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.

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 semiconductor laser resonator configured to generate a laser beam includes a gain medium layer including a semiconductor material and comprising at least one protrusion formed by at least one trench to protrude in an upper portion of the gain medium layer. In the semiconductor laser resonator, the at least one protrusion is configured to confine the laser beam as a standing wave in the at least one protrusion.

A semiconductor laser resonator configured to generate a laser beam includes a gain medium layer including a semiconductor material and comprising at least one protrusion formed by at least one trench to protrude in an upper portion of the gain medium layer. In the semiconductor laser resonator, the at least one protrusion is configured to confine the laser beam as a standing wave in the at least one protrusion.

A quantum cascade laser is configured with a semiconductor substrate, and an active layer provided on a first surface of the substrate and having a multistage lamination of unit laminate structures each of which includes an emission layer and an injection layer. 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 constituting an external cavity for generating the first pump light and configured to be capable of changing the frequency .sub.1, outside an element structure portion including the active layer. Grooves respectively formed in a direction intersecting with a resonating direction are provided on a second surface of the substrate.

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 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.

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.