Observation of bound states of solitons in an L-band passive mode-locking ring fiber laser

We have experimentally observed a single-pulse soliton in an L-band passive mode-locking ring fiber laser, and have observed a weak filter which arose from the balance between the nonlinear phase shift and the gain dispersion. Different bound states of solitons have also been obtained only by carefully adjusting the polarization controller in the cavity. To the best of our knowledge, this is the first time that bound states of the solitons were observed in the L-band fiber laser mode-locked with nonlinear polarization rotation.