Generation of soliton molecules in a normal-dispersion fiber laser

Though normal-dispersion mode-locked fiber lasers generally work on a single-pulse regime due to the large chirp of the pulses, we observe bound states of one kind of dissipative solitons in the laser. The separation between the bounded pulses in such soliton molecules increases with the pump power, and also varies with the settings of polarization controllers, which is consistent with other kinds of bounded pulses such as solitons. Our study indicates that the bound state of pulses is an intrinsic feature of mode-locked lasers and is independent of pulse profiles, stimulated by overdriving of mode-locking mechanisms.