Research On Optical Coherence Tomography With Its Autofocus

Based on partial interference theory,optical coherence tomography(OCT)integrates many technologies such as optics,electronics,automatic control,digital image processing and so on,and becomes a new biomedical imaging technology with the advantages of three-dimensional real-time imaging,no intrusion damage,micron resolution and so on.OCT is suitable for characterizing the structure and function of biological tissue,as well as detecting the development degree of related lesions,and has important application value in biomedical imaging fields,such as dermatology,dentistry,ophthalmology and so on,which can be called as "optical biopsy" and "gold standard" for diagnosis of fundus lesions.The imaging target of OCT is the depth information inside the sample,so that the image quality of OCT is related to the imaging depth of the OCT system,and the penetration depth of sample light in the sample.During the imaging process,adjusting the distance between the imaging system and the sample,and changing the position where the sample light focus falls inside the sample,that is focusing,is beneficial to ensure the maximum distinguishable imaging depth,improve the penetration depth of sample light in the sample,obtain more depth information,and then improve OCT image quality.At present,most of the OCT equipment used in the market complete the focusing process manually.Manual focus will not only increase the extra time and labor cost,but also bring some artificial errors.As a result,this paper studies the OCT technology and its automatic focusing.The main research contents are as follows:1.The principle,classification,development and application of OCT technology are introduced.After that,combined with formula derivation and simulation,the theory of OCT technology in time domain and Fourier domain are analyzed,and the importance of uniform sampling in wavenumber domain of Fourier domain OCT is placed on emphasis.The definiens of spatial resolution,imaging depth,imaging speed and sensitivity are listed;2.A SD-OCT imaging system has been designed and built,which consists of broadband light,interferometer,spectrometer,computer and so on.The theoretical depth resolution of the SD-OCT imaging system is 2.895 ?m,the theoretical lateral resolution is 14.823 ?m,and the theoretical imaging depth is 4.491 mm in air.Wavelength calibration and depth calibration have been completed,and the maximum distinguishable imaging depth in air is ±2.2 mm,where the superposition of depth information and mirror noise can be observed.The uniform sampling method in wavenumber domain is designed and simulated.The imaging results of nail,fingerprint,cherry-tomato and onion of the system are shown and the reasons for the bright long line and short line in the depth direction of the imaging results are analyzed;3.A SD-OCT automatic focusing system has been designed and built,which is composed of the two micro motion modules under the reference arm and the sample arm in the SD-OCT imaging system,as well as the automatic focusing algorithm.The automatic focusing algorithm is mainly composed of image quality evaluation algorithm,zero optical path difference position automatic adjustment algorithm,and sample light focus automatic adjustment algorithm.In evaluating the imaging results of nail,fingerprint,cherry-tomato and onion,the calculation results of the image quality evaluation algorithm and the standard variance reach a good agreement in the numerical values.During the automatic focusing experiment of onion,the zero optical path difference position automatic adjustment algorithm realizes the zero optical path difference position near the sample surface,which guarantees the maximum distinguishable imaging depth of the sample,after that,the sample light focus automatic adjustment algorithm realizes the collection of more depth information,which improves the image quality evaluation calculation result by about 3.6% and the standard deviation calculation result by about 2.6%.