Linewidth is measured over the threshold transition of a vertical-cavity surface-emitting laser and results are compared to a Fokker–Planck semiconductor laser model. The linewidth shows non monotonous behaviour at threshold similar to that previously observed in distributed feedback lasers. The behaviour agrees with the Fokker–Planck model if the linewidth enhancement factor alpha= 5:0. Measurement of the relaxation oscillation frequency and the damping rate using an injected optical probe allows, together with the Fokker–Planck model, the extraction of major laser parameters.