Patent ID: 8654809

Claim:
A quantum cascade laser comprising: a semiconductor substrate; and an active layer which is provided on the semiconductor substrate and has a cascade structure formed by alternately laminating quantum well emission layers and injection layers by multistage-laminating unit laminate structures each including the quantum well emission layer and the injection layer, wherein the unit laminate structure included in the active layer has, in its subband level structure, a first emission upper level, a second emission upper level as an energy level higher than the first emission upper level, a single emission lower level, and a relaxation level as an energy level lower than the emission lower level, the first emission upper level, the second emission upper level, and the emission lower level relating to emission are energy levels in the quantum well emission layer, the quantum well emission layer includes at least, from the side of the injection layer of the preceding stage, a first barrier layer, a first well layer, a second barrier layer, and a second well layer, and the first well layer has a layer thickness that is thinner than other well layers in the quantum well emission layer so that a ground level of the first well layer, when the first well layer is present as a single quantum well layer, is the second emission upper level of the quantum well emission layer, the first barrier layer is an injection barrier layer being a barrier layer thickest in the unit laminate structure, the second barrier layer has a layer thickness that is thicker than 1.8 nm and thinner than the injection barrier layer, wave functions of the first and second emission upper levels are coupled under conditions of the operating electric field, and these emission upper levels behave like one emission upper level with an energy width, light is generated by intersubband transitions of electrons from the first emission upper level and the second emission upper level to the emission lower level in the quantum well emission layer, and electrons after the intersubband transitions are relaxed from the emission lower level to the relaxation level, and injected from the injection layer into the quantum well emission layer of the unit laminate structure of the subsequent stage via the relaxation level, and wherein the unit laminate structure included in the active layer has, in its subband level structure, a plurality of emission upper levels including at least the first emission upper level and the second emission upper level, concerning emission intensities of intersubband transitions from the plurality of emission upper levels to the emission lower level, the weakest emission has an emission intensity not less than ⅕ of that of the most intense emission, and an energy width of the whole of the plurality of emission upper levels is designed to be smaller than the energy of the longitudinal optical phonon.