Evaluation of the coupling coefficient of a distributed feedback laser with residual facet reflectivity

We show that the stop-band width of a distributed feedback (DFB) laser with both facets having antireflective coatings is affected by residual facet reflectivity. In the weakly coupled case the stop-band width of a DFB laser with residual facets having a power reflectivity of about 1.0% is narrower than that with no reflective facets. When the residual power reflectivity of the facets is less than 0.01%, the stop-band width is the same as that with no reflective facets. It is therefore essential to consider the effect of reflective facets to evaluate correctly the coupling coefficient.