Research Of Micro-displacement Sensing Method Base On Symmetrical Defocus Confocal System

With the development of micro-structure fabrication technology, the machiningaccuracy transforms from normal to micron, submicron even to nanometer, thisarises a higher demand for ultra-precision measurement of three-dimension micro-structure. During the numerous technology of microstructure measurement, by thevirtue of high accuracy, high resolution, non-contact and three-dimension imagingcharacteristics, confocal microscopy system is widely used in the field of micro-electronics, micro-optics, bio-engineering. In several research directions of theconfocal system, the method to improve the system’s spatial resolution becomes theresearch focus of the researchers’.Based on the research on the action of confocal microscopy system’s primaryspherical aberration to its axial and transversal intensity distribution characteristics,this paper gives a symmetrical defocus structure to achieve super-resolution. Forthe reflective confocal microscopy, the article studies the realization of thesymmetrical defocus structure specifically. The main content of this paper is asfollows:1. Theoretical analysis of symmetrical defocusFrom the properties of the symmetrical defocus is the conclusion produced bythe lighting arm multiply the feeler arm.（the multiplicative of the system）, herepropose the symmetrical defocus model of the confocal microscopy. With thestructure of the model, it can deduce the distribution function along the axis andtransverse direction in the situation of symmetrical defocus, then this paper gives adetail analysis of the super-resolution properties produced by the symmetricaldefocus of illumination and detection arm. The simulation shows that this methodcan improve the resolution ability along the axis.2. Establishment of performance evaluation indicators of symmetrical defocusSince the evaluation indicators of pupil filter （including the transverse super-resolution factor GT, the axial super-resolution factor GAand the Strehl ratio S）proposed by Juana is based on the2rd order expansion of pupil optical filter, itcan’t be directly used to evaluate the super-resolution got by the symmetricaldefocus. To cover it, this paper constructs three indicators to evaluate thesymmetrical defocus structure including Strehl ratio S, the value of full width halfmaxim FWHM and side lobe suppression ratio M. On this basis, the calculationmodel of these three indicators is built up. With the MATLAB software and thecomputing model built up, the paper simulates the trend of the three evaluationindicators changed with the symmetrical defocus coefficientW₂₀^Dand optimizes the interval of symmetrical defocus coefficient.3. Realization of symmetrical defocus structureIn the actual reflective confocal microscopy system, microscope objective areused as dual-task including illumination and light collection, it can’t achieve todetach the focal of microscope objective so that it can make a symmetricaldefocus constructure. So two specific implementations are put forward. The one isachieved by quadric phase pupil, this paper deduces the axial and transverseintensity distribution characteristics. The other one is based on the focal shifteffect. Firstly, from the analysis, a group of phase filters which are used toconstruct a symmetrical defocus structure should have3basic characters,including the value of axial focal shift uFshould be opposite number, axial super-resolution factor GAand Strehl ratio S should be equal. In addition, this papergives two sufficient conditions to meet the3basic characters, and attach the stric tproof. Finally, a group of phase pupil filters fit the sufficient conditions areproposed to verify the validity of the proof. The third one is based on differentialconfocal system. The article mainly explains the implementation process, andgives the simulation of axial characteristics of the system.4. Experimental verification of symmetrical defocus featuresThe paper completes the selection of experimental devices required to build aplatform and the optical experiment laboratory. With the VC++6.0and the ADQAPI function compete the software design. To ensure the testing capability of theexperiment laboratory, this paper completes the stability test of laser source,fluctuation test of the piezoelectric ceramics（PZT） and the resolution of theexperiment laboratory. And it also completes symmetrical defocus axial charactertest and the test about variation trend of the max light intensity to verify thecorrectness of the symmetrical defocus thought.