Passively mode-locked fiber lasers, which are important for the development of fiber lasers, generate ultrashort pulses with wide spectral bandwidths and high peak power levels. They offer significant research value and are widely used in fields such as biosensing, communication, medicine, and military. The generation of dissipative solitons in these lasers requires a delicate balance among dispersion and nonlinear effects, gain and loss, spectral filtering effects of the gain medium, and saturable absorbers, thus rendering them an excellent platform for investigating nonlinear effects and soliton dynamics. Stimulated Raman scattering (SRS), which is a common nonlinear effect, has garnered widespread attention owing to its ability to significantly extend the wavelength range of ultrashort pulses. The development of time-stretch dispersive Fourier transform (DFT) techniques has enabled more dynamic soliton phenomena to be identified. Using DFT techniques, this study demonstrates double-soliton collisions in dissipative systems, as well as soliton collisions under SRS effects. The effect of SRS on the collision dynamics of solitons is demonstrated, thus expanding the study of nonlinear dynamic processes in soliton evolution dynamics.

In our experiment, we construct a ytterbium-doped passively mode-locked fiber laser with an entire laser cavity comprising positive-dispersion elements. We utilize a 50-cm-long ytterbium-doped fiber as the gain medium and exploit the birefringence filtering effect of the fiber as a filter. To provide a sufficient filtering bandwidth, we employ a 30-m-long single-mode fiber. A polarization-dependent isolator (PD-ISO) ensures the unidirectional transmission of pulses within the cavity and operates in conjunction with polarization controllers PC1 and PC2 to achieve nonlinear polarization locking. The cavity length is 40.5 m, with a net dispersion of approximately 0.99 ps2 and a repetition rate of 4.6114 MHz. We simultaneously measure the spectral, temporal, and real-time spectral characteristics using an optical spectrum analyzer, a high-speed oscilloscope, and DFT techniques. The dispersion required for DFT is provided by a 20-km-long single-mode fiber, which is a multimode fiber for the 1030 nm wavelength band, but higher-order modes can be effectively eliminated over long-distance transmission, thus ensuring reliable detection. The total dispersion is approximately 700.4 ps/nm. Using a fixed pump power, we successfully achieve dual-wavelength soliton collisions within a dissipative system by adjusting the polarization controller. Additionally, we achieve soliton collisions under Raman effects.

We first analyze the results of soliton collisions in the absence of Raman effects (Fig. 3). By calculating the drift rate of the two solitons, we can determine that the collision is a dual wavelength soliton collision, and the corresponding transient evolution process is analyzed. The collision process shows pulses gradually approach each other until they overlap. After the overlap, the trailing pulse does not immediately vanish but continues to propagate before disappearing. Subsequently, the energy of the leading pulse increases, accompanied by broadening. After propagating for a certain duration, the pulse splits and drifts away from each other gradually, thus resulting in a decrease in the original soliton energy and an increase in the secondary soliton energy. Next, we illustrate soliton collisions under Raman effects (Fig. 5). The Raman effect influences the soliton collision, which causes both pulses to possess the frequency component of the main spectrum and the Raman-induced frequency-shift component. Consequently, the collision does not exhibit a significant relative drift, which is typically observed in conventional soliton collisions, owing to the approximate frequencies. Instead, constant pulse spacing is maintained during transmission. However, the frequency components of the two pulses are different, thus resulting in collision during propagation. Moreover, during the recovery after the collision, pulse reconstruction becomes difficult because of the partial energy gained by the Raman component during the collision, thus resulting in an unstable recovery.

This study presents the construction of a passively mode-locked ytterbium-doped fiber laser using a nonlinear polarization rotation (NPR) locking technique. Under certain pump power levels and polarization controller settings, soliton collisions are achieved and soliton collisions under Raman effects are reported. The differences between soliton collisions with and without Raman effects are elucidated to explain the influence of Raman effects on soliton collisions. The results of this study provide a clear understanding of soliton collisions and contribute to the understanding of the impact of Raman effects on complex nonlinear soliton dynamics. This understanding can further facilitate investigations into the potential applications of mode-locked fiber lasers.