Reporting in the May 12 issue of Science, scientists at the University of Rochester in Rochester, N.Y., Manhattan College in New York and the University of Michigan in Ann Arbor describe the observation of 1550-nm laser pulses that propagate backward through an erbium-doped fiber prepared with a negative group velocity, with the peak of a given pulse exiting the fiber before it enters. The exotic phenomenon, which ultimately may find application in telecommunications, does not violate causality and does not involve a backward flow of energy, being instead an effect of the time dependence of gain saturation in the fiber.

In their experiments, the investigators pumped the fiber with 980-nm laser radiation to yield a dip in the gain profile such that the material displayed a group velocity of –75 km/s and a group index of –4000. They reconstructed the time evolution of the 1550-nm pulses propagating through the fiber by cutting 25-cm segments from the end of the 9-m-long fiber and measuring the output after each cut. They found that the peaks of the pulses exited the fiber before they entered and that a backward-propagating pulse in the fiber linked the exiting and entering peaks.

Using another setup incorporating three 3-m-long erbium-doped fibers separated by bidirectional taps that extracted 1 percent of the propagating radiation, they confirmed the existence of the backward pulses but determined that the energy flow was only in the forward direction.