Revelation of the Mode‐Resolved Dynamics of 3D Breathers in a Spatiotemporal Mode‐Locked Fiber Laser

ABSTRACT 3D solitons in spatiotemporal mode‐locked (STML) fiber lasers provide a unique platform for exploring high‐dimensional nonlinear physics and complex mode interactions. As a special type of 3D solitons, 3D breathers possess abundant evolutionary characteristics, yet the mode‐resolved dynamics remain unexplored. Here, two distinct types of 3D breathers are experimentally realized in an STML fiber laser, and the real‐time dynamical evolution of individual transverse modes is captured via mode‐resolved measurement. The two breathers exhibit markedly different energy exchange mechanisms: one involves energy exchange among transverse modes (asynchronous transverse modal energy evolution), while the other features energy exchange among spectral components (asynchronous energy evolution of spectral components). Due to energy exchange among spectral components and bandwidth disparities among spectral peaks, energy modulation depth is remarkably lower than spectral intensity modulation depth. Numerical simulations qualitatively reproduce both experimentally observed 3D breathers. Linear mode coupling (LMC) dominates synchronous transverse modal energy evolution, while asynchronous transverse modal energy evolution relies on both LMC and propagation effects (including intermodal gain competition and nonlinear effects such as intermodal four‐wave mixing) in Yb‐doped fiber. This work reveals individual modal dynamics and their complex interactions during 3D soliton breathing, deepening understanding of intricate spatiotemporal dynamics in STML fiber lasers.