Quantum cascade laser

1:

• A quantum cascade laser comprising:
• an active layer including a plurality of injection quantum well regions and a plurality of light-emitting quantum well regions, each of the plurality of injection quantum well regions and each of the plurality of light-emitting quantum well regions being alternatively stacked, the each of the plurality of light-emitting quantum well regions emitting laser light by optical transition between sub-bands of a carrier of an electron or a hole, and the each of the plurality of injection quantum well regions mitigating the carrier after the optical transition between sub-bands to a mini-band level, and injecting the carrier into a light-emitting quantum well region downstream, the active layer including a first portion and a second portion, a thickness of the first portion and a thickness of the second portion being the same;
• a first and a second cladding layer sandwiching the active layer, and having a refractive index lower than an effective refractive index of each of the plurality of light-emitting quantum well regions;
• and
• an optical guide layer interposed between the first portion and the second portion in a stacking direction, having a refractive index higher than the effective refractive index of each of the plurality of light-emitting quantum well regions, and having a thickness greater than thicknesses of all well layers of quantum well layers of each of the plurality of light-emitting quantum well regions.