Vectorial Nature in Nonlinear Multimode Interference Based Ultrafast Fiber Lasers

Recent research on nonlinear multimode interference (NL-MMI) based mode-locking method provides a new approach for ultrafast all-fiber lasers. Here, we report on the polarization dynamics of group velocity locked vector solitons (GVLVSs) and noise-like vector pulses (NLVPs) in a graded-index multimode fiber (GIMF) based all-fiber laser. Due to the non-polarization sensitive characteristic of the NL-MMI, emitted pulses automatically evolve into the vector multi-pulse complexes constituting two orthogonally polarized components. Beyond the traditional polarization resolved measurement, dispersive Fourier transform (DFT) and polarization beam splitting techniques are jointly adopted to enable insights into the transient polarization dynamics of the vector solitons. It is found in real time that the two orthogonally polarized components oppositely shift their central wavelengths to compensate the polarization dispersion. In addition, stationary and pulsating NLVPs are respectively unfolded by the DFT based polarization resolved measurement. Especially, the pulsating NLVPs before and after polarization resolved measurement are accompanied with the identical periodic spectral breathing process. All these findings reveal the vectorial nature in NL-MMI based ultrafast fiber lasers, as well as highlight the transient polarization dynamics and the long-term pulsation in dissipative systems.