Patent ID: 6888869

Claim:
A semiconductor laser comprising: a semiconductor substrate; a core region defined by an active layer formed on one side of the semiconductor substrate; and a clad region defined by at least one clad layer overlaying the active layer, wherein the core region has a gain region with a length not smaller than 18 micrometers and not greater than 200 micrometers along an optical axis or at least the core region or the clad region; at least one of the core region and the clad region has a stripe shape and a stripe width modulated in a direction parallel with a surface of the substrate and perpendicular to the optical axis such that the center portion of the stripe, which works as a multi-mode interference waveguide enabling a lateral multi-mode, has a wider width than vicinity portions of the stripe, each of which works as a mono-mode waveguide, said multi-mode interference waveguide has a rectangular plane shape with a lateral width W and a waveguide length L which are decided so that a light intensity distribution at an output terminal of said multi-mode interference waveguide becomes a single-hill lowest order mode, and W, L, an effective refractive index n of a laser waveguide, and an operation wavelength Î» are decided so as to satisfy a formula as follows: 0.9 nW 2 /Î»â‰¦L â‰¦1.1 nW 2 /Î».