Multiple wavelength quantum cascade light source

A quantum cascade (QC) superlattice (SL) laser comprising:

• a core region including a QC active region which comprises a multiplicity of repeat units, each repeat unit including a unipolar, undoped radiative transition (RT) SL region and an injection/relaxation (I/R) region, each of said RT SL regions comprising a plurality of GaInAs quantum well (QW) layers interleaved with a plurality of AlInAs barrier layers, said QW layers having energy states characterized by upper and lower minibands,
• upper and lower cladding regions bounding said core region,
• an optical cavity resonator, said core region being disposed in said resonator,
• electrodes for applying current above the lasing threshold to said active region effective to cause said RT regions to generate stimulated emission of light at energies determined by upper and lower energy levels within said upper and lower minibands, respectively, and
• means for rendering said minibands essentially flat in the presence of said applied field, characterized in that
• said lower miniband has first and second lower energy levels and said upper miniband has third and fourth upper energy levels, said energy levels montonically decreasing in energy from the fourth to the first, below said lasing threshold electron transitions between said fourth and first levels generate light at a first spontaneous emission line having a center wavelength .lambda..sub.1 and a line broadening first energy and between said third and second levels generate light at a second spontaneous emission line having a center wavelength .lambda..sub.2 and a line broadening second energy,
• the energy separation of said center wavelengths is greater than the larger of said first and second line broadening energies, and
• the energy separation of said upper levels is less than the energy of an optical phonon in said active region.