Unintentional Polarization Dependent Pulsewidth of Graphene Mode-Locked Er-Doped Fiber Lasers

Abstract

The unique polarization dependent pulsation phenomenon in a graphene mode-locked erbium-doped fiber laser (ML-EDFL) is explored by the interaction between intracavity weak nonlinear polarization rotation and strong saturable absorption. With unintentional polarization dependent loss (PDL) induced by twisting single-mode fiber (SMF) in an intracavity polarization controller (PC), the unique pulsewidth tunability in a polarizer-free graphene ML-EDFL can be observed. Numerical and experimental procedures are established to combine the hybrid effects and to realize the possible detuning range of intracavity polarization angle for polarization dependent mode-locking dynamics. Twisting SMF induced unintentional PDL in the polarizer-free EDFL contributes a tiny self-amplitude modulation (SAM) coefficient of ${\gamma _{\rm NPR}} = 5.36 \times {10^{ - 4}}\sin 2{\theta _P}$ . The mode-locking stability still relies on the graphene saturable absorption to optimize the SAM up to ${\gamma _{\rm total}} = 1.46 \times {10^{ - 3}}$ and shorten the EDFL pulsewidth to 330 fs. These clearly elucidate the enigma on polarization dependency of most polarizer-free ML-EDFLs with built-in PC. Even an unintentional weak polarization dependency in the EDFL without any polarizer can induce a residual PDL to introduce a polarization angle-dependent SAM coefficient, which declares the importance on controlling intracavity polarization for stabilizing and optimizing the graphene ML-EDFL with unintentional nonlinear polarization rotation induced PDL.