  Cross phase modulation (XPM) could induce soliton trapping in nonlinear
medium, which has been employed to achieve vector soliton, optical switching
and optical analog of gravity-like potentials. These results are generally
within the definition in Hamilton system. Here, we report on the observation of
a XPM-forced frequency-oscillating soliton (XFOS) whose wavelength exhibits
redshift and blueshift periodically like dancing in a mode-locked fiber laser
under moderate birefringence. XFOS consists of two orthogonally polarized
components exhibiting simultaneous frequency oscillation driven by XPM and gain
effect, which allows withstanding higher pulse energy. The pulse trapping is
maintained by differentiating the frequency-shift rate. Numerical simulations
agree very well with experimental results, revealing an idiosyncratic evolution
dynamic for asymmetry pulses in nonlinear dissipative system and envisaging a
technique to control pulse feature with preset pulse chirp. XFOS may exist
generally in polarization-independent ultrafast lasers, which enriches soliton
family and brings useful insights into nonlinear science and applications.
