The nature of pulse propagation through a material with a negative value of the group velocity has been mysterious, as simple models seem to predict that pulses will propagate "backward" through such a material. Using an erbium-doped optical fiber and measuring the time evolution of the pulse intensity at many points within the fiber, we demonstrate that the peak of the pulse does propagate backward inside the fiber, even though the energy flow is always in the forward direction.

Abstract-A review of current fiber-based technologies capable of producing slow-light effects is presented, with emphasis on the applicability of these technologies to telecommunications. We begin with a review of the basic concepts of phase velocity, group velocity, and group delay. We then present a survey of some of the figures of merit used to quantify the engineering properties of slow-light systems. We also present a description of several of the physical processes that are commonly used to induce a slow-light effect. Finally, a review of some recent advances in this field is presented.

When a pulse superposed on a cw background propagates through an erbium-doped fiber amplifier with a negative group velocity, either pulse broadening or pulse compression can be observed. These effects can be explained in terms of two competing mechanisms: gain recovery and pulse spectrum broadening. The distortion of the pulse shape caused by these effects depends on input pulse width, pump power, and background-to-pulse power ratio. With the proper choice of these three parameters, we can obtain significant pulse advancement with minimal pulse distortion.