Vortex soliton molecule in a fiber laser.

We report the generation of vortex soliton molecules (VSMs) in a passively mode-locked fiber laser based on a mode selective coupler (MSC). ±1-order VSMs with variable numbers of molecules are observed. By adjusting the polarization state of the light in the cavity, we further demonstrate the process in which one VSM splits to multiple. During this process, the number of the solitons inside the VSM also varies and their separation gradually increases while the spectral modulation being unobservable, and vice versa. The obtained results have potential applications in fields of optical communications, especially in information coding.