Complex-coupled DFB lasers: Advantages and drawbacks of gain and radiationloss grating

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