Period-doubling and quadrupling of bound solitons in a passively mode-locked fiber laser

We report on the experimental observation of regular, period-doubled, period-quadrupled and chaotic states of bound solitons in a passively mode-locked fiber laser. We show that not only the solitons in the laser can easily bind together forming stable states of bound solitons, but also the formed bound solitons as an entity can exhibit complicated nonlinear dynamics, such as the period-doubling route to chaos. Our experimental results further suggest that the bound solitons formed in the laser can have strong binding energy, and consequently behave like ordinary solitons.