Beam Self‐Cleaning of 1.5 μ$\umu$m High Peak‐Power Spatiotemporal Mode‐Locked Lasers Enabled By Nonlinear Compression and Disorder

Abstract The realization of beam self‐cleaning in a cavity is more challenging than in the outside cavity. The peak power of intracavity pulses needs to be high to reach the threshold of beam self‐cleaning, which usually relies on additional diffraction grating to compress pulses in a positive‐dispersion cavity. Here, it is first experimentally and numerically demonstrated that self‐cleaning can be observed in an all‐fiber high peak‐power Er‐doped spatiotemporal mode‐locked (STML) laser at all‐negative‐dispersion. Through the nonlinear compression of graded‐index multimode fiber, the pulses are compressed along with the emergence of beam self‐cleaning. Besides, the inherent disorder of multimode fiber accelerates the self‐cleaning process. The intracavity pulse energy of ≈13 nJ with a pulse duration of 734.5 fs is derived under a highly multimode excitation, with an output pulse energy of 2.33 nJ. The pulse energy is a nearly fourfold improvement over the previous report in all‐fiber STML at 1.5 m. Temporal‐dependent characteristics and nonlinear polarization dynamics of beam self‐cleaning are also experimentally uncovered. It is demonstrated that the STML fiber laser will enable new insights into nonlinear pulse propagation in cavities and related applications.