Three-dimension Topography Measurement Using Digital In-line Holography

Micro-nano elements are widely applied in microelectronics,optical communication,astronomy,metrology and structural biology.They provide a value of irreplaceability in national defense technology and national economy and promote technological innovation and social development.The high precision three-dimension topography measurement of micro-nano elements is the basis and guarantee for machining accuracy,it is also the prerequisite for the development of micro-nano machining technology.Optical measurement methods have the advantages of non-contact,fast,high accuracy.But they mostly used scanning imaging,accuracy suffers the influence of wafer stage,system structure is complex or it demands hash measuring circumstance.As a flourishing optical branch,digital holography provides a new solution way to the measurement field.In digital in-line holography,image sensor is used as recording medium to collect the interference fringes pattern and store it in a computer.Reconstruction algorithm presents the complex amplitude distribution of object.The characters of simplest optical path,flexible in recording and reconstructing,fast imaging,no-contact,and wide field of view make it a promising tool for the studies of medicine,biology,and material sciences.In this thesis,comprehensive study of digital in-line holography is carried out,comprising of theory,simulation analysis,and experiments,to explore a new type of microscopy imaging system.Firstly,the advantages of digital in-line holography are analyzed from the aspect of digital holography,and the hot research and applications are briefly described.Secondly,the theory and simulation,including the angular spectrum theory of diffraction and Fresnel paraxial approximation,are introduced in detail.Besides,several methods in improving the imaging quality and resolution are analyzed respectively.Then,contrast hologram is utilized to restore the object through the reformed reconstruction algorithm.We propose introducing Fresnel zone plate to split the incident light into two parts: one spreads along the original direction as reference light,another gathers to be a focus.The focus illuminates the sample as a point source generating the scattered object light.Differential-interference fringes pattern is formed by the two beams,real image and virtual image is separated along the axis,and the twin image is disappear in background because of the sensitive distance of reconstruction algorithm.Next,the proof-of-concept experiments have carried from pinhole source,traditional point source,and differential-interference,respectively.We propose an ideal spherical generating method,the laser focuses into an infinitesimal focus through laser beam expander and long working distance microscope objective.Pinhole permutation with different size is utilized to match the focal point,and emerges an ideal spherical wave.Finally,combining mechanical design,hardware circuit and software design,a new digital in-line holography three-dimension measuring system is formed,and the microscopy function is verified by biological samples.The results show that the imaging system has high resolution,wide field of view and fast measurement speed,and it has extensive application prospects.