Complex-coupled DFB lasers: advantages and drawbacks of gain and radiation loss grating

Two types of InGaAs-GaAs-AlGaAs complex-coupled distributed feedback laser structures grown by molecular beam epitaxy are compared. The first structure uses interrupted quantum wells (QW's) grown over a grating. Its properties strongly depend on the interface cleaning resulting in nonreproducible threshold current densities. The second structure includes three uniform QW's, the gain coupling being induced by the vertical radiation of the second order grating. This last structure has reproducible, low continuous wave threshold current densities (jth=300 A/cm/sup 2/). The coupling strength and its origin are described for the two structures. Both structures have high mono-mode yield (>75%) and side-mode suppression ratios larger than 40 dB.