Higher dimensional oscillations of soliton molecules in ultrafast fiber laser

Optical soliton molecules manifest stationary as well as oscillating and vibrating dynamics, showing an interesting analogy with some behaviors of the molecules of matter. Nevertheless, the soliton molecules observed in ultrafast lasers remain fundamentally different from their matter counterpart, since they arise from dissipative systems [1] . The major experimental focus is set on the retrieval of the dynamics of the relative pulse temporal separations and phases within a given soliton molecule [2] . Whereas these few internal degrees of freedom appear as the most relevant, they may not be sufficient to picture complex soliton molecule dynamics, as was recently pointed out in a numerical study [3] . In our experiment, we employ the dispersive Fourier-transform (DFT) technique [4] to record in real time the optical spectra of a two-soliton molecule generated by an ultrafast fiber laser ( Fig. 1 a,b). Besides retrieving the relative phase and separation between the two solitons (see Fig. 1 c,d), we also obtain new information about their chirp difference, inferred from the curvature of the phases computed after fast-Fourier transform of the DFT traces ( Fig. 1 e,f).