Plasma emission spectroscopy for online monitoring of laser processing: signals, models and critical perspectives

Laser processing employs high-energy-density beams to induce material melting, vaporization, ablation, joining, and surface modification. Owing to its non-contact nature, high precision, and high efficiency, this technology has been widely applied in aerospace, automotive manufacturing, semiconductor devices, and the nuclear industry. However, the complex laser–material interactions are highly susceptible to fluctuations in laser parameters, material properties, and processing conditions, which may give rise to defects such as cracks, porosity, lack of fusion, and dimensional deviations. Plasma emission spectra generated during laser processing originate from material vaporization, ionization, and plume evolution, and can provide in situ information on energy coupling, material states, and defect formation, thereby showing considerable potential for online monitoring. Focusing on representative laser processing scenarios, including laser welding, additive manufacturing, cutting, etching, drilling, and cleaning, this review critically examines recent advances in online monitoring using plasma emission spectroscopy with respect to spectral information, measurement reliability, data interpretation models, and monitoring objectives. Particular emphasis is placed on the evidence and limitations of this technique in composition monitoring, depth prediction, process-state identification, endpoint determination, and readiness for closed-loop control. Finally, the challenges and future directions of plasma emission spectroscopy for industrial integration and intelligent control are discussed.