  We present theoretical and experimental evidence of a universal mechanism
through which temporal cavity solitons of externally-driven, passive, Kerr
resonators can form robust long-range bound states. These bound states,
sometime also referred to as multi-soliton states or soliton crystals in
microresonators, require perturbations to the strict Lugiato-Lefever mean field
description of temporal cavity solitons. Binding occurs when the perturbation
excites a narrowband resonance in the soliton spectrum, which gives long
oscillatory tails to the solitons. Those tails can then interlock for a
discrete set of temporal separations between the solitons. The universality of
this mechanism is demonstrated in fiber ring cavities by providing experimental
observations of long-range bound states ensuing from three different
perturbations: third-order dispersion (dispersive wave generation), the
periodic nature of the cavity (Kelly sidebands), and the random birefringence
of the resonator. Sub-picosecond resolution of bound state separations and
their dynamics are obtained by using the dispersive Fourier transform
technique. Good agreement with theoretical models, including a new vector
mean-field model, is also reported.
