Highly-multimode spatiotemporal mode-locking fiber laser based on a spatial alignment structure

The spatiotemporal mode-locking (STML) offers a viable solution to address the energy-limit of 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 non-linear polarization evolution and material SAs, which restrict output parameters due to complex spatial structures or low damage thresh-olds. To the best of our knowledge, we report the first experimental realization of highly-multimode CS laser (dominated by the LP21 mode) using a spatial alignment structure (SAS). This SAS, con-sisting of just two aspherical lenses, functions simultaneously as an SA, spatial filter, and attenuator. This configuration enhances sys-tem compactness and introduces additional degrees of freedom for adjustment. By simply modifying the alignment of the SAS, vari-ous MM nonlinear dynamics can be observed, including center wavelength shifting, spectrum and spatial evolution, harmonic STML, soliton molecules, and multi-color STML. Our system shows clear advantages in pulse energy (＞3 nJ), stability, and tunability compared to other 1.5 μm STML lasers. Incorporating an SM output coupler allows for the simultaneous generation of SM-CS and multimode convention soliton (MM-CS) with similar spec-tral profiles and pulse durations nearing the transform-limit. This indicates that soliton-like pulse shaping is crucial for achieving highly multimode STML. The pulse energy of SM-CS, measured at 7.05 nJ (35.25 nJ intracavity pulse energy), represents nearly a tenfold increase compared to previous SM-CS fiber lasers. This STML system with the new SA not only offers a valuable platform for exploring complex multimode nonlinear dynamics but also pro-vides a promising approach for achieving high-energy soliton lasers.