MEASURED PULSE VELOCITY GREATER THAN c IN A NEON ABSORPTION CELL

The pulse velocity in a Ne absorption cell placed inside a self-locked He–Ne (6328 Å) laser has been found to exceed c, the free-space velocity of light, by about 3 parts in 104. Conversely, the pulse velocity in a He–Ne gain cell was less than c by about the same fraction. These results are consistent with the classical group velocity dω/dk, where the frequencies are confined to the central region of the resonant line.