Dependence of external optical feedback sensitivity on structural parameters of DFB semiconductor lasers

External optical feedback sensitivity in distributed feedback (DFB) semiconductor lasers is analyzed in this paper with special attention to phase-shifted and complex-coupled lasers. The effects of variations of structural parameters such as index and/or gain coupling strengths, facet reflectivities, and corrugation phase angles on external optical feedback sensitivity are studied. Quarter-wave-shifted index-coupled DFB lasers exhibit low external optical feedback sensitivity for large index coupling coefficients and high facet reflectivities. Pure gain-coupled DFB lasers perform better than the uniform index-coupled DFB lasers without any phase-shifting structure but worse than λ/4 phase-shifted index-coupled lasers with high coupling strengths. External optical feedback sensitivity of complex-coupled lasers is a sensitive function of the structural parameters and varies significantly with the index-to-gain coupling ratio and the total coupling. Presence of a small amount of index coupling relative to the gain coupling in a gain-coupled laser improves the external optical feedback sensitivity