Research On Pupil Spliting Snd Focus Shifting Detection Method In Heterodyne Confocal Microscope System

With the development of nano machining technology,nano metamaterials and the increasing complexity of ultra precision structures,the demand for three-dimensional measurement of micro spatial structures is increasingly urgent.Confocal microscopy has the advantages of unique three-dimensional tomography ability,high resolution,no special requirements for the measured samples,non-contact measurement will not damage the surface of the sample,and can be widely used in three-dimensional microstructure morphology measurement.At present,heterodyne confocal is still one of the main methods to improve the axial resolution of confocal microscope system.However,due to the hard pinhole combined with photodetector,the system has many problems,such as difficult adjustment of pinhole defocusing position,easy blockage and so on,which makes the measurement performance of the system difficult to achieve the best,and lacks the three-dimensional coherent imaging model of the system under the tilt of the collected light.In order to solve the above problems and further improve the measurement performance of the system,the main work of this project is as follows(1)A three-dimensional coherent imaging model of heterodyne confocal with pupil separation and focal shift is established.The method of pupil separation and focal shift is introduced into the measuring arm of confocal system to simplify the optical path of the detecting arm.The soft pinhole detection method is adopted to solve the problem of hard pinhole detection.The three-dimensional coherent imaging model of the tilted collecting optical path is established.Because the grating is added to the detection arm of the confocal system,the collected light after splitting is tilted into the imaging lens,and the distribution of the diffractive light field on the detection plane is analyzed.Based on the theory of infinite distance confocal microscopy,the three-dimensional coherent transfer function model of the whole system is established,The effect of beam tilt on the axial response of the system is discussed.(2)The design of symmetrical phase shift pupil filter is carried out.Based on the modulation characteristics of phase pupil filter,the pupil optimization model under inclined illumination is established,and the pupil structure is optimized by using genetic algorithm.The simulation results show that the designed conjugate phase pupil filter can produce the optimal positive and negative defocusing at the focal plane of imaging lens,and the radial resolution is increased by 6%,The axial resolution of the system is increased by 74%by differential processing,which can effectively expand the measurement range of the system.(3)Since the SLM is introduced to replace the actual pupil filter in the system,in order to make the SLM pose not affect the pupil modulation,this paper analyzes the influence of the pupil phase diagram translation and rotation on the modulation.The simulation results show that the pupil offset is less than 38?m.In order to ensure the accuracy of phase map loading,a method of SLM phase map alignment based on Fresnel zone plate is proposed.The position of phase map is determined by the alignment of parallel beam spot center and Fresnel zone plate focused beam.The experimental results show that the final alignment accuracy is 6.5?m.It can meet the requirements of optical alignment accuracy.(4)Finally,the experimental platform is built,and the corresponding experiments are carried out on the designed pupil filter and the overall performance of the system.The experimental results show that the detection result of confocal system will not be affected by the oblique incidence of light beam caused by grating splitting;The axial super-resolution factor G_A=1.042,and the radial super-resolution factor G_T=1.052;Finally,the heterodyne confocal system with pupil separation and focus shifting is built.The results show that the system has 25%performance when the plane lens is used as the measured sample and the measuring objective lens NA=0.2 Compared with the common confocal system,the measurement range is increased by 47%,and the slope of the linear region is increased by 66%.When using soft pinhole detection,the axial resolution of30nm can be achieved.The accuracy of the 3D coherent imaging model with the collected light tilt is verified.The above results show that the proposed method can simplify the optical path of the detection arm,make the system easier to integrate,and solve the problems of hard pinhole;At the same time,through the modulation of the conjugate pupil,the measurement range of the heterodyne confocal system is expanded,and the three-dimensional resolution of the system is further improved.