XPM-forced frequency-oscillating soliton in mode-locked fiber laser

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.