Femtosecond passively Er-doped mode-locked fiber laser with WS₂ solution saturable absorber

This study presents the femtosecond fundamental and harmonic mode-locked Er-doped fiber laser with WS₂ solution saturable absorber (SSA). The SA is fabricated based on a D-shaped fiber (DF) embedded in WS₂ nanosheets solution. Such WS₂ solution has virtues of good antioxidant capacity, excellent scattering resistance, high heat dissipation, and high damage threshold. This kind of SA shows a modulation depth of 11%, a saturable intensity I_sat of 5 MW/cm², and nonsaturable loss of 18%. By employing DF-WS₂ SSA, a stable mode-locked fiber laser is achieved with repetition rate of 10.2 MHz and pulse duration of 660 fs. At the pump power of 350 mW, 460.7-MHz repetition rate harmonic mode-locking (HML) operation is also obtained, which corresponds to 45th harmonics of the fundamental cavity repetition rate. The pulse duration is 710 fs and signal-to-noise ratio is 66 dB, showing the excellent performance in HML fiber laser with SA. The results indicate that DF-WS₂ solution can work as a potential SA for ultrafast nonlinear optics.