Full-Field Optical Coherence Tomography

Researching the structural and functional information of biological tissue through imaging approach can provide impersonal basis for clinical diagnosis. As a recently developed imaging technique, optical coherence tomography (OCT) based on low coherence interferometer combining with confocal scanning microscopy and heterodyne detection technology, provides non-invasive cross-sectional image with micrometer resolution and millimeter imaging depth in real time. Since it was first introduced, OCT has been an attractive tool for biomedical imaging and thus become the research focus.To avoid the image distortion caused by environmental turbulences and sample motion, full-field optical coherence tomography (FF-OCT) was proposed as a high-speed imaging method that produces en-face tomographic images without lateral scanning. Many groups are doing research on FF-OCT due to its advantages such as high resolution, simple signal processing and high mechanical stability. However, to our knowledge, the relevant results have never been reported so far in China. So the works presented in this dissertation mainly focus on FF-OCT, especially on several problems such as eliminating optical cross-talk and achromatically phase shifting, and further a FF-OCT system is developed.The main work and innovations in the dissertation are summarized as following:1. The forming mechanism of the optical cross-talk inherently occurred in the parallel imaging systems with spatially coherence illumination is analyzed, and a novel imaging method and system to eliminate the cross-talk is proposed by using digital micromirror device combining with smart coding. In our system, point-by-point scanning imaging is achieved at any local area which can suppress the cross-talk, and the maximum number of points can be imaged simultaneously on the full area which ensures the imaging efficiency.2. Phase shift error exists when PZT phase shifter whose phase shift is dependent on wavelength is used to implement the phase shifting of broadband spectrum. To quantificationally understand how the phase shift error influences on the imaging result, an expression according to Hariharan algorithm is presented under the condition that the original phase difference is zero. The result of numerical calculation demonstrates that the influence is unnegligible.3. An achromatic phase shifter based on a rotating half-wave plate and its implementation configuration in FF-OCT system are proposed to meet the demand of achromatically phase shifting of broadband spectrum. The phase shifter is almost independent of the wavelength over a broad range, and can provide phase shift at eight times of the rotating angle of the half-wave plate. Since it's difficult to estimate the azimuth of wave plates and the rotating direction of the half-wave plate in practice, the configuration of the shifter is thus extended and eight types shifter are obtained. Numerical analysis on phase shifting error and amplitude ratio between orthogonal polarization components versus wavelength over a range of 180 nm is conducted for phase shifts required for the Hariharan and three-step phase-shifting algorithm respectively. The FF-OCT system with the proposed achromatic phase shifter has several advantages compared with previous systems.4. The FF-OCT system based on the proposed achromatic phase shifter is developed, and several experiments have been done on it. The practical phase shift is measured and the veracity of phase shift is validated by Hariharan algorithm. The results demonstrate the feasibility of the proposed theoretical model. In addition, the dynamically focusing experiments are conducted.5. In the research of endoscopic OCT system, a Fizeau-type common path OCT system using a fibre-based Michelson interferometer to outside compensate the optical path difference between the reference and sample light in the Fizeau interferometer is developed. Experiments of path length compensation and vibration are conducted, and the results demonstrate that this path length compensation method is feasible and the system can avoid environmental turbulences. Therefore, this OCT imaging approach is best suitable for endoscopic imaging. The detailed endoscopic OCT system expanded from this common path system with outside path length compensation is also presented.