An optical semiconductor element including a semiconductor substrate, a first cladding layer of a first conductive type provided on the semiconductor substrate, an active layer provided on the first cladding layer, a second cladding layer of a second conductive type provided on the active layer, a first mesa constituted of a part of the first cladding layer, the active layer, and the second cladding layer, an auxiliary cladding layer of the second conductive type provided on the first mesa, a second mesa constituted of the auxiliary cladding layer, and a semi-insulating layer provided on the first cladding layer and on both sides of the first mesa and both sides of the second mesa, wherein a width of the second mesa is greater than a width of the first mesa.

There is provided an optical transmitter having high optical feedback resistance even at the time of high-output operation and capable of suppressing deterioration of optical waveform quality and transmission characteristics. The optical transmitter includes a DBR laser, an EA modulator, a passive optical waveguide, and an SOA that are monolithically integrated on a same substrate, the DBR laser including an active region where current is injected and optical gain is obtained, and two DBR regions that are formed on opposite ends of the active region, the EA modulator optically modulating laser light from the DBR laser, the passive optical waveguide being for guiding modulated light from the EA modulator, the SOA optically amplifying the modulated light from the passive optical waveguide.

The upper surface of the semiconductor substrate has a slope descending from the projection in the second direction at an angle of 0-12° to a horizontal plane. The mesa stripe structure has an inclined surface with a slope ascending from the upper surface of the semiconductor substrate at an angle of 45-55° to the horizontal plane, the mesa stripe structure having an upright surface rising from the inclined surface at an angle of 85-95° to the horizontal plane. The buried layer is made from semiconductor with ruthenium doped therein and is in contact with the inclined surface and the upright surface. The inclined surface is as high as 80% or less of height from the upper surface of the semiconductor substrate to a lower surface of the quantum well layer and is as high as 0.3 μm or more.

A quantum cascade laser includes a first mesa waveguide provided on a substrate, the first mesa waveguide including a first core layer, a second mesa waveguide provided on the substrate, the second mesa waveguide including a second core layer, a first electrode electrically connected to the first mesa waveguide, and a second electrode electrically connected to the second mesa waveguide. The first mesa waveguide and the second mesa waveguide extend in a first direction. The first mesa waveguide and the second mesa waveguide are apart from each other by a first distance in a second direction, the second direction intersecting with the first direction. The first electrode and the second electrode are apart from each other by a second distance. The second distance is larger than the first distance.

A quantum cascade laser includes a first and a second mesa waveguides disposed on a substrate, a first electrode, a second electrode, and a current blocking region disposed burying the first and second mesa waveguides. The first and second mesa waveguides extend in a first direction. The first and second mesa waveguides are arranged apart from each other by a distance in a second direction intersecting with the first direction. The current blocking region has a first portion disposed between the first and second mesa waveguides and a second portion disposed on the first portion. The end of the first electrode and the end of the second electrode are facing each other in the second direction. The second portion protrudes from a reference plane which includes a surface of the end of the first electrode and extends in the first and second directions.

A QCL includes a semiconductor substrate and an active layer provided on the semiconductor substrate. The active layer has a cascade structure in which a unit laminate including a light emission layer which generates light and an injection layer to which electrons are transported from the light emission layer is laminated in multiple stages. The light emission layer and the injection layer each have a quantum well structure in which quantum well layers and barrier layers are alternately laminated. A separation layer including a separation quantum well layer having a layer thickness smaller than an average layer thickness of the quantum well layers included in the light emission layer and smaller than an average layer thickness of the quantum well layers included in the injection layer is provided between the light emission layer and the injection layer in the unit laminate.

A semiconductor laser is provided with: an active layer that excites a transverse electric (TE) mode and a transverse magnetic (TM) mode of light and constitutes at least a part of a resonator guiding the TE mode and the TM mode of light; and a diffraction grating as a frequency difference setting structure that sets the difference in oscillation frequency between the TE mode and the TM mode of light higher than a relaxation-oscillation frequency

The present application is provided with: a ridge laminated with a first conductivity type cladding layer, an active layer, and a second conductivity type first cladding layer in order and having a top portion formed to be flat; a first buried layer buried on both side areas of the ridge; a second buried layer covering the first buried layer and protruding toward the center of the ridge and toward a top portion of the ridge to form an opening formed by protruding portions facing each other; and a second conductivity type second cladding layer buried on the second buried layer and in the opening, wherein a surface of the second buried layer on a side to the top portion of the ridge is formed so as to fit within a surface of the second conductivity type first cladding layer.

An optical semiconductor device outputting a predetermined wavelength of laser light includes a quantum well active layer positioned between a p-type cladding layer and an n-type cladding layer in thickness direction. The optical semiconductor device includes a separate confinement heterostructure layer positioned between the quantum well active layer and the n-type cladding layer. The optical semiconductor device further includes an electric-field-distribution-control layer positioned between the separate confinement heterostructure layer and the n-type cladding layer and configured by at least two semiconductor layers having band gap energy greater than band gap energy of a barrier layer constituting the quantum well active layer. The quantum well active layer is doped with 0.3 to 1×10.sup.18/cm.sup.3 of n-type impurity.

A quantum cascade laser including: a laser structure having a first region including a first facet, a second region including a second facet, an epitaxial surface, and a substrate surface; an insulating film disposed on the second facet and the epitaxial surface; an electrode disposed on the epitaxial surface and the insulating film and in contact with the epitaxial surface; and a metal film disposed over the second facet and the epitaxial surface and separated from the electrode and the substrate surface. The insulating film is disposed between the metal film and the second facet and between the metal film and the epitaxial surface. The second region includes a semiconductor mesa. The second facet is located at a boundary between the first region and the second region. The first region includes a connecting surface. The connecting surface connects the second facet to the first facet.