MoS2 nano-flake doped polyvinyl alcohol enabling polarized soliton mode-locking of a fiber laser
By inserting a MoS2 nano-flake doped ultrathin polyvinyl alcohol film (PVA:MoS2 nano-flake) into a non-polarized Er-doped fiber laser (EDFL), the hybrid mode-locking of an EDFL system with nonlinear saturable absorption (NSA) and weak nonlinear polarization rotation (NPR) effects is demonstrated. The dominating mechanism responsible for the effect of intracavity polarization angle detuning on the evolution of hybrid mode-locking is investigated. In the EDFL without NSA, the weak NPR induces a polarization dependent loss (PDL) that is sinusoidally changed by varying the oriented angle of intracavity polarization with an extinction ratio of 3.6 dB, which inevitably perturbs the mode-locking stability owing to the intrinsic birefringence fluctuation. The PVA:MoS2 nano-flake based NSA film nonlinearly increases the transmittance by 2% enlarging the peak intensity of the pulse up to 20 MW cm−2, corresponding to an equivalent self-amplitude modulation (SAM) coefficient of 2.75 × 10−2. By incorporating both the weak NSA and NPR mechanisms in the EDFL cavity, the polarization angle dependent extinction ratio of the residual NPR induced PDL effect can be reduced to 1.9 dB. This NSA/NPR hybrid mechanism greatly stabilizes and optimizes the mode-locking behaviour of the EDFL by up-shifting the additive SAM coefficient beyond the mode-locking threshold, as elucidated from the modified Haus master equation. The stabilization on the passively mode-locked EDFL pulse relies strictly on inserting the PVA:MoS2 nano-flake based NSA even with a weak NSA at the initial stage, and the pulsewidth shortening is optimized to 330 fs by properly detuning the oriented angle of intracavity polarization to precisely achieve both the additive SAM and the enhanced self-phase modulation (SPM) effects at the second stage.