Period-doubling bifurcation and polarization dynamics of vector solitons in a fiber laser

In a mode-locked fiber laser with a saturable absorber, two types of period doubling of vector solitons are numerically observed: One is the asynchronous period doubling of two orthogonal polarization components, and the other is a synchronous one under different operation conditions. The spectral evolution of the asynchronous period doubling of vector solitons is characterized by the appearance of additional sidebands in the spectrum, with the polarization state locked to multiples of the cavity length. In contrast, the synchronous period doubling of vector solitons evolves more slowly, with their polarization locked at approximately $\ensuremath{-}\ensuremath{\pi}/2$, indicating circular polarization. Additionally, using the soliton distillation method removes the continuous-wave background and sidebands, revealing the dynamics of asynchronous and synchronous period doubling of pure vector solitons.