To improve the performance of InAs/GaInAs quantum-dot lasers emitting near 1.3 mum, the influence of growth parameters and of the dot layer design were investigated. As starting point a symmetrically designed "dots in a well" (DWELL) structure was used. The thickness of the Ga0.85In0.15As layer grown before the InAs dot layer formation turned out to be one of the key parameters to control the dot morphology. The best optical properties were achieved by an asymmetric quantum well design with a 1 nm thick GaInAs layer below and 5 nm above the InAs dot layer. With this design the linewidth of the dot ground state transition could be reduced to 30 meV and the level separation could be increased to 75 meV. In comparison to lasers with symmetric DWELLs, the threshold current could be reduced by 50%, and an increased T-0 value of 130 K up to 40degreesC could be obtained. (C) 2002 Elsevier Science B.V. All rights reserved.