Monitoring and Maintaining Laser Surface Texture Quality Based on Acoustic and Optical Process Emissions during Direct Laser Interference Patterning

Direct laser interference patterning (DLIP) is a promising technique for fabricating periodic surface structures on large areas, but controlling process quality can be challenging due to laser power fluctuations. This study presents a novel monitoring approach using acoustic and optical emissions to diagnose and correct these fluctuations during DLIP structure fabrication. Experiments are conducted using a nanosecond-pulsed IR laser to create 6 μm periodic microstructures on stainless steel. Significant laser power fluctuations are observed over time, resulting in noticeable surface texture inhomogeneity across larger areas. Acoustic and optical emissions, recorded via microphone and photodiodes respectively, are found to correlate well with local structure depth, surface roughness, and macroscopic appearance of the textured surface. A strategy for in-process correction is demonstrated using a PI controller to adjust laser power based on acoustic emission feedback in the 18–22 kHz range during processing. Implementing this closed-loop control system achieves a homogeneous texture with a consistent 2 μm structure depth, compared to significant variations between 0.8 and 2.4 μm without the controller. This monitoring and control approach offers a simple, cost-effective solution for ensuring quality and consistency in large-area DLIP fabrication processes, potentially improving the reliability and efficiency of surface texturing applications.