More curious features of nonlinear pulse propagation in single-mode optical fibers

Abstract

Among the curiosities of nonlinear pulse propagation in single-mode optical fibers are optical solitons—pulses with the appropriate intensity and shape so that the self-phase modulation resulting from the intensity-dependent refractive index of the fiber exactly compensates for the effects of its group velocity dispersion (GVD), and the pulse propagates with a constant shape and intensity. Bright solitons, which exist for anomalous GVD, have been studied for some time, but only recently has it been possible to observe dark solitons, which exist for normal GVD. Dark solitons, short depressions or holes in the intensity of a background signal, have the curious feature that they are odd or antisymmetric pulses. When, as in recent experiments, the background is a pulse only a few times wider than the dark pulse, the background will evolve rapidly, decreasing in intensity and developing a frequency chirp. We show that, even with such a rapidly evolving background, dark pulses have the curious feature that they can propagateas dark solitons, adapting adiabatically to the changing background.