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Polarisation-sensitive optical coherence tomography for material characterisation and testing

Optical Coherence Tomography (OCT) is an emerging technology for high-resolution non-contact imaging of semitransparent structures. Originally developed for medical diagnostics, we extend the OCT-technique to problems posed in material testing and characterization. Layer thickness and refractive indices as well as internal structures of polymer parts have been determined within this study. An extension of OCT, namely polarisation-sensitive OCT (PSOCT) has been used to identify regions of increased anisotropy within injection-moulded plastic parts and to determine quantitatively internal strain within translucent materials in a nondestructive way. Introduction and experimental: Optical Coherence Tomography (OCT) is an emerging technology for high-resolution non-contact imaging of semitransparent structures. The measurement principle of OCT is based on an interferometric detection of path-length distributions of low-coherence light back-reflected from interfaces within the sample [1-3]. The original application of OCT is the imaging of the human retina [1] and has been extended to the characterisation of a variety of biological tissues [3]. Anyhow, the applications of OCT outside the biomedical sector, especially for material characterisation and testing are so far only marginally touched: e.g. OCT imaging of glass fibre composites and the detection of subsurface defects and cracks in other non-biological materials, like in ceramics [4,5]. Polarisation-Sensitive OCT (PS-OCT) is an extension to the classical OCT method and maps in addition to the light intensity distribution the polarisation state of light within the sample [6-8]. Thus, additional physical parameters (like birefringence) and enhanced structural information, that is difficult to resolve with other imaging techniques, can be obtained. Recently, we extended for the first time classical OCT measurements to PS-OCT to characterise and non-destructively test injection-moulded polymer parts for a qualitative determination of internal strain fields and regions of material anisotropy [9]. In the current study we present new applications of classical OCT, like non-destructive thickness measurements of lacquer-layers and discrimination between individual layers, even when of same consistence and composition. Furthermore, we performed, simultaneously to PS-OCT, tensile stress-strain experiments on different polymeric materials to measure the stress-optical coefficient in the perspective of a quantitative and non-destructive determination of residual and induced internal stress in samples. For the presented study we used a PS-OCT set-up as schematically depicted in Fig. 1. The beam of a low coherent light source is linearly polarised (vertically) and divided equally into two arms of a Michelson interferometer. In the reference arm a scanning mirror varies the optical path length. The quarter wave plate in the reference arm provides after back-reflection from the mirror linearly polarized light rotated by 45° to its original polarisation direction. The sample under investigation is placed in the other arm and is illuminated by circularly polarized light with the help of a second quarter wave plate.