Polarized extreme asymmetric vector solitons with normal and ultra-narrow spectra in a mode-locked laser

We report the experimental observation of novel, to the best of our knowledge, steady-state and breathing polarization-asymmetric vector solitons (PAVS) in a fiber laser. The vector pulse is composed of two markedly distinct orthogonally polarized components, one of which exhibits the typical spectral features of conventional solitons, while the other is an extremely strong narrow peak. The extremely narrow spectrum can stably propagate in the cavity, facilitated by the continuous replenishment of new sub-pulses via polarization coupling from conventional solitons, which counteracts the cavity losses. In the breathing state, the two polarization components undergo anti-phase energy oscillations. Numerical simulations accurately reproduce the experimental results and further reveal that the extremely narrow spectrum temporally corresponds to a double-period intensity modulated pulse with a pronounced trailing tail. This work not only enriches the family of vector solitons but also provides deeper insight into the dynamical properties of PAVS in nonlinear dissipative systems.