We present a detailed experimental study on the influence of dispersion on the pulse formation in passively modelocked vertical-external-cavity surface-emitting lasers (VECSELs). We have demonstrated that the shortest pulse duration requires slightly positive dispersion to balance the nonlinear phase shift induced by strong semiconductor gain and absorber saturation. This is in contrast to soliton modelocking in ion-doped solid-state lasers - but similarities remain and will be discussed. Our results are in good qualitative agreement with numerical simulations and confirm the quasi-soliton modelocking mechanism of ultrafast VECSELs.
(c) 2010 Optical Society of America.