Non-Invasive Optical Detection And Imaging With Scattering Data

Optical scattering is a basic phenomenon found in nature, and researchers around the world have paid much attention to the techniques of non-invasive detection and imaging based on optical scattering. The purpose of this thesis is focused on the research of non-invasive optical detection and imaging by use of scattering data.The thesis consists of two parts. The first part describes the measurement of the scattered field or intensity distribution at the two end face of the waveguide, and the reconstruction formulas to find the location and optical properties of the defects in the waveguide. The second part describes the study of optical coherence tomography (OCT)—a very powerful imaging technique for high scattering media. The history and principles are reviewed, and several numerical models, based on Mie theory and Monte Carlo techniques, are given. By numerical simulations, we have found that proper space filtering and polarization gating can increase the contrast and the resolving power of an OCT system. We have developed OCT systems and do some imaging experiments; and we also have developed an algorithm to compensate the dispersion of the samples.The thesis is divided into eight chapters, the summaries of which are as followes:Chapter 1 is about introduction. In this chapter we review the history of the research of optical scattering since modern science starts. The scattered lights are characterized according to the time domain and the space domain, and different research methods, such as analytical theory, transport theory, experimental and numerical simulation, are introduced. Subsequently we give the introduction of techniques based on optical scattering, diffused optical tomography (DOT) and OCT.Chapter 2 - 4 are about the analytical reconstruction method which is useful to find the location and optical properties of the defects in waveguides. In Chapter 2, we first deduce the two dimension (2D) partial equation for the optical scattering in a waveguide, based on Maxwell's equations; and then introduce a numerical method -methods of the moments (MOM) to solve the 2D partial equation. In Chapter 3 and 4, we show the algorithms to reconstruct the location and optical properties when there is only one point defect or one thin-strip defect in the waveguide, based on the knowledge in Chapter 1. When a guided mode is excited inside a planar waveguide,the presence of the defect (a point defect or a thin-strip defect) will cause a distortion of the field distribution in the waveguide. Such a field distortion can be measured (with, e.g., a CCD camera) at the two end faces of the planar waveguide and used to predict the location and width of the thin strip defect.Chapter 5 is about principles of OCT systems. We first introduce the principles of low coherence interferometry, and deduce some formulas for the interferometric signals. Then we give the classical structure and some performance coefficients of OCT systems.Chapter 6 is about simulation of OCT. We have developed a numerical model of OCT, based on Mie theory and Monte Carlo technique; by simulation, we have verified resolving power of OCT system; and found that space filtering and polarization gate could increase the contrast and resolving power of OCT system. Finally, we used our model to study the lateral resolution when the sample is illuminated by finite-spot source. We have found the shower curtain effect, which was observed by other researcher in experiments.Chapter 7 is about OCT experiment system. At first, we introduce the OCT system developed by ourselves, including hardware and software. We have done some imaging experiments by use of our OCT system, and the samples include suspension, segments of onion, tooth of human being, and jade, and our system performed well. At the end, we have discussed the influence to OCT signal of dispersion caused by samples, and then we develop a numerical algorithm to compensate it.Chapter 8 is the conclusion and expectation. We draw a conclusion and give some key problems and developing trends for future work of non-invasive optical detection and imaging by use of scattering data.