Harmonic mode locking of bound solitons.

We systematically investigate the passive harmonic mode locking (HML) of the bound states of two solitons in a fiber laser that has been mode locked by nonlinear polarization rotation (NPR). The experiment shows that the stable HML state of the bound solitons (BSs) with a fixed and discrete separation is obtained. The repetition rate changes by increasing the pump power and slightly altering the polarization state in the cavity. The dynamic is similar to the HML of a single-pulse operation. In our experiment, the repetition rate can be turned from the fundamental mode locking up to ninth-order HML when the pump power is increased from 168 to 476.1 mW. Once the BSs are obtained, their separation is fixed at 1.5 ps, regardless of the HML order. Under the direct BS interaction, the BS trains are very stable and easily reproducible. This HML behavior of BSs confirms that the BS is another intrinsic feature of the laser, except for the single soliton.