Polarization-controlled switching of pulse regimes in spatiotemporal mode-locked fiber laser via a hybrid saturable absorber

We present an operation switchable spatiotemporal mode-locked fiber laser using a hybrid saturable absorber. The laser cavity incorporates a single-mode fiber-graded-index multimode fiber-single-mode fiber structure acting as an artificial saturable absorber, combined with the nonlinear polarization rotation mechanism. The proposed hybrid saturable absorber enables switching among multimode Q-switching, Q-switched mode-locking, and spatiotemporal mode-locking solely by adjusting the polarization controller at a fixed pump power. We demonstrate that under 800 mW pumping, the laser evolves via polarization tuning from a 48.24 kHz Q-switching state to a Q-switched mode-locking regime featuring mode-locked pulses at 18.72 MHz nested within a Q-switched envelope of 46.88 kHz, and finally to stable spatiotemporal mode-locking at 18.72 MHz. This operational flexibility is attributed to the tunable intracavity loss modulation induced by the hybrid saturable absorber mechanism. Moreover, numerical simulations of the intracavity pulse evolution further confirm the critical role of the hybrid saturable absorber. To the best of our knowledge, this study for the first time achieves various operation regimes solely through polarization control at constant pump power using the hybrid saturable absorber, thus offering a uniquely flexible platform for diverse pulse outputs.