| Optical coherence tomography (OCT) is a fast developing technology for medical diagnosis in vivo. The resolution of optical coherence tomography (OCT) depends on the spectral properties of the light sources used in OCT systems. The minimum distance that can be resolved by this technique is inversely proportional to the spectral width of the light source. Using broadband light sources, ultrahigh-resolution OCT can achieve axial image resolutions on the few micron scale. However, if the tissue under investigation is dispersive, the envelope of interference signal broadens and the resolution decreases. So this problem is raised and challenges us to find methods to improve the resolution of OCT image.In this thesis, based on the investigation on the development and application of Optical Coherence Tomography technology, especially the limit of current dispersion compensation method, the dispersion effect in OCT is studied both theoretically and experimentally. Through theoretical analysis and numerical calculation, the reason why the dispersion in OCT happens is found, the dispersion originated from RSOD and Sample is discussed, the influence of group velocity dispersion and third order dispersion on signal are compared. Further more, the experimental setup of OCT is built and the envelope broadening caused by dispersion is observed, the theory of dispersion is testified. If the dispersion is not compensated, the resolution of OCT signals degrades. Thus, on the ground of theory analysis and experiment test, a new numerical compensation method is presented. Our experiment results have shown that the presented numerical dispersion compensation method is efficient. Using this automatic iterative optimizationmethod, the unbroadened signal can be regenerated with a high resolution. The factors such as the test error and noise that affected the quality of the OCT signal are also considered.The content and the main achievements of this thesis cover the following aspects:In theory, the dispersion of Rapid Scanning Optical Delay Line and some samples are calculated respective, the group velocity dispersion and third order dispersion of BK7 glass and a RSOD with fixed parameters are numerically simulated, the OCT signals of the back surface of different sample are simulated.An experimental OCT setup is built, composing of Super Luminescent Diode as the broadband light source; a fiber Michelson interferometer as heterodyne system to detect the signal, one dimension axial scanning realized by a step motor controlled with a computer, signal acquisition and input to computer using PCI board card. In order to improve the performance of OCT system, intellectualized software are developed to control the setting of step motor and PCI board card digitally and conveniently, the amplification and low frequency electrical circuits are designed to amplify the OCT signal and decrease the noise. The axial resolution of our OCT system is 12.1um and the lateral resolution is 24.2um.In the application of our experimental OCT setup, the interference signals of different samples are observed. The experiment results testify the theoretical achievement on dispersion. The amplitude and phase in frequency domain are analyzed with Fast Fourier Transformation and a new numerical method is presented to compensate dispersion in OCT. The golden section method is employed to find the optimal coefficient for dispersion compensation in the present method. The coherence width of interference signals through dispersive samples is tremendously reduced after processed with the present method. The feasibility of this dispersion compensation method is testified with experimental results. Some factors such as the scanning speed, the data acquisition speed that affect the quality of the OCT signal isalso considered.Finally, I make a conclusion of this thesis and plan what will be done in the future. |
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