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We investigate the behavior of fast light pulse propagation in an N-type Doppler-broadened 4-level atomic system using double Raman gain processes. This system displays novel and interesting results of two controllable pairs of the double gain lines profile with a control field. The detailed physics of the processes are explored having multiple controllable anomalous regions in the medium. In this set up, the system exhibits significant enhancement in the probing Gaussian pulse through the medium as compared with Ref. [L. J. Wang, A. Kuzmich, and A. Dogariu, Nature \textbf{406}, 227(2000)]. The advance time of the retrieved Gaussian pulse is always greater than the advance time studied in the above said experiment. We analyzed that the pulse propagating through the medium with larger negative group index, $7.32\times10^8$, leaves the medium almost undistorted and sooner by time $76.12 \ ms$ than the pulse which leaves the medium of Wang \emph{et al.}. The Gaussian pulse always remains almost undistorted at output due to lossless characteristic of the medium. We also underlined the ways to suppress incoherences generated by the Doppler-broadening effect in the system. The limitations of the recently developed applications require to explore mechanisms for ultimate speed of a superluminal probe light pulse. In this connection, the proposed scheme may be helpful and can be easily adjusted with the current technology., Comment: 11 pages, 7 figures, enhancement of superluminal probe light pulse

We investigate nonlinear Kerr-induced coherence effect on a superluminal probing light pulse in a gain-assisted N-type 4-level atomic system via an intense monochromatic laser field. The dispersion exhibits a novel, interesting and useful two-paired double gain lines processes. The system displays lossless characteristics similar to [L. J. Wang, A. Kuzmich, A. Dogariu, Nature \textbf{406}, 277 (2000)] but with advantages of \emph{multiple} \textbf{controllable} anomalous regions, \emph{significantly enhanced} superluminal behavior and \textbf{relaxed} temperature, states of matter regardless of its isotropic or anisotropic conditions. Unlike the instability in [A. M. Akulshin, S. Barreiro, and A. Lezama, Phys. Rev. Lett. \textbf{82}, 4277 (1999)], the present system also \emph{overcomes the instable-limit} by the Kerr-induced coherence effect in the system. Indeed, the coherence enhances the group velocity remarkably by at least $-30 ms$ more than of an instable Kerr-free system with almost negligible distortion in the retrieved pulse., Comment: 8 pages, 7 figures, Stable enhanced superluminality of a probe Gaussian pulse