A model, based on the transfer matrix method and extended to above-threshold conditions in order to be used in the analysis and design of distributed feedback (DFB) structures, is presented. It provides the oscillation parameters as lasing frequency and net material gain, the linewidth estimation and output power, the photon density, carrier density, and effective refractive index profiles. This model can be used with or without facet reflectivities and with a number of symmetric or asymmetric phase shifts. It takes into account nonuniform envelope material gain and nonuniform index saturation. It is easily adaptable to any structure modifications.