Generation and observation of period-doubling and harmonic mode-locking solitons in a linear erbium-doped fiber oscillator with a Sagnac filter

Mode-locked fiber lasers provide an ideal platform for investigating nonlinear phenomena in dissipative optical systems. In this study, we experimentally demonstrate a linear mode-locked erbium-doped fiber laser incorporating a Sagnac loop mirror, which enables tunable spectral filtering by precise polarization control. Through gradually tuning the intracavity polarization state, the laser exhibits rich transitions from stable single-wavelength mode-locking state into a distinct period-doubling vector soliton and harmonic mode-locking soliton states. Among them, the pulse train undergoes doubling and harmonic behaviors of the fundmental repetition rate, accompanied by alternating pulse intensities induced by the intracavity polarization rotation. Real-time dispersive Fourier transform (DFT) technology provides real-time measurement of the spectral evolution during the transition, confirming that the vector soliton evolution and harmonic dynamics can be enabled by polarization-induced modulation. Furthermore, dual-wavelength period-doubling vector solitons are observed in the Sagnac-filter-based fiber laser for the first time, with both wavelength channels exhibiting synchronized vector soliton dynamics. These results establish a clear link between intracavity polarization perturbations and nonlinear vector soliton pathways, and further highlight dual-wavelength fiber lasers as versatile platforms for exploring complex dynamical behaviors in nonlinear photonics and advancing applications in dual-comb metrology and ultrafast signal processing.