Superluminal Light Pulse Propagation in Active Nonlinear Media

The advent of lasers and novel experimental techniques have made it possible to conduct experiments on the already solved but ever "intriguing" problem of the "superluminal velocity of light pulses" ^1,2 in various experimental situations (see review ³): anomalous dispersion near an absorption line ⁴, linear gain spectral lines ^5-11, and tunneling barriers ^12,13. In essence, these experiments have only confirmed the usefulness of the five different kinds of wave velocity, namely, the phase velocity, the group velocity, the energy velocity, the "signal "velocity, and the "front" velocity, introduced by L. Brillouin ² for absorptive dispersion media. In this connection, I would like to return to the discussion of this problem for the interesting case of propagation of a short laser pulse in a nonlinear (saturable) amplifying medium in the wider context of propagation of instability autowaves in an active nonlinear medium.