Observation of bound state solitons in tunable all-polarization-maintaining Yb-doped fiber laser

We report on the experimental observation of various bound states of solitons (BSs) in a tunable all-polarization-maintaining (PM) Yb-doped fiber laser. The results show that the BSs can be obtained when a slightly mechanical perturbation is introduced into the mode-locked fiber laser. When the coupling ratio of backward light into the cavity is increased, the separation of the BSs is also increased and the capacity of the spectral modulation is weakened. The tightly BSs degrade into loosely BSs when the coupling ratio achieves its maximum and a mode-locked soliton-like spectrum appears.