This numerical study shows that an effective way to reduce the threshold current density (Jth) in the quantum well (QW) and bulk types of gain-coupled distributed feedback (DFB) lasers is to reduce the active volume. Although the threshold current per unit cavity width (Ith) is smaller for the quantum-disk type than it is for the quantum-wire or QW type of gain-coupled DFB laser, both the optimum number of vertically stacked quantum structures (Nst) for minimizing Ith when cavity length (L) is fixed and the optimum L for minimizing Ith when Nst is fixed are the largest for the quantum-disk type. When L=500 μm, for example, the optimum Nst is 7 for the quantum-disk type, 5 for the quantum-wire type and 3 for the QW type. Threshold current density (Jth) is almost independent of facet reflectivity (r), and this implies that all these lasers oscillate at the Bragg mode. The Jth of the bulk laser, however, is influenced most by r because this type has the smallest differential gain.© 1998 American Institute of Physics.