研究了经无像差透镜聚焦的高斯光束在焦平面附近的群速度与相速度，阐明了由于衍射导致相位变化从而产生的超光速传播现象。给出了透镜几何焦点与光束束腰间距的微小变化对相速度与群速度的影响。

The phase velocity and the group velocity of focused pulsed light beams are studied in the vicinity of the focus of an aberration-free lens analytically and numerically. The phase anomaly caused by the diffraction leading to superluminal pulsed beam propagation is illustrated. The effect of the small shift in position between the geometrical focus and the beam waist of the focused beam on the group velocity and the phase velocity is given.