Depth monitoring during laser ablation of ceramic coatings with self-mixing interferometry

In this paper self-mixing interferometry for measuring ablation depth during laser percussion drilling of TiAlN coating is studied. The accuracy of self-mixing interferometry measurements was investigated in a large processing range producing blind holes with depths below and beyond the average coating thickness. The self-mixing measurements were compared to a conventional measurement device based on focus variation microscopy to evaluate the measurement error. The self-mixing signal defects as well as measurement error classes were defined. The results depict that the measurement error was independent from the processing condition and for 76% of cases it was below the intrinsic resolution of the device (λ/2=0.393 μm).