  We report on the experimental evidence of four wave mixing (FWM) between the
two polarization components of a vector soliton formed in a passively
mode-locked fiber laser. Extra spectral sidebands with out-of-phase intensity
variation between the polarization resolved soliton spectra was firstly
observed, which was identified to be caused by the energy exchange between the
two soliton polarization components. Other features of the FWM spectral
sidebands and the soliton internal FWM were also experimentally investigated
and numerically confirmed.
