Energy levels, wave functions, and matrix elements of optical dipole transitions have been numerically
calculated for superlattice quantum-cascade structures. An expression for the gain has been obtained with
regard to the complete set of energy levels in different models of spectral-line broadening. A universal relation
between the gain and emission spectra for unipolar laser structures has been proposed. The effect of spectral
broadening on the shape of emission spectra is estimated. The electroluminescence spectra are compared with
the calculated spontaneous recombination spectra and good agreement between the results is shown
