Real-time observation of vortex mode switching in a narrow-linewidth mode locked fiber laser

Physical systems with energy oscillation possess resonant modes that mainly include temporal and spatial domains. In the laser field, enormous progress has been made toward controlling the interactions of many longitudinal modes and locking them to form temporal mode locking, which paves the way for diverse applications. Recently, it has evolved from a curiosity to the mode locking mechanism of spatial modes that reveals the new effects of spatial modes interactions in the mode locking formation process. In this paper the first observation of vortex mode switching and the corresponding pulse evolution dynamics in a narrow-linewidth mode locked fiber laser has been demonstrated. The vortex mode switching dynamics has four stages prior to the final mode locking of another vortex mode including quiet-down, relaxation oscillation, quasi-mode locking and energy recovery. The wavelength shifting evolution dynamics has been observed via time-stretch dispersion Fourier transform method. The spatial mode competition through optical nonlinearity induces energy fluctuation on the time scale of ultrashort pulses, which plays an essential role in the mode switching dynamic process. This investigation is performed by making use of a dual-resonant acousto-optic mode converter. The result has great implications in mode locking mechanism study and ultrashort laser applications.