Monitoring Material Modification using Inline Coherent Imaging
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Laser machining is a commonly used method for materials processing. Focusing laser energy onto a sample can lead to material modifications and achieve feature sizes on the order of micrometres. However, designing a machining platform capable of producing high quality, repeatable, and accurate results is a key challenge because the final outcome can be variable, even when using fixed laser parameters. Therefore, in order to understand and monitor the process, real-time in situ metrology is required. In this work, a coherent imaging technique analogous to spectral domain optical coherence tomography (SD-OCT) was applied inline with a machining laser in order to monitor the cut development of various materials for industrial and biomedical applications. Such inline coherent imaging (ICI) provides axial resolution on the order of ones to tens of micrometers as well as temporal resolution on the order of microseconds. In stainless steel, the machining front was observed to have very different responses to pulsed lasers operating in different ablation regimes. Applying shorter pulse duration with higher peak intensity led to more deterministic material removal with little relaxation between pulses, while longer pulses revealed periodic melting and refilling behaviour. In addition, improvement of depth sensitivity to nanometre scales was explored by accessing phase information for Doppler processing techniques. For poorly absorbing materials, ICI provides the ability to observe structures below the surface. This is a very important characteristic for biomedical applications, such as guiding ablation in biological tissue. By monitoring the ablation of bone tissue in real-time using ICI, the operator was able to terminate exposure from the machining laser 50 μm before perforation into a natural inclusion in the tissue. ICI was able to anticipate the inclusion 176 ± 8 μm below the ablation front with signal intensity 9 ± 2 dB above the noise floor. With added real-time depth control, many applications will benefit whether it is achieving higher precision cuts in industrial materials, or limiting the possibility of damaging organs at risk below the cutting surface in surgical intervention.