Bound states of solitons in a fiber laser mode locked with carbon nanotube saturable absorber

We report on the experimental observation of bound states of solitons in an erbium-doped fiber laser passively mode-locked by the carbon nanotube saturable absorber. Bound states of solitons with various pulse separations are obtained. While the tightly bound solitons always exhibit the same set of fixed discrete pulse separations with π or π/2 phase difference, the feature becomes less obvious for the loosely bound solitons. The result that various states of the bound solitons were obtained in the same fiber laser makes a systematic experimental study on them and a comparison of their properties possible. Our results once again show that the bound soliton emission is an intrinsic feature of the mode-locked soliton fiber lasers.