Additional modal selectivity induced by a localized defect in quarter-wave-shifted DFB lasers

The effects of a localized impurity upon the threshold spectral properties of quarter-wave-shifted distributed feedback (QWS-DFB) lasers are analyzed. The singularity, which can exhibit absorbing, dephasing, as well as reflecting properties, is mathematically described by a transfer matrix so that its inclusion in the usual matrix formalism of coupled-wave equations is easy. The exact location of the singularity, its strength, as well as the grating phase with respect to the origin are found to affect greatly the modal behavior of the structure. The localized defect interacts differently with each mode due to their intracavity stationary field pattern, so that a modally selective center is produced. A correct optimization of the singularity parameters thus enables an enhancement of the laser performance.