As a member of the chalcogenide family, NiSe exhibits a direct bandgap of 1.74 eV, making it a promising candidate for nonlinear optical devices. However, its potential in the near-infrared region of the telecommunication band has not been fully explored. In this study, a well-coupled saturable absorber (SA) device is fabricated for the first time using NiSe nanosheets, and it is applied to an ultrafast fiber laser, achieving an ultrashort pulse laser output with an optical conversion efficiency of 13.9%. The laser based on NiSe SA achieves tunable multisoliton mode locking, including conventional solitons, bound-state solitons, dual-wavelength solitons, and second to fourth harmonic solitons, over a wavelength range of 1528.5-1556 nm by adjusting the resonator's polarization state through the polarization controller and controlling the pump power. Numerical simulations and soliton dynamic analysis in the study of NiSe SA reveal the intricate details and behaviors of ultrafast soliton pulse locking. The results indicate that the well-coupled NiSe SA, characterized by a modulation depth of 36.73%, a saturation intensity of 0.287 MW/cm2, and excellent spectral broadband absorption properties, can enhance intracavity nonlinear effects and enable the realization of stable and tunable multisoliton mode-locked pulses. Additionally, soliton collisions with group velocity differences are investigated under stable dual-wavelength soliton output, especially vector dual-wavelength soliton. This demonstrates the excellent application potential of NiSe SA in fields such as ultrafast optical communications and information encryption.
Keywords: NiSe SA; bound-state soliton; harmonic mode locking; light to light conversion efficiency; vector dual wavelength; wavelength tunable.