Static and dynamic measurements are performed with GaAs oxide-confinedvertical-cavity surface-emitting lasers (VCSELs), using multimodefibers with a core diameter of 50 and 62.5 /spl mu/m and differentnumerical apertures (NAs). They show that a small NA can have a severeimpact on the eye opening and thus also on the bit-error rate. Themeasurements are analyzed with a spatiotemporal two-dimensional (2-D)multimode VCSEL model. The required parameter extraction for themodel is verified with small- and large-signal measurements. Theanalysis shows that the change of the eye opening can be explainedby the interaction between the mode- and the current-injection profile,carrier diffusion, and intermodal gain compression (IGC). IGC increasesdifferences in the modal power distribution caused by the interactionbetween the mode profiles and the current-injection profile. Carrierdiffusion is able to compensate these increased differences of themodal power distribution. Its impact, however, on dynamic changescaused by IGC is moderate.