The Influence Law And Process Control Of The Surface Figure Of Large Optics In Continuous Polishing

Using the high-power laser facility to carry out inertial confinement fusion research is a frontier research field in the world today,which has important strategic significance in the fields of military and energy.Thousands of large and high-precision optical elements used in the high-power laser facility represent the highest level of ultra-precision processing equipment and technology nowadays.Full-aperture continuous polishing is one of the most important processes in fabricating large flat optical elements,which has significant advantages in controlling the mid-spatial frequency errors,improving processing efficiency,and reducing processing costs.The large pitch polishing lap used in continuous polishing is difficult to achieve selective removal of high and low areas of the optic surface,so the control of low frequency surface figure mainly depends on the operators’ skill and lacks theoretical guidance and quantitative control methods.Thus,continuous polishing has long been a bottleneck in the field of advanced optical manufacturing.This paper focuses on controlling surface figure in the continuous polishing,the influence of polishing process factors on the optics’ surface figure was deeply studied by theoretical analysis and polishing experiment,deterministic control methods of polishing process factors were proposed,and then a comprehensive control method of surface figure of continuous polishing was established.This study first analyzed the converging mechanism of the surface figure in continuous polishing,explored the conditions that the optical element needs to meet to achieve a stable surface figure under the steady polishing condition based on the material removal equation.It is revealed the dependence of the surface figure on process factors such as the relative velocity,polishing pressure and material removal coefficient.In order to obtain the distribution uniformity of the velocity and its influence on the surface figure,a kinematic model of the continuous polishing was established,and the effective sliding distance was proposed to analyze the velocity and material removal uniformity.The influence of the polishing motion parameters and the groove pattern of the polishing lap on the effective sliding distance distribution of each point on the optic surface is studied.The results show that the closer the rotating speed of the optic and lap and the greater the offset distance of the optic,the better the uniformity of the sliding distance.When the annular groove is used,the annular texture on the optic surface is easily caused by the annular structure feature,which affects the uniformity of material removal.The radial groove,square groove and spiral groove are beneficial to improving the uniformity of velocity distribution at each point on the optic surface.For the polishing pressure,a polishing pressure model based on elastic contact conditions was established,and the influence of the mechanical properties and surface shape of the polishing lap on the contact pressure distribution was analyzed.For the surface shape of the polishing lap,according to the mechanical structure and movement characteristics of the continuous polishing machine,a laser displacement sensor was used to scan the polishing lap surface with a spiral path,and then an interpolation algorithm was used to generate the three-dimensional shape of the polishing lap surface.The composite correction method using precision milling,sub-aperture correction and fullaperture correction was established,and the application effects of the precision milling and sub-aperture correction are verified through polishing experiments.For the full-aperture correction of the polishing lap shape,in order to explore the shape development of the pitch lap under the loading of the large-size conditioner,the viscoelastic mechanical properties of the pitch lap were studied,and the viscoelastic creep model of the pitch lap was established.The model explored the viscoelastic deformation mechanism of the pitch lap under the loading of the conditioner,revealed the dynamic shape and the development rule of the polishing lap in the polishing contact area,and established a method for controlling the shape of the conditioner to the polishing lap,and then the full-aperture correction method for the polishing lap shape is verified through the polishing experiment.For the material removal coefficient,it is influenced by the optical element,polishing slurry and polishing lap in the polishing system.The contact chemical reaction mechanism in the polishing process was explored,and the influence of optical element material and polishing slurry p H on material removal coefficient was analyzed.The monitoring method of polishing lap surface glazing state was established.The change of polishing lap surface glazing state during polishing process and its influence on material removal coefficient were studied.The repair method of polishing lap surface glazing state was proposed.On the basis of the above researches,an ultra-precision continuous polishing machine was developed.A comprehensive control method and an integrated process system for the surface figure of optical elements were established,and a systematic verification experiment was carried out.The developed ultra-precision continuous polishing machine adopts an ultra-large air floating turntable as the polishing lap.The air floating turntable is driven by an external gear ring geared with a servo motor gear to achieve high-precision and stable operation of the polishing lap.A multi-station bridge mechanism is set above the polishing lap to realize the multi-functional integrated control of optical element polishing,polishing lap detection and correction.The proposed comprehensive control method for surface figure controls the polishing conditions from the three aspects of relative velocity,polishing pressure and material removal coefficient.An integrated process system was established to realize intelligent monitoring,analysis and control of the polishing process,thereby significantly improving the polishing accuracy and efficiency of the optical element surface figure.