and an active layer having a multistage lamination of emission layers and injection layers. The active layer is configured to be capable of generating first pump light of a frequency .sub.1 and second pump light of a frequency .sub.2, and to generate output light of a difference frequency by difference frequency generation. An external diffraction grating is provided for generating the first pump light, outside an element structure portion including the active layer, and an internal diffraction grating is provided for generating the second pump light, inside the element structure portion. The frequency .sub.2 is set to be fixed to a frequency not coincident with a gain peak, and the frequency .sub.1 is set to be variable to a frequency different from the frequency .sub.2.

A semiconductor optical device includes: a first conductive type semiconductor layer; an active layer; a second conductive type semiconductor layer including a ridge portion; a pair of first grooves, formed on bottom surfaces of both sides of the ridge portion and dividing the active layer; an optical functioning part including the first and second conductive type semiconductor layers, converting a state of light, and having a height higher than a height of the bottom surface of the ridge portion; and a second groove, at least a part thereof being formed on the optical functioning part, an end portion thereof being connected to the first groove, the second conductive type semiconductor layer being divided, and the maximum height of an inner wall surface thereof being higher than the maximum height of an inner wall surface of the first groove.

A semiconductor optical device includes: a first conductive type semiconductor layer; an active layer; a second conductive type semiconductor layer including a ridge portion; a pair of first grooves, formed on bottom surfaces of both sides of the ridge portion and dividing the active layer; an optical functioning part including the first and second conductive type semiconductor layers, converting a state of light, and having a height higher than a height of the bottom surface of the ridge portion; and a second groove, at least a part thereof being formed on the optical functioning part, an end portion thereof being connected to the first groove, the second conductive type semiconductor layer being divided, and the maximum height of an inner wall surface thereof being higher than the maximum height of an inner wall surface of the first groove.

A semiconductor laser may include an off-cut substrate having a cut angle of at least approximately 6 degrees (). The semiconductor laser may include an epitaxial structure over the off-cut substrate. A first sidewall formed by the off-cut substrate and the epitaxial structure may be parallel to a second sidewall formed by the off-cut substrate and the epitaxial structure. A front facet formed by the off-cut substrate and the epitaxial structure may be parallel to a back facet formed by the off-cut substrate and the epitaxial structure. The cut angle of the off-cut substrate may cause the first sidewall to be non-perpendicular to epitaxial layers of the epitaxial structure. The cut angle of the off-cut substrate may cause the second sidewall to be non-perpendicular to the epitaxial layers of the epitaxial structure.

A semiconductor laser may include an off-cut substrate having a cut angle of at least approximately 6 degrees (). The semiconductor laser may include an epitaxial structure over the off-cut substrate. A first sidewall formed by the off-cut substrate and the epitaxial structure may be parallel to a second sidewall formed by the off-cut substrate and the epitaxial structure. A front facet formed by the off-cut substrate and the epitaxial structure may be parallel to a back facet formed by the off-cut substrate and the epitaxial structure. The cut angle of the off-cut substrate may cause the first sidewall to be non-perpendicular to epitaxial layers of the epitaxial structure. The cut angle of the off-cut substrate may cause the second sidewall to be non-perpendicular to the epitaxial layers of the epitaxial structure.

A semiconductor light-emitting element includes a multilayer body including a first end surface and a second end surface which are opposed to each other, wherein a first semiconductor layer, a light emitting layer, and a second semiconductor layer are stacked; a pair of recesses that are formed on the second semiconductor layer, separated from the second end surface, and separated from each other in the direction parallel to the first and second end surfaces; a ridge portion that is a protrusion between the pair of recesses and extends along the direction perpendicular to the first and second end surfaces; a band-shaped electrode disposed on the ridge portion; and a light guide layer formed on the second semiconductor layer between the ridge portion and the second end surface and guides light from the light emitting layer.

A semiconductor light-emitting element includes a multilayer body including a first end surface and a second end surface which are opposed to each other, wherein a first semiconductor layer, a light emitting layer, and a second semiconductor layer are stacked; a pair of recesses that are formed on the second semiconductor layer, separated from the second end surface, and separated from each other in the direction parallel to the first and second end surfaces; a ridge portion that is a protrusion between the pair of recesses and extends along the direction perpendicular to the first and second end surfaces; a band-shaped electrode disposed on the ridge portion; and a light guide layer formed on the second semiconductor layer between the ridge portion and the second end surface and guides light from the light emitting layer.

A tunable laser source includes a mirror, a tunable filter, and a semiconductor optical amplifier integrated device including first, second, and third semiconductor optical amplifiers between a first end face facing toward the tunable filter and a second end face facing away from the first end face. The first amplifier is closer to the first end face than the second and third amplifiers. The semiconductor optical amplifier integrated device further includes a partially reflecting mirror and an optical divider that are disposed between the first amplifier and the second and third amplifiers. The partially reflecting mirror is closer to the first amplifier than the optical divider. The optical divider includes first and second branches connected to the second and third semiconductor optical amplifiers, respectively. The tunable filter and the first amplifier are disposed in an optical path between the partially reflecting mirror and the mirror that form a laser resonator.