Modeling of Distributed Feedback Semiconductor Lasers with Axially Varying Parameters.

We describe a numerical model that is capable of predicting the important laser characteristics such as the threshold gain and the gain margin between the main and side modes for a distributed feedback (DFB) semiconductor laser of arbitrary complexity. The method consists of solving the coupled-mode equations with axially varying parameters iteratively until the boundary conditions at the two facets are satisfied. We apply the numerical model to discuss two DFB laser structures. In the case of a multiple phase-shift DFB laser our results show that such devices can have a more uniform axial distribution than that of a conventional quarter-wave-shifted DFB laser while maintaining sufficient gain margin between the main and side modes. In the case of a dual-pitch DFB laser we show that the incorporation of a slightly different grating period (~0.1%) over a small section can provide a gain margin that is comparable to that achieved in quarter-wave-shifted DFB lasers.