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We demonstrate that polarization-sensitive optical coherence tomography (PS-OCT) is suitable to map the stress distribution within materials in a contactless and non-destructive way. In contrast to transmission photoelasticity measurements the samples do not have to be transparent but can be of scattering nature. Denoising and analysis of fringe patterns in single PS-OCT retardation images are demonstrated to deliver the basis for a quantitative whole-field evaluation of the internal stress state of samples under investigation., 10 pages, 6 figures; Copyright: Blackwell Publishing Ltd 2008; The definitive version is available at: www.blackwell-synergy.com

Optical coherence tomography (OCT) is a technique for contactless and non-destructive imaging of internal structures within semi-transparent materials. When OCT is performed polarisation-sensitively (PS-OCT), additional information about internal anisotropies and strains is obtained. Originally developed for the biomedical sector, OCT increasingly attracts interest for material characterisation. In this work, we apply PS-OCT imaging for non-biological samples using a novel transversal ultra-high resolution PS-OCT set-up. We demonstrate the advantages of this technique for structural analysis and strain mapping for different types of samples: we evaluate photoresist moulds for the fabrication of micro-electromechanical parts (MEMS), and we investigate the glass-fibre composite outer shell of helicopter rotor blades where defects have formed.

Mittels optischer Kohärenztomografie (OCT) können in kontaktfreier Weise Querschnittsbilder von semitransparenten Proben aufgenommen werden. In diesem Artikel werden instrumentelle Erweiterungen der OCT vorgestellt, welche in der zerstörungsfreien Materialcharakterisierung zur Anwendung kommen können. Messungen an diversen Materialproben, wie z. B. an Fotolackschichten für die Fabrikation von mikromechanischen Teilen, wurden mit verschiedenen OCT-Aufbauten durchgeführt, um die individuellen Vorteile der ultrahochauflösenden OCT, der schnellen Fourier-Domain-OCT und der polarisationssensitiven OCT aufzuzeigen.

LIGA is the basic idea of promising developments for the manufacturing of microelectromechanical system parts containing high aspect ratio microstructures. Aim of the work is a brief discussion of the starting-points for inline process inspection within a direct LIGA technology using deep X-ray lithography for the production of micromechanical gear wheels with critical dimensions of ∼35 μm width at ∼1 mm height as well as to show methodic and technical measuring possibilities. Firstly, results of the determination of residual solvent content distribution within ultra-thick SU-8 films are shown obtained from refracted near field optical measurements. Furthermore, the capability of X-ray computer tomographic imaging is discussed and measurements for the determination of the three-dimensional shape of high aspect ratio microstructures are practically demonstrated with microscopic and interferometric optical methods. Finally, first results demonstrate the potential of the optical coherence tomography for several further important measurement tasks, among others, e.g. for the imaging of the distribution of mechanical stress at the resist–substrate interface. The results show that much information which is essential in the LIGA process can be achieved with recently available measurement methods. However, further development of non-destructive measurement techniques would be desirable for an effective inline process control of mass production of micromechanical parts.

Optical coherence tomography (OCT), originally developed and so far nearly exclusively used for biomedical applications (e.g., [1,2]), is a contact-free, non-destructive technique based on low-coherence interferometry to image structures within translucent and turbid materials. Commonly, cross-sectional reflectivity images with a depth-resolution determined by the coherence length of the near-infrared light source are obtained. When OCT is performed in a polarization sensitive way (PS-OCT), additional information about birefringence within a material is obtained by mapping the retardation between ordinary and extraordinary rays [3]. Because birefringence is induced when strain occurs, PS-OCT provides depth resolved information about the internal stress within a sample.

Optical coherence tomography (OCT) is a contactless and non-invasive technique nearly exclusively applied for bio-medical imaging of tissues. Besides the internal structure, additionally strains within the sample can be mapped when OCT is performed in a polarization sensitive (PS) way. In this work, we demonstrate the benefits of PS-OCT imaging for non-biological applications. We have developed the OCT technique beyond the state-of-the-art: based on transversal ultra-high resolution (UHR-)OCT, where an axial resolution below 2 μm within materials is obtained using a femtosecond laser as light source, we have modified the setup for polarization sensitive measurements (transversal UHR-PS-OCT). We perform structural analysis and strain mapping for different types of samples: for a highly strained elastomer specimen we demonstrate the necessity of UHR-imaging. Furthermore, we investigate epoxy waveguide structures, photoresist moulds for the fabrication of micro-electromechanical parts (MEMS), and the glass-fibre composite outer shell of helicopter rotor blades where cracks are present. For these examples, transversal scanning UHR-PS-OCT is shown to provide important information about the structural properties and the strain distribution within the samples.

Optical coherence tomography (OCT), so far mainly used in the biomedical field, has a high potential as non-destructive and contactless technique for material characterization and analysis. For these applications, OCT systems with ultra-high resolution in the micrometer range and capable of high imaging speeds are required. In this work, we combine ultra-high resolution imaging using a femtosecond Ti:sapphire laser as light source with the concepts of transversal OCT. Based on acquisition by heterodyne detection via acousto-optic modulators (AOMs), and by using an xy-galvano scanner unit we are able to obtain en-face scans with sizes as large as 3 x 3 mm2 within a few seconds. The ultra-high resolution of our OCT system of 2.95 mm axially and 4 mm laterally, both in air, is shown to be essential for imaging of different compounds and fibre materials. We demonstrate the benefits of en-face scanning OCT for various applications in material investigation where in-plane information is of interest which can hardly be obtained by cross-sectional OCT.

Optical coherence tomography (OCT) and its extension, polarization-sensitive (PS-)OCT, are techniques for contactless and nondestructive imaging of internal structures. In this work, we apply PS-OCT for material characterization. We use a transversal scanning, ultra-high resolution (UHR-)PS-OCT setup providing cross-sectional as well as inplane information about the internal microstructure, the birefringence and the orientation of the optical axis within the material. We perform structural analysis and strain-mapping for different samples: we show the necessity of UHR imaging for a highly strained elastomer sample, and we discuss the effect of large birefringence on the PS-OCT images. Furthermore, we investigate high-aspect ratio photoresist moulds for the production of microelectromechanical parts (MEMS), demonstrating that transversal UHR-PSOCT is a promising tool for non-destructive strain-mapping.