Inline Coherent Imaging Applied to Laser Micromachining
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Laser processing has the advantage of minimal sample contact and thus little tool wear over time compared to conventional machining. However, this leads to the difficulty of real-time depth monitoring and control. To help understand the process and achieve automation of laser micromachining, a coherent imaging technique adapted from spectral domain optical coherence tomography (SD-OCT) is applied “inline”with a machining laser to monitor the depth changing information. The axial resolution of the inline coherent imaging (ICI) system is 7–8 microns and the acquisition rate is up to 230 kHz. The capture time is as low as 1.5 microseconds. 3D laser machining usually requires ultrafast lasers and homogeneous materials. With ICI, a feedback system is developed for 3D sculpture suitable even for heterogeneous materials without any sophisticated material characterization. 3D patterns with sizes up to 1 mm × 1 mm × 0.2 mm are sculpted in bone and wood with a ps UV laser. 3D patterns with sizes up to 6 mm × 6 mm × 2 mm are sculpted in bone with a CW IR laser. Many laser applications require high scan speed facilitated by scanning optics. The versatility of ICI is also demonstrated in a galvo-telecentric beam delivery system. ICI is used in a process of trench (as long as 10 mm) etching of steel to monitor the intrapulse and interpulse morphology changes as well as the sweep-to-sweep (up to 36 sweeps) depth changes. High scan speed (up to 375 mm/s) trench etching of silicon are also monitored and the parameter space is explored without destructive post-processing. Motion during imaging capture time (>1.5 microseconds) may cause contrast degradation. To reduce the motion artifacts, preliminary experiments on stroboscopic ICI based on a kHz pulse repetition rate femtosecond laser are described. By “sampling” the motion of the machining front discretely with a “sampling time” as short as the imaging pulse duration, our results demonstrate that stroboscopic ICI is a promising way to improve the ICI contrast against motion artifacts.