Functional laser texturing by means of Direct Laser Interference Patterning is one of the most efficient approaches to fabricate well-defined micro textures, which mimic natural surfaces, such as the surface of the lotus leaf for self-cleaning properties or shark skin for reduced friction. The fabrication of DLIP surface patterns with micro- and submicrometer resolution necessitates advanced monitoring strategies in order to provide optimal ablation quality as well as to ensure DLIP-pattern repeatability. Additionally, the employed monitoring systems require in-line capabilities to allow for closed-loop processing. A possible approach that can be utilized to identify the working position for DLIP is the use of acoustic emission (AE) generated by the laser-interaction with the surface [1].
In this work, a systematic study of the airborne acoustic emission during Direct Laser Interference Patterning is shown for aluminum surfaces using a microphone. The focus is on the analysis of the acoustic information extracted from the audio signal, in particular the process-inherent characteristics of the interference volume. The results show that the size of the ablation spot can be correlated with the amplitude and spectral shape of the AE. A linear relationship between the spot diameter and the laser fluence was observed and could be correlated with the ablation threshold depending on the spatial period. This approach circumvents manual measurement of the spot size, and is an important step towards a fully automated initialization of surface processing in the micrometer range.

[1] Steege, T., Alamri, S., Lasagni, A. F. and Kunze, T., “Detection and analysis of photo-acoustic emission in Direct Laser Interference Patterning,” Scientific Reports, 11(1), 14540, (2021).