Patent ID: 7792170

Claim:
A semiconductor laser having an oscillation wavelength λ (nm) and comprising at least a substrate, a first-conduction-type clad layer having an average refractive index N 1cld , an active layer structure having an average refractive index N A , a second-conduction-type clad layer having an average refractive index N 2cld , wherein a first-conduction-type subwaveguide layer having an average refractive index N 1SWG is present between the substrate and the first-conduction-type clad layer, a first-conduction-type low-refractive-index layer having an average refractive index N 1LiL is present between the subwaveguide layer and the substrate, these refractive indexes satisfy all the following formulae: N 1cld <N A , N 2cld <N A , N 1cld <N 1SWG , N 2cld <N 1SWG , N 1LIL <N 1SWG , and wherein the first-conduction-type subwaveguide layer is configured such that, when an input current is increased, a red-shift of the longitudinal-mode due to a red-shift of the gain spectrum in the active layer structure is inhibited by the plasma effect of reducing the refractive index of the subwaveguide layer.