Research On Single Fiber Focused Spot Digital Scanning Imaging Technology

The pathology and micro invasive surgery request the endoscope to probe into the vivo with tiny probe and cell-level imaging capabilities. It drives the development of the endoscope toward fiber-optic, miniaturization and high resolution. For the single single-mode fiber optic endoscope, the scanning unit is integrated with the probe. For single-mode fiber bundle endoscope, the numble of the fibers enlarge the probe volume. The interval between the fiber cores influences the image resolution. The single-mode fiber optic image limits the minimize miniaturization and high resolution of fiber endoscope. Multimode fiber can transmit several physical quantities parallel and realize local imaging. Research on single multimode fiber imaging technology has important theoretical value and practical significance for the development of the endoscope.The existing multimode fiber imaging method has poor noise resisting ability and needs the mechanical scanning unit. How to achieve multimode fiber digital scanning imaging with no lens and mechanical scanning unit is an important scientific problem. The existing multimode fiber focused spot shaping method needs iterative computations and only one focused spots can be formed in each optimizing process. How to form a lot of focused spots with no-iterative calculation in an optimizing process at multimode fiber output is an urgent scientific problem. Some blind spots exist in the reference light from the multimode fiber, which affect the quality of the focused spot. The phase shifting error of liquid crystal spatial light modulator, the number of liquid crystal spatial light modulator subregion and the ideal spot focus range are the key item in focused spot shaping method which affect the quality of the focused spots. How to eliminate the blind area and the optimization method of these key parameters in multimode fiber focused spot shaping method is another important scientific problem.The research work in this paper is carried out to solve the above problems. The main contents are as follows:Firstly, single multimode fiber focused spot digital scanning imaging(FSDSI) method is studied. Single multimode fiber FSDSI model is established according to the incoherent imaging theory and the turbid media transfer theory. The Gauss-Seidel iteration method is applied to solve the imaging model and reconstruct the object image. Single multimode fiber FSDSI system is developed, in which liquid crystal spatial light modulator is used to control the input light field of multi-mode fiber to form the scanning focused spots in multimode fiber output in order to realize multimode fiber digital scanning imaging without the mechanical scanning unit. The FSDSI system has the advantage of fiber optics, miniaturization and high-resolution.Secondly, multimode fiber focused spot shaping(MMFFSS) method based on adaptive parallel coordinates(APC) algorithm is proposed to form a lot of focused spots. The MMFFSS model based on APC algorithm is established based on light field mode decomposition and turbid media transfer theory. Online interference field recording and offline phase optimization are adopted with little amount of calculation and no iteration. A lot of focused spots are formed by APC in single optimizing process. The problem of forming one focused spots in each optimizing process with large amount of calculation and iterations is solved. The time to shape focused spots is shortened greatly. Compared with adaptive sequence coordinate ascent(SCA) algorithm, the overall performance of APC algorithm is close to SCA algorithm. The APC to SCA ratio of the time taken to form 100 spots is 1/64. The ratio becomes smaller with the increased focus on the number of spot shaping.Thirdly, the blind area elimination method is proposed based on complementary reference modes, in which genetic algorithm is applied to quickly obtain the optimal complementary reference modes. The problem that the quality of the focused spots in the blind areas is bad is solved. The reason for the phase shift error is analyzed. The suitable phase shift algorithm is chosen to elimate the phase shift error. The number of SLM subregion and the ideal spot focus range is optimized and the optimal values of these parameters are obtained by theoretical analysis and experimental optimization. The quality of focused spots is improved.Finally, the experimental apparatus of single multimode fiber FSDSI system is built. The calibration method of the system impulse response function is researched. The scanning focused beam spots are used to form the moving point light sources on the object surface, which are used to calibrate the impulse response function. A lot of scanning focused spots are formed by the MMFFSS method based on APC algorithm and used to scan the resolution target. The object image is reconstructed. The experiments show that the view field is a circle with 50μm diameter, the work distance is 60μm, the resolution of the system is 2.19μm, which is close to the system theoretical resolution 2.07μm. The measurement time of the FSDSI method is about 0.45 as the time taken by the random spot sampling imaging method.The influence of the object position and the sampling plane axial offset on the system resolution is analyzed.