Pulse velocity in a resonant absorber

Experiments are reported that measure the pulse velocity of a mode-locked 6328-Å He-Ne laser pulse in a resonant absorber as a function of the pulse intensity. At low intensities, pulse velocities greater than the velocity of light in the passive medium are observed. These results are compared with a simple theory based on the saturation of the dispersion associated with the absorption. At higher intensities where the pulse approaches a 2π pulse, pulse delays (that is pulse velocities less than the velocity of light in the passive medium) are observed.