Quantum cascade laser

A quantum cascade laser comprising:

• a substrate, and
• an active layer including a plurality of emission regions and a plurality of injection regions, each emission region including an injection barrier layer, and a light-emitting quantum well layer that has three continuous well layers emitting infrared light by undergoing an intersubband transition,
• each injection region including an extraction barrier layer, and a relaxation quantum well layer that creates an energy level for relaxing the energy of carriers from the each emission region, the active layer being provided on the substrate,
• a lattice constant of at least one barrier layer of the each emission region being smaller than a lattice constant of the substrate,
• a well layer on a side of the injection barrier layer being deeper than a well layer on a side of the extraction barrier layer and shallower than an intermediate well layer in the three continuous well layers,
• a wavelength of the infrared light being less than 4.3 μm,
• the each emission region and the each injection region being alternately stacked.