Semiconductor laser

A semiconductor laser, comprising:

• a substrate;
• an active layer formed in or under a mesa structure formed on the substrate;
• a cladding layer formed above the active layer formed in the mesa structure;
• a diffraction grating layer formed in the cladding layer, the diffraction grating layer including a plurality of first refractive index regions and a plurality of second refractive index regions;
• and
• a low-reflection facet coating film provided on both facets in a direction in which the mesa structure extends,
• wherein the mesa structure includes, in plan view, a first region having a first width and a second region having a second width that is wider than the first width,
• wherein the first region includes a first diffraction grating region in which the plurality of first refractive index regions and the plurality of second refractive index regions for use in reflecting a light beam having a Bragg wavelength are alternately arranged,
• wherein the second region includes a second diffraction grating region in which the plurality of first refractive index regions and the plurality of second refractive index regions are alternately arranged, and a non-diffraction grating region which transmits the light beam having the Bragg wavelength,
• wherein the first diffraction grating region and the second diffraction grating region form a resonator, and
• wherein the first region has a normalized coupling coefficient that is larger than a normalized coupling coefficient of the second region.