Comment on “Superluminal solitons in a Λ-type atomic system with two-folded levels”

Abstract

We show that the analysis of the optical solitons in a resonant, lifetime-broadened Λ-type atomic system with two-folded lower levels, given recently by D. Han et al. [J. Opt. Soc. Am. B24, 2244 (2007)], is incorrect. Although in the anomalous dispersion regime near resonance one may obtain a superluminal optical soliton, such a soliton suffers serious absorption and hence cannot propagate a significant distance. However, by choosing appropriate system parameters to work in the normal dispersion regime and hence inside the transparency windows of double dark resonance, an ultraslow optical soliton with very low light intensity can form and propagate stably for a fairly long distance.