Compound pulse solitons in a passively mode-locked fiber laser

We demonstrate experimentally a novel form of double-pulse solitary waves in a passively mode-locked fiber ring laser. The new solitons are characterized as that they consist of two single solitons with fixed, discrete pulse separations. Through studying the interaction between bound-soliton pairs observed in a passively mode-locked fiber laser, we firstly revealed that the bound soliton pair behaves as a unit, and has exactly the same interaction properties as those of the single pulse soliton in the laser. Numerical simulations confirm the existence of the new form of solitons in the laser.