Spatiotemporal Mode-Locked Multimode Soliton Fiber Laser Based on a Spatial Alignment Structure

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