Unveiling the Internal Dynamics of Rectangular Pulses in an Ultrafast Fiber Laser with MoS2 Saturable Absorber

In this work, we demonstrate the generation and evolution behaviors of rectangular pulses in a passively mode-locked erbium-doped fiber laser incorporating a molybdenum disulfide (MoS2) saturable absorber. At lower pump power, the laser operates in a stable fundamental mode-locked state. With the gradual increase in pump power and appropriate polarization adjustment, the rectangular-pulse regime with spectral sidebands emerges, and the corresponding temporal width broadens from 0.34 ns to 0.85 ns as the pump increases. The dispersive Fourier transform measurements are employed to capture the real-time spectral evolution of the generated rectangular pulses, revealing that such pulses consist of chaotic sub-pulses with randomly varying intensities. Furthermore, the numerical simulations are in reasonable agreement with the experimental observations. Our results shed light on the pulse dynamics governing the transition and temporal width broadening, contributing to a deeper understanding of the underlying nonlinear phenomena in ultrafast fiber lasers.