  We report on the observation of various bound states of dispersion-managed
(DM) solitons in a passively mode-locked Erbium-doped fiber ring laser at near
zero net cavity group velocity dispersion (GVD). The generated DM solitons are
characterized by their Gaussian-like spectral profile with no sidebands, which
is distinct from those of the conventional solitons generated in fiber lasers
with large net negative cavity GVD, of the parabolic pulses generated in fiber
lasers with positive cavity GVD and negligible gain saturation and bandwidth
limiting, and of the gain-guided solitons generated in fiber lasers with large
positive cavity GVD. Furthermore, bound states of DM solitons with fixed
soliton separations are also observed. We show that these bound solitons can
function as a unit to form bound states themselves. Numerical simulations
verified our experimental observations.
