A Distributed Feed-Back (DFB) QCL has a diffraction grating grown into the active region of the semiconductor laser. This grating provides narrow band optical feedback, distributed along the length of the waveguide, eliminating the need for discrete mirrors to form an optical cavity. DFBs allow for semi-stable fixed wavelength sources. A Fabry-Perot (FP) QCL uses the cleaved facet ends of the chip to form the 2 reflective surfaces of the laser cavity. Since all wavelengths are reflected equally in a FP QCL, all wavelengths in the gain profile of the chip are available for lasing.