A Vertical-Cavity Surface-Emitting Laser at Threshold

Linewidth is measured over the threshold transition of a vertical-cavity surface-emitting laser and results are compared to a FokkerPlanck semiconductor laser model. The linewidth shows nonmonotonous behavior at threshold similar to that previously observed in distributed feedback lasers. The behavior agrees with the FokkerPlanck model if the linewidth enhancement factor $\alpha \approx 5.0$. Measurement of the relaxation oscillation frequency and the damping rate using an injected optical probe allows, together with the FokkerPlanck model, the extraction of major laser parameters.