Bound solitons with various phase differences and pulse separations were observed in a net-anomalous-dispersion erbium fiber laser mode-locked by carbon nanotubes, including two-soliton bound states with phase differences close to p (out of phase), 0 (in phase), and±/2, as well as triple-soliton bound states. To our knowledge, this is the first time that in-phase two-soliton bound states have been obtained experimentally in fiber lasers. Evolution of bound states with change of pump power was investigated, and stability of different bound states was compared. Mechanisms why and how various bound states appeared in the same fiber laser are discussed and analyzed.
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