Advances in In Situ Monitoring of Ultrafast Laser Subtractive Processing: From Process‐State Sensing to Quality Control

ABSTRACT Ultrafast laser processing can sometimes suffer from quality instability, making reliable in situ process monitoring necessary. Significant advances have been achieved in optical, acoustic, coherent imaging, and data‐driven monitoring technologies; however, existing reviews have largely been organized around sensing modalities or computational approaches, while a unified understanding linking quality degradation, process‐state perception, machining objectives, and control strategies remains lacking. To address this gap, this Review establishes a framework that explicitly links quality degradation, state sensing, task‐oriented monitoring and quality control in ultrafast laser processing. Key quality issues, including geometric deviations, structural damage, and endpoint failures in heterogeneous materials, are first analyzed to define their monitoring requirements. Major sensing approaches, including spectroscopic and photodetector‐based monitoring, acoustic sensing, coherent imaging, and ultrafast spatiotemporal imaging, are then evaluated based on their observable quantities, capabilities, and application boundaries. Recent advances in multimodal fusion, intelligent state reconstruction, quality prediction and physics‐informed data‐driven control are further discussed. Future developments will rely on robust sensing, low‐dimensional state representation and real‐time closed‐loop architectures, enabling a transition from empirical parameter optimization towards intelligent and adaptive ultrafast laser manufacturing.