Influence of a localized scattering center upon the spectral characteristics of a distributed-feedback structure

We investigate the influence of a process-induced localized impurity upon the spectral characteristics of distributed-feedback (DFB) structures in the frame of a transfer matrix formalism. Its interaction with the intracavity field is shown to exhibit a modally selective behavior, possibly leading to a modal degeneracy lift at threshold. This effect, very sensitive to the location of the impurity with respect to the grating phase, could prove of interest in enhancing the performance of DFB lasers or filters.