The non-destructive sensing and imaging of the inner structure of materials is essential in many industrial processes for quality control and the development of new materials.
Standard non-destructive measurement methods, like ultrasound, exhibit a resolution of approximately 100 µm and often require direct contact between the probe and sample (e.g. with the aid of contact gels).
Optical coherence tomography (OCT) is an emerging optical imaging method, which allows the acquisition of cross-sections in a non-destructive and contactless manner. Today, OCT systems provide axial resolutions ranging from 1 – 10 µm and allow real-time in situ and in vivo imaging.
Optical coherence tomography is based on the phenomenon of white light interferometry and is very sensitive to small changes in the refractive index of the sample. Therefore it delivers complementary information to other imaging techniques like X-ray computed tomography (CT), and magnetic resonance imaging (MRI). The principle behind OCT is depicted in figure 1. Near infrared light waves are partially reflected at different depths within the sample, and their arrival times at a detector are interferometrically compared with a reference wave. The detected signal contains information on the position of the scatterers within the sample, such as their reflectivity, velocity, and polarization properties. Cross-sectional images can be reconstructed by collecting depth scans at different adjacent positions.