Study On Key Techniques Of Aspheric Topography Measurement Based On White Light Interferometry

Aspheric optical components are extremely widely used in modern optical systems owing to their miniaturization and high degree of freedom,but their fabrication and measurement are much more difficult than traditional optical components such as spherical mirrors or flat mirrors.Advanced surface measurement equipment plays an important role in improving the machining quality of aspheric surface,while the current traditional aspheric topography measurement technologies are difficult to give consideration to precision,versatility and efficiency.The development of white light interferometry technology has realized two-dimensional and three-dimensional measurement of surface processing surface shape,and can more accurately describe the surface morphological details characteristics,such as surface roughness contour,texture,geometry measurement,which has important reference significance for the quality control of aspheric surface precision machining.This paper focused on the development of white light scanning interferometer and key technologies,proposed a design method of white light scanning interferometric system,focused on the pre-processing method of white light interferometric signal and signal peak value calculating method,built the hardware system of white light interferometric measurement instrument,developed the relevant testing software,and proved the accuracy and repetitiveness of the proposed testing system through system calibration experiment and aspheric surface measuring experiment.The main work accomplished in this paper can be summarized as follows:1)Different methods and principles of quality measurement of optical components were studied,the principles of current common aspheric surface quality testing methods and their advantages and limitations were analyzed,and the advantages and significance of white light scanning interferometry in aspheric surface measurement were described in detail.2)Based on the two-beam interference theory,this paper analyzed the formation process of white light coherent scanning interference signal,proposed a design method of white light scanning interference measuring system,selected the system components,built the measurement system,and studied the key algorithms according to the actual measurement requirements,and developed a set of white light interference measurement software integrating interferometer hardware control,data processing and quality analysis functions.3)The effects of scanning sampling process,internal dispersion phenomenon and environmental noise on the interferometric signal of the interferometric microscope system were analyzed.In order to improve the acquired signal quality,on the basis of the analysis of the signal preprocessing method of central differential in combination with filter,this paper proposed an adaptive signal preprocessing method based on Empirical Mode Decomposition（EMD）,in which the normalized index K is synthetically built with the normalized mutual relationship number,mean square error and peak signal-to-noise ratio as the threshold reference of EMD decomposition order,and the signal decomposition reconstruction is carried out according to the flow path,and the quality evaluation index of the reconstructed signal is calculated each time,and the best decomposition reconstruction order can be accurately located and the optimal pre-processing effect of white light interference signal can be achieved in the end.The effectiveness of the proposed algorithm was validated by comparing the proposed method with existing methods through system simulation and analysis methods.4)The current common peak resolution method of white light interference signal was studied.Based on the signal demodulation technique,an energy operator based on forward-to-backward difference averaging was proposed to improve the robustness of signal demodulation,and the acquired envelope amplitude signal was post-processed to improve the smoothness and symmetry of the local envelope curve and the resolution accuracy of the detection value was further enhanced by cubic spline interpolation and gaussian fitting on the local peak function.5)Calibration experiments of the measurement system were conducted using VLSI standard steps to ensure the measurement accuracy and stability of the instrument.In order to test the performance of the measurement system,test experiments were conducted on aspheric samples.After comparing and analyzing several sets of actual measurement data,it was verified that the designed white light interferometer can achieve the measurement of aspheric surface contour and texture roughness with sub-nanometer accuracy,the measurement repeatability error was less than0.1nm,and the measurement accuracy and stability meet the design requirements.