Research On High Resolution Microsphere Digital Holography Imaging Method

Digital holography is characterized as a real-time, full field of view, non-contact and quantitative phase contrast imaging approach and has been applied in cell morphology observation, micro-nano element, temperature field, the particle tracking. With the rapid development of the life science, micro processing technology and microelectronics technology, the profile measurement of micro-nano structure becomes a hot research spot. It is of great significance to improve the resolution of the digital holography. There are some superresolution digital holography technologies, such as synthetic aperture, the subpixel sampling method, angular multiplexing, grating method. These methods need to record multiple holograms and complex integrated reconstruction algorithm. In recent years, it is shown that microsphere can also improve the resolution of the imaging system, but this approach is only used in combination with white light microscopy and only intensity images are obtained. To improve the resolution of the digital holography and to expand the phase-contrast imaging, we introduce a new method combined the microsphere with digital holography. The dissertation has been proposed as follows:The influence of the microsphere parameters such as refractive index and diameter on imaging feature is studied. First, we deduce the relationship between the spherical aberration, numerical aperture and the microsphere parameters. The theoretical analysis shows that the spherical aberration is proportional to the radius of the microsphere and increases with the relative refractive index first, and decreases when the numerical aperture reaches 1. So if the radius and refractive index of the microsphere are appropriately applied, the resolution of the image system will be high. Second the relationship between the microsphere radius and the paraxial focal length of the microsphere is analyzed. The results indicated the microsphere paraxial focal length is proportional to the radius of the microsphere and is inversely proportional to the microspheres refractive index, which is verified by COMSOL Multiphysics simulation. Finally, we verify that microsphere can convert the high spatial-frequency evanescent wave into magnified propagating waves for far-field detection by simulation.The experiment of microsphere super-resolution digital holographic microscopy is studied. And the imaging principle of MO imaging system combined the microsphere is analyzed. The numerical aperture of the system with the microsphere is quantitatively analyzed. An inverted setup of off-axis Frernel pre-magnificated digital holographic microscopy based on microsphere is designed and built. Then we obtain a series of intensity phase reconstruction images of the various samples by phase unwrapping and surface fitting phase processing algorithms. The experimental results indicate that the pre-magnification off-axis image digital holography system based on microsphere not only achieves the super-resolution by single exposure, but also gains the phase image of the sample. Finally, the ability to the microsphere self-assembly experiments is conducted to verify the ability to expand field-of-view.