Measuring the Progress of Laser Drilling by Means of OCT and Validation Using X-Ray Synchrotron Imaging

The progress of laser drilling of deep holes was observed by means of optical coherence tomography (OCT) and simultaneously by high-speed imaging with an X-ray synchrotron. A good agreement of the depths measured by OCT and the ones extracted from the X-ray images was found for boreholes with high aspect ratios up to an observation depth of 2 mm. It was verified for different borehole geometries generated by percussion drilling, helical drilling, using circular and linear polarization, and a wide range of different laser parameters. Moreover, the OCT can retrieve a plausible depth even when the tip of the borehole is accessible only through multiple reflections of the probe beam. This proves OCT to be an expedient tool for monitoring laser drilling even under challenging conditions.