Ultrafast Flat-Top Beam Emission with an All-Fiber Multimode Laser

Flat-top beams can eliminate the adverse effects of traditional Gaussian beams due to an uneven energy distribution. It is widely used in laser processing, image processing, and biomedical fields. The generation of pulsed flat-top beams using fiber lasers has gradually become a hot research topic in recent years. In this work, we propose an effective strategy to directly generate ultrafast flat-top beams with an all-fiber multimode laser without additional beam shaping. Utilizing the inherent characteristics of the multimode fiber, misalignment splicing of fibers, and tapered fibers in the cavity, we experimentally realize ultrashort pulses with flat-top beam shape through spatiotemporal mode-locking (STML). Numerical simulations based on the generalized multimode nonlinear Schrödinger equation further confirm that the flat-top beam is due to the coherent superposition of different transverse modes. To the best of our knowledge, this is the first time that the picosecond-level pulses with a flat-top beam shape are experimentally realized in all-fiber lasers. This work is of great significance for the development of ultrafast flat-top beams and also helps to broaden the applications of flat-top beams and multimode fiber lasers.