Discrete set of separations between phase-locked soliton pairs in a passively mode-locked fiber laser

This work shows experimentally and numerically that for a given laser configuration, only a discrete set of separations is accessible. The model used in the numerical simulations is similar to the experimental setup. It is based on parameter management technique with periodic pulse propagation in various parts of the cavity, and includes nonlinearity, dispersion, birefringence, gain saturation and spectral filtering. This model reproduces well the formation of stable soliton pairs. Numerical results demonstrate the formation of a stable phase-locked soliton pair with fixed separation and fixed phase difference, from an initial condition of two pulses with an arbitrary separation and an arbitrary phase difference within certain limits. When starting from initial separations and phase differences beyond those limits, the pulse pair evolves towards phase-locked states having discrete possible values for their separation. These separations are regularly spaced. Experimental results confirm the existence of these quantized separations of the phase-locked soliton pairs.