High-energy, high-beam quality spatiotemporal mode-locked fiber laser with precise transverse mode control

Spatiotemporal mode-locked lasers offer unique dynamics and high-energy potential through transverse mode collaboration. However, current schemes lack precise transverse mode control, limiting the simultaneous improvement of single-pulse energy and beam quality. This paper employs a transverse mode control technique, which uses a mode multiplexer/demultiplexer to realize transverse mode division and can achieve precise control of all the transverse modes participating in spatiotemporal mode-locking (STML). Experimental results of a four-mode step-index (STIN) fiber spatiotemporal mode-locked laser demonstrate stable mode-locking at a single repetition frequency across multiple transverse modes via intermodal dispersion control using compensating fibers. Transverse mode energy superposition enables a high single-pulse energy output of 7.6 nJ in the 1.5 μm band. Furthermore, polarization controllers in each branch dynamically optimize intermodal coupling, significantly reducing the M 2 value from 4.8 to 1.4, achieving Gaussian-like high-beam-quality output in a STIN fiber spatiotemporal mode-locked laser for the first time. The technique is theoretically extensible to any number of transverse modes, enabling precise control of dispersion, phase, and polarization per mode for coordinated energy and beam quality optimization in spatiotemporal mode-locked lasers.