Optical coherence tomography for process control of laser micromachining

Laser-Laboratorium Göttingen e.V., Hans-Adolf-Krebs-Weg 1, D-37077 Göttingen, Germany.
The Review of scientific instruments (Impact Factor: 1.61). 03/2010; 81(3):033705. DOI: 10.1063/1.3356080
Source: PubMed


In situ surface imaging for nondestructive evaluation (NDE) by optical coherence tomography (OCT) before, during, and after ablative laser processing is presented. Furthermore, it is shown that the ability of in situ characterization is beneficial for samples such as optical fibers, which are difficult to handle in the standard analysis. Surface images taken by the OCT are compared with these common analysis tools such as scanning electron microscopy (SEM), reflected-light, and confocal microscopy. An axial resolution of approximately 126 nm for surface detection and a lateral resolution <2.5 microm are obtained and the potential of the setup to imaging structures with high aspect ratio is demonstrated.

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    • "Polarization-sensitive OCT, which is a functional extension of OCT for birefringence measurements, has been used for depth-resolved stress measurements in injection-molded polymer samples [6], and strain maps in glass-fiber composite materials [7]. Wiesner et al. used ultra-high-resolution OCT system for in situ subsurface nondestructive examination of laser micro-machined surfaces [8]. Ultra-high-resolution OCT has also been used for non-destructive analysis of tablet coatings [9], sub-surface inspection of silicon integrated chips [10] and strain mapping [11]. "
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    ABSTRACT: An application of optical coherence tomography (OCT) for measuring shape and the radius of curvature of deeply curved metallic surfaces in precision mechanical components is demonstrated. Measurements carried out by OCT technique on concave and convex hemispherical metallic surfaces generated by diamond turning process is presented. The technique is compared with a two-beam laser interferometric method for determinations of shape errors and radius of curvature. It is shown that OCT is a useful tool for three-dimensional shape determinations for deeply curved surfaces such as hemispheres and has advantages over two-beam laser interferometric method.
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    • "A gas flow nozzle separates the imaging optics from the sample to provide a protective and transparent stream of nitrogen gas to the working area on the target surface. The necessary precise control of the focal plane position is accomplished by the integration of an optical coherence tomography (OCT) module into the sample-alignment optics of the processing system [Wiesner et al. (2010)]. "
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    • "Nevertheless an innovative monitoring technique for laser machining based on optical coherence tomography / imaging is able to acquire a high precision depth profile during or after the process. Applications were reported for monitoring laser micromachining [17] and drilling [18] [19] as well as to control laser micromachining with [20] and without [21] focusing lens and for laser drilling [22] [23] [24] [25]. None of the presented works consider the effects of the optical machine system (scanner unit and the focusing lens) on the measurement beam. "
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    ABSTRACT: Laser surface structuring is a rapidly developing manufacturing technology. This technology offers a high level of flexibility regarding the geometrical design and a wide range of machinable materials. However, current systems used for laser structuring have a crucial deficit. The existing process control systems have no sufficient feedback infor- mation of the machined surface topology before, during and after the process, not being able to detect errors/devia- tions and so to adapt the structuring strategy while processing. Therefore a novel scanner based inline metrology system has been developed to control the laser surface structuring process by a process monitoring software.
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