Numerical investigation of soliton molecules with variable separation in passively mode-locked fiber lasers

Soliton molecules evolution is numerically investigated in a passively mode-locked fiber laser based on the nonlinear polarization rotation (NPR) technique. Peak-to-peak separation of soliton molecules can be controlled by changing either pump strength or cavity linear phase delay appropriately. Moreover, soliton molecules with intensity-independent evolution, separation-independent evolution and large intensity-vibrating evolution are numerically found, respectively. The characteristics of soliton molecules evolution versus linear phase delay or pump strength are given. Periodic stable evolution regimes are found. The separation-controllable soliton molecules can be attributed to the mutual effects of phase delay, Kerr nonlinearity and other parameters of the cavity.