Non-destructive Testing And Evaluation Of Multi-Layered Thin-film Structures Based On Time-domain Optical Coherence Tomography System

Optical Coherence Tomography(OCT)is a powerful cross-section imaging technique.It is more superior to the other optical imaging techniques due to its high resolution and sensitivity.As a new non-destructive testing method,OCT uses visible and near-infrared light sources and therefore the health safety will not be an issue.After rapidly applied to the fields of medical testing in the past decades,it begins to be applied to the non-medical fields recently.In this thesis,we developed and optimized a time-domain optical coherence tomography system and investigated its applications in non-destructive testing and evaluation of multi-layered thin-film structures.According to the balanced detection principle,a broadband low-coherence white light source was employed to build a sub-micron resolution time-domain optical coherence tomography system.For the unsatisfactory reconstruction results of current algorithms,the Stationary Wavelet Transform(SWT)algorithm was introduced innovatively to deal with the OCT images.This method could extract more distinct interface interference signal from the SWT detail coefficients.The primary feature of system was the balanced detection of the hardware,and the stationary wavelet transform of the software.To eliminate the DC noise of the system and improve the signal to noise ratio(SNR),the balanced detection principle was considered in the developed OCT system.The results showed that the SWT-based method greatly improved the imaging resolution,so that the system had the obvious advantages comparing with other systems.In the thesis,the establishment of the optical coherence tomography system(including light source module,coupler module,two-dimensional scanning module and detector module)was analyzed in detail.The component selection of each modules and the optics design were optimized.In order to boost optical coherence tomography(OCT)research,Matlab and ActiveX control were employed in the development of time-domain OCT imaging system(including linear platform control module,NI acquisition module-signal processing module and image display module).In the OCT software,scanning module of system was used to perform depth scan in accordance with the required timing and parameters.NI data-acquisition card was used to obtain OCT interference signal synchronously.OCT signals were processed using some signal processing methods,including stationary wavelet transform denoising etc.The full width at half maximum(FWHM)of the interference waveform was calculated to achieve the resolution of the developed OCT system.The processed OCT data was reconstructed to 2D OCT images which could be used for internal structural characterization of materials.The self-developed time-domain optical coherence tomography system was used comparatively to investigate different samples characterization,such as plasma spray coatings etc.The influence of light source to the system was analyzed comparatively by using two light sources:a broadband white light and a Super-luminescent Diode(SLD)with a center wavelength of 850 nm.Because of the wider bandwidth and smaller center wavelength,the white light system achieved a higher resolution of 0.93 ?m,it's more suitable for the internal structure and defect detection of multi-layered thin-film structures;However,SLD light has relatively narrower bandwidth and larger center wavelength,a lower resolution could be achieved by the SLD-based OCT system,but SLD has better penetration that made it suitable for detection of plasma spraying coating,some other thicker coating materials,and the imaging of some biological samples.The time-domain optical coherence tomography system investigated in the thesis has effectively improved the quality of image reconstructions.Our present works has extended the OCT applications to some non-medical fields.