Spatiotemporal\nMode-Locked Multimode Soliton\nFiber Laser Based on a Spatial\nAlignment Structure

The spatiotemporal mode-locking (STML) offers a viable\nsolution\nto address the energy limit of the single-mode conventional soliton\n(SM-CS). While the potential for generating highly multimode conventional\nsoliton lasers has been predicted, experimental demonstrations are\nstill lacking. Moreover, existing STML systems have relied on conventional\nsaturable absorbers (SAs), such as nonlinear polarization evolution\nand material SAs, which restrict output parameters due to complex\nspatial structures or low damage thresholds. To the best of our knowledge\nwe report the first experimental realization of a multimode CS laser\n(dominated by the LP21 mode) using a spatial alignment structure (SAS).\nThis SAS consists of two aspherical lenses, which function simultaneously\nas an SA, spatial filter, and attenuator. This configuration enhances\nsystem compactness and introduces additional degrees of freedom for\nadjustment. By modifying the alignment of the SAS, various MM nonlinear\ndynamics can be observed, including center wavelength shifting, spectrum\nand spatial evolution, harmonic STML, soliton molecules, and a multicolor\nSTML. Our system shows clear advantages in pulse energy (>3 nJ),\nstability,\nand tunability compared to other 1.5 μm STML lasers. Incorporating\na SM output coupler enables the simultaneous generation of both a\nSM-CS and a multimode convention soliton (MM-CS), which exhibit similar\nspectral profiles and pulse durations approaching the transform limit.\nOur results indicate that soliton-like pulse shaping is crucial for\nachieving multimode soliton pulses. The pulse energy of SM-CS, measured\nat 7.05 nJ (35.25 nJ intracavity pulse energy), represents nearly\na 10-fold increase compared to previous SM-CS fiber lasers. This STML\nsystem with the new SA offers a valuable platform for exploring complex\nmultimode nonlinear dynamics and provides a promising approach for\nachieving high-energy soliton lasers.