Experimental observations of separation-changeable soliton pairs in a fiber laser mode-locked by nonlinear polarization rotation technique

The soliton pairs with changeable temporal separation in a passively mode-locked fiber laser are investigated experimentally. Temporal separations of pulses are as large as tens of nanoseconds, which are almost four orders of magnitudes larger than the pulse duration. The mechanism of the changeable separation mainly attributes to the variation of the linear phase delay in the cavity. Soliton pairs with large temporal separations and their variable characteristics are different from the operation of typical harmonic or bound state of solitons. These novel states in this report can be considered as the quasi-bound states.