Ablation characteristics of a high-power multimode flat-top pulsed laser under defocus and oblique incidence

High-power multimode flat-top pulsed lasers provide millimeter-scale footprints for large-area surface processing, but defocus and oblique incidence can substantially alter their spatial energy distribution and usable processing area. In this study, single-pulse experiments, microscopic characterization, and process-effective beam modeling were combined to investigate the ablation behavior of a 1064 nm multimode pulsed laser on 201 stainless steel. An equivalent multimode Gaussian-array model was established to describe the imaging, overlap, and separation of Gaussian-like sub-spots. By fitting the measured footprint sizes at 23 normal-incidence defocus distances, a process-effective beam-quality parameter of Meff2≈16 was obtained, with R2 = 0.980 and a root-mean-square error of 0.153 mm. A nominal visible surface-modification threshold of 0.088 J/mm2 was determined experimentally. Cross-sectional observations showed that the laser-affected depth decreased from approximately 248 μm at nominal focus to 45 μm at a defocus distance of 160 mm, while no clearly distinguishable modified zone was observed at or beyond 170 mm. At large defocus distances, interference-like spatial modulation redistributed energy into alternating high- and low-fluence regions, producing grid-like ablation patterns and interrupting the continuous surface response. Oblique incidence further caused projection stretching, fluence dilution, and asymmetric footprint distortion. A threshold-based method was, therefore, proposed to extract the regular usable region from the calculated fluence field and experimental footprint. Under normal incidence, a substantially continuous surface-processing range was maintained within 0–110 mm, whereas approximately 120–140 mm represented a transitional range. The proposed method provides a practical basis for determining usable footprint dimensions, scanning spacing, and overlap under variable defocus and incidence conditions.