Effect of facet reflections on the normalized threshold gain margin of complex-coupled DFB lasers

We have compared the effect of facet reflections on the normalized threshold gain margin (GM) of purely gain-coupled (GC), partly GC, and QWS DFB lasers. The GM is defined as (α_th1-α_th0)L, where α_th0 and α_th1 are the threshold amplitude gain of the main mode and the most intense side mode, respectively, and L is the laser length. Partly GC DFB lasers have the minimum dependence of the GM characteristics on the facet reflectivities and the grating phase. There is an optimum value of the ratio, χ_g/χ_i, which gives the maximum GM for a given |χ|L