Dynamic evolution of Nyquist-pulse solitons from a mode-locked fiber oscillator

Nyquist pulses, characterized by their sinc-shaped temporal and rectangular spectral profiles, have found widespread applications in high-speed optical communication and optical storage. Traditional methods for generating Nyquist pulses, which typically rely on active spectral modulation, often experience deviations from the ideal rectangular spectrum and encounter increased system complexity. In this work, the direct generation of Nyquist-pulse solitons (NPSs) from a passively mode-locked fiber oscillator is numerically demonstrated. The dynamic evolution of the pulses is systematically investigated, highlighting the pivotal role of gain in regulating pulse dynamics. By tuning the gain saturation energy (Eₛₐₜ), controllable transitions among dissipative solitons, unstable states, and NPSs are revealed. At Eₛₐₜ = 13.5 pJ, a NPS with a pulse duration of 3.6 ps is achieved, exhibiting 99.96 % fidelity to the ideal sinc function. Furthermore, the spectrum of NPSs evolves from concave-topped to flat-topped and ultimately to convex-topped with increasing Eₛₐₜ, unveiling a gain-driven nonlinear shaping mechanism dominated by self-phase modulation (SPM) and spectral filtering. These findings demonstrate that gain not only defines the accessible pulse regimes but also serves as a precise control parameter for tailoring both the temporal and spectral characteristics of NPSs. This establishes a pathway for high-fidelity NPSs generation directly from the oscillator and lay a theoretical foundation for advanced applications in photonic neural networks, quantum key distribution, and ultrafast laser science.