The state filling effect in semiconductor quantum well lasers significantly affects the modulation dynamics. The state filling effect strongly depends on the optical confining layer structure. As a direct consequence of the reduction of the state filling effect in a properly designed graded index separate confinement heterostructure, a record 3dB bandwidth in excess of 9GHz has been obtained in uniformly pumped single quantum well GaAs/AlGaAs lasers by a careful tailoring of the device parameters.