Patent ID: 8817835

Claim:
A quantum cascade laser comprising: a plurality of active layers, each of active layers including a first barrier layer, a first quantum well layer, a second barrier layer, a second quantum well layer, a third barrier layer, a third quantum well layer, and a fourth barrier layer provided in this order along a predetermined direction; a plurality of injection layers; and a core layer having the active layers and the injection layers, the active layers and the injection layers being alternately provided along the predetermined direction to form a cascade structure, wherein the first quantum well layer has a film thickness larger than a film thickness of the second quantum well layer, the film thickness of the first quantum well layer being in a range of not less than 1.05 times and not more than 1.15 times the film thickness of the second quantum well layer, the film thickness of the second quantum well layer is larger than a film thickness of the third quantum well layer, the second barrier layer has a film thickness smaller than a film thickness of the third barrier layer, and the first barrier layer of at least one of the active layers is immediately adjacent to one of the injection layers and to the first quantum well layer of said at least one active layer, and wherein said at least one of the active layers has a band structure including an upper energy level of electrons and a lower energy level of electrons, the upper energy level has a wave function with even parity, the lower energy level has a wave function with odd parity, and each of the wave functions of the upper and lower energy levels is distributed substantially symmetrically in an area of the first quantum well layer, the second barrier layer, and the second quantum well layer in the predetermined direction.