Two-pulse interference and superluminality

We examine how the interference of a coherent light-pulse with its slightly time-delayed copy may generate a pulse nearly identical to the original one and ahead of it. The simplicity of this 2-pulse system enabled us to obtain exact analytic expressions of the pulse distortion, valid for a wide class of pulse shapes. Explicit results are given for the pulses usually considered (gaussian, hyperbolic-secant) but also for more realistic pulses of strictly limited duration. We finally show that, for a given distortion, the 2-pulse system can produce pulse advancements comparable to those which have been actually demonstrated in the optical range with other superluminal systems.