A quantum cascade laser is configured with a semiconductor substrate, and an active layer provided on a first surface of the substrate and having a multistage lamination of unit laminate structures each of which includes an emission layer and an injection layer. The active layer is configured to be capable of generating first pump light of a frequency .sub.1 and second pump light of a frequency .sub.2, and to generate output light of a difference frequency by difference frequency generation. An external diffraction grating is provided constituting an external cavity for generating the first pump light and configured to be capable of changing the frequency .sub.1, outside an element structure portion including the active layer. Grooves respectively formed in a direction intersecting with a resonating direction are provided on a second surface of the substrate.

A QCL includes a gain region that generates radiation. An optical waveguide feeds the radiation back to the gain region. A first order and/or a second order Bragg reflector are formed in the optical waveguide. The first order Bragg reflector narrows a spectrum of the radiation. The second order Bragg reflector outcouples the radiation through a large facet.