A Review on Passively Mode‐Locked Ultrafast Lasers: Mechanisms, Soliton Behavior, Stability, Emerging Materials, and Applications

This work presents an in‐depth and unified exposition of the physics, mechanisms, and emerging trends governing passively mode‐locked ultrafast lasers. It systematically reviews the fundamental principles of mode locking, emphasizing the role of both real saturable absorbers—such as SESAMs, quantum dots, and low‐dimensional 2D materials—and artificial mechanisms, including nonlinear polarization rotation (NPR). The dynamics of pulse formation are analyzed across distinct dispersion regimes, elucidating the generation and evolution of solitons, dissipative solitons, and similaritons alongside their characteristic energy and duration scaling laws. Practical instabilities, including Q‐switching and multipulsing, are critically examined with their physical origins and suppression techniques. Advanced characterization and modeling approaches, such as real‐time time‐stretch dispersive Fourier transform (TS‐DFT) diagnostics and numerical functional mapping, are discussed in relation to pulse stability and evolution. The report further explores next‐generation materials like MXenes, perovskites, and quantum‐dot‐based absorbers, as well as emerging paradigms in machine‐learning‐assisted cavity optimization and sub‐10 fs pulse generation. Overall, this study provides a comprehensive framework linking material engineering, nonlinear dynamics, and laser design strategies, offering insights relevant to the development of high‐performance ultrafast sources for spectroscopy, precision machining, biomedical imaging, and optical communications.