title: Development of a simple full field optical coherence tomography system and its applications
Creators: name: Li, C
Publisher: name: University of Liverpool Repository
Year: 2016

description: Optical coherence tomography (OCT) is a versatile and powerful imaging technique widely used in biomedical applications. It employs non-destructive radiation and performs non-contact micrometre-scale cross-sectional imaging of the sample structure. However, classic OCT systems generally apply a single-point detection scheme, which creates inefficiencies in terms of the experimental alignment of system, the point-by-point signal acquisition process and the measurement speed. One of its variants, full-field optical coherence tomography (FF-OCT) employs parallel illumination and directly acquires en-face images with a complementary camera, hence omitting the need of electromechanical lateral scans as in classic OCT systems. Current FF-OCT systems could offer more efficient measurement procedures as well as superior imaging performance, however, they are neither economically viable nor universally applicable to different applications. There is a need for a simplified low cost system to make such a powerful technology readily available for a wide range of applications. In this thesis, the development of a low cost simple FF-OCT system is described from its system setup, experimental procedures, data analysis, and system performance. The system consists of only essential components including probing lens and a beam-splitter, together with a low cost infrared LED source and CMOS camera. During the measurement, the system only requires to control the axial movement of the sample arm and the image acquisition by the camera. For the imaging of a sample with a depth of 100 um, the FF-OCT measurement only takes less than two minutes. The time-efficient measurement with the simple system offers great advantage over the developed phase-shifting FF-OCT system, which requires lengthy measurement and excessive operations, despite the decoupling of signal strength and instantaneous phase with penetration depth. Therefore, compared to state-of-the-art systems, it has the advantage of being low-cost, fast image acquisition speed and simple experimental operations. For the data analysis of tomographic imaging, the axial position of a structural feature is determined by that of the envelope, which is obtained by processing raw FF-OCT signal with Hilbert transform. The imaging performance of the simple system is measured to have a spatial resolution of 3.6 x 10.3 um2 (axial x lateral) and a system sensitivity of 74 dB. The characterisation of small-size pharmaceutical pellet coatings, bovine corneal layers and paint films is to demonstrate the potential of the simple FF-OCT system for the tomographic imaging. The layered structures and internal morphology features can be revealed by analysing the measured FF-OCT B-scan images and A-scan signals. First of all, the simple FF-OCT system is capable of performing accurate and quick measurements of pellet coatings, which are validated by the XuCT technique. FF-OCT imaging can provide a spatial characterisation of coating layers, an accurate determination of coating thickness, and an estimation of coating uniformity and porosity, making the simple system a powerful tool for the coating evaluation of similar pharmaceutical pellets. Secondly, the simple system can detect corneal surfaces and the two anterior layers of bovine cornea. This could permit the prediction of the corneal oedematous state and epithelial erosions by the analysis of the FF-OCT results of the corneal structure. Thirdly, the simple system is capable of revealing the surface and subsurface of basecoat and clearcoat films. The measurement of their paint thicknesses is also verified by the reference profilometry results. FF-OCT imaging can provide further spatial evaluation of a paint film and the areal thickness map could be obtained. The study of these paint samples with the simple system might provide an indication for the FF-OCT measurement of industrial automotive paint. For the data analysis of the surface topography, the axial position of the surface is obtained by applying interpolation and a minimum search algorithm to the raw FF-OCT signal. This allows sub-micrometre depth precision to be obtained with the simple system. In the validation of the measurement of the surface topography, a less than 10 nm deviation of the FF-OCT measurement is found compared to the AFM measurement of a nanostructured step-like surface. The capability of the simple system for the surface topography is further illustrated by the determination of the electrode thickness of semiconductor microelectronics. By analysing the phase change upon reflections and the optical path lengths during the measurement, the step-like structure and the sandwich configuration can be revealed from the measured FF-OCT surface maps. The usefulness of the simple system is presented in the surface topography of PMMA models. It is demonstrated that the areal refractive power can be obtained by analysing the 2-D curvature of the FF-OCT measured surface map, which is useful in the identification of surface irregularity.