In-situ monitoring of hole evolution process in ultrafast laser drilling using optical coherence tomography

Abstract

In-situ monitoring and control of the drilling process are critical for achieving high-quality structure fabrication, optimizing the process, and gaining a deeper understanding of drilling dynamics. Optical coherence tomography (OCT), an advanced interferometric measurement technique, is now widely used for in-situ monitoring of depth information in laser welding or additive manufacturing. However, due to the complex trajectories and evolution dynamics of trepanning drilling, in-situ observation of hole evolution in ultrafast laser trepanning drilling of metals has not been realized. Here, we report a real-time monitoring technology for in-situ measurement of hole depth in ultrafast laser trepanning drilling using spectral-domain optical coherence tomography (SD-OCT). To achieve the position-synchronized acquisition of depth information, a position-encoded scanning method is proposed. Then the spatiotemporal correlation between the drilling process and the measurement results is systematically analyzed. Considering the spatial correlation and temporal continuity of laser drilling, a new method, called spatiotemporal correlation depth extraction (STC-DE) is first proposed to achieve the automatic, position synchronous in-situ measurement of hole depth in laser trepanning drilling. Finally, the effectiveness and generalization ability of this method are verified under different processing parameters. Experimental results demonstrate the measurement accuracy can reach micron level. This study fully presents the feasibility of SD-OCT in in-situ measurement of hole depth and real-time monitoring of hole evolution process in ultrafast laser drilling. It highlights the potential of this approach in revealing complex machining phenomena, optimizing processes, and achieving precise manufacturing control.