Theoretical Study of Fast Light with Short sech Pulses in Coherent Gain   Media
  We investigate theoretically the phenomenon of so-called fast light in an unconventional regime, using pulses sufficiently short that relaxation effects in a gain medium can be ignored completely. We show that previously recognized gain instabilities, including superfluorescence, can be tolerated in achieving a pulse peak advance of one full peak width. 
B.D. Clader and J.H. Eberly