Temporal and spectral self-shifts of dark optical solitons.

We report the discovery of temporal and spectral self-shifts of dark optical solitons propagating in single-mode fibers. These shifts become increasingly pronounced as the intensity and the fiber length are increased. We show that our data are in quantitative agreement with numerical solutions to a modified nonlinear Schrödinger equation that includes the Raman contribution to the nonlinear refractive index.