Research On Certain Key Technologies Of Freeform Optical Imaging System

With the development of machine vision,remote sensing,and wearable displays,the applications of optical imaging systems are becoming more and more complex,and the requirements for imaging performance is becoming higher.By breaking the constraint of rotational symmetry and get more degree of surface shape freedom,freeform surfaces have brought new solutions to the optical imaging system.However,a series of key technical problems in realizing freeform imaging systems have emerged.To realize the freeform imaging systems easier,researchers have been researching the key technologies for years.Among the key technologies,the analyzing method of freeform imaging system design and optical freeform surface shape testing are two of the most important technologies.Regarding to the analyzing method of freeform imaging systems design,with more design freedoms,the design process of freeform imaging systems become more complicated and the design methods become more diverse.A principle should be followed when improving the performance of the imaging system with freeform surfaces: one should use freeform surfaces as few as possible.Therefore,the designer needs to conduct careful analysis on the selection of the optimal freeform surface locations.However,most of the existing methods for selecting the locations of freeform surface are still based on the empirical judgment or paraxial approximate index.Obviously,for the design of non-paraxial freeform imaging systems with increasingly complicated optical structures,the lack of means to effectively analyze the locations of freeform surface has increased the complexity of the design.Regarding to the optical freeform surface testing,since the freeform surface shapes are always non-rotational symmetric,the traditional optical surface testing method is difficult to meet high-efficiency and high-precision at the same time.Computer Generated Hologram(CGH)testing provides a new solution for null testing of optical freeform surfaces,and has been favored by researchers because it can provide references for system alignment while testing.The realizing steps of CGH mainly include design,encoding and fabricating.Among them,a good encoding method can effectively reduce the volume of encoded data while ensuring the accuracy of CGH encoding,which could reduce the burden on processing equipment.However,few related researches are presented,a simple and universal method is lacking for highprecision encoding of CGH,which has limited the popularization of CGH testing.This article is carrying out researches on the above key technologies and their difficulties.In the analyzing method of the freeform optical imaging system,basing on the characteristics of the freeform optical imaging system,the selecting method of the freeform optimal locations during the design process is mainly studied.In the freeform surface shape testing,the high-accuracy CGH testing method is studied and the research is mainly focuses on high-accuracy encoding method of CGH.The contents of the researches are:1.According to the characteristics of the non-rotational symmetry of free-form surface,the elements in the design process are studied,and the key points of selecting the freeform surface's optimal location are discussed.Combined with the characteristics of non-paraxial imaging system,the concepts of Pupil Ray Deviation and Field Ray Eccentricity based on imaging ray path tracing are proposed.Basing on the ray eccentricities,a method for analyzing the optimal location of the freeform surface while optimizing the freeform optical imaging system is proposed.This method provides an effective analysis method for selecting the optimal location of freeform surface in the design process of freeform optical imaging system.2.A high numerical aperture off-axis reflection microscope objective is designed,the system was analyzed with the ray eccentricities analyzing method,and the optimal free surface location was selected.After optimized with one freeform surface,the fullfield numerical aperture of the system reaches 0.59,and at 1000lp/mm the central field's MTF is better than 0.5,the marginal field's MTF is better than 0.36.The enumeration method was used to study and compare the different freeform surface optimization results of the system,the validity of the location selecting results based on the ray eccentricity was verified.3.The technologies of high-accuracy CGH testing are studied and discussed.Aiming at the problem of data compression,a CGH encoding method based on nonmaximum suppression is proposed combining with the characteristics of the CGH pattern.The principle of this encoding method is studied in detail,and the nonmaximum suppression coding error is analyzed by simulation.The high-accuracy CGH based on non-maximum suppression was verified by experiment.With the condition that the encoding error is less than 140 nm,a ?50mm CGH is completed.The processing data volume is 12.9GB and the encoding file volume is 120 MB.4.The non-maximum suppression encoding CGHs was used to complete a highaccuracy surface testing of an off-axis parabolic mirror.Starting from the design of the testing scheme,to optical design,encoding,error estimation,and finally to the measurement of the mirror,we presented the entire process of CGH testing.The encoding error is less than 85 nm,and the testing accuracy reaches 0.021 ? RMS.Compared with the testing results of the Stigmatic Null Test,the deviation of the results is within 0.004 ?.The result verified the non-maximum suppression encoding CGH's high-accuracy testing ability on the optical surface with non-rotational symmetry.