Observation of superluminal and slow light propagation in erbium-doped optical fiberA. Schweinsberg, N. N. Lepeshkin, M. S. Bigelow, R. W. Boyd and S. JaraboEurophys. Lett., 73 2 (2006) 218-224DOI: https://doi.org/10.1209/epl/i2005-10371-0