Analysis of light propagation in an anomalously dispersive medium

We analyse the propagation of a probe laser pulse though a medium of atoms in the double Λ configuration with two closely spaced upper levels. The medium is irradiated with a strong coherent field and is additionally incoherently pumped. Depending on the parameters of the system the dispersion of the medium may change alternatively from normal to anomalous with absorption varying from positive to negative (gain). The incoherent optical pump was found to be the switch between the sub- and the superluminal regime. A laser field propagating through such an optical system can experience alternatively the negative or positive group velocity. The dynamics of propagation in the case of positive but larger than c group velocities is investigated in particular. In this paper we show how the description in terms of the group velocity breaks down in the superluminal regime. (paper)