Analysis Of Flow And Temperature Field Of Optical Elements And Error Control Technology In Continuous Polishing

The development of laser inertial confinement fusion energy based on controlled thermonuclear fusion technology is a cutting-edge research field in the world.It has important strategic significance in military and energy fields.Among them,thousands of large-aperture planar optical elements used in high-power laser devices depend on advanced optical material research and development and ultra precision processing technology of large-diameter planar optical glass elements.Continuous polishing is one of the key technologies to realize the global planarization of large aperture planar optical elements.It has significant advantages in fast convergence of low-frequency surface shape,suppression of intermediate frequency waviness error and reduction of processing cost.The processing tool of continuous polishing is a large size polishing lap,it is difficult to selectively remove different areas on the surface of optical elements.At present,the control of low-frequency surface shape of optical elements mainly depends on the experience of technical workers,and lacks theoretical guidance and mature quantitative control methods,which restricts the development of continuous polishing and even advanced optical manufacturing.Focusing on the problem of surface shape and medium frequency error control in continuous polishing,the paper reveals the influence of polishing parameters on the surface shape error of optics.The paper also proposes an active control method for low frequency surface shape of large aperture planar optical elements based on the comprehensive analysis of heat flow field.The vibration is introduced into continuous polishing to suppress the medium frequency error of large aperture planar optical elements and improve the material removal efficiency.The influence of the fluid field on the surface shape of optical elements during continuous polishing was studied.Based on the contact mode between the optical element and the polishing lap in the continuous polishing process,it is preliminary predicted that the lubrication mode of the liquid film of the polishing interface belongs to mixed lubrication.Using the load sharing method,the simulation models of the polishing pressure distribution and material removal distribution under mixed lubrication were established.The correctness of the simulation model of liquid film characteristics is verified by the measurement experiment of the friction coefficient and the optics surface shape.The experimental results showed that under the effects of flow field,the material removal rate at the edge of the optical element in continuous polishing was greater than that of the center,and when the rotation speed of the optics/polishing lap,the total load of the element and the surface roughness of the polishing lap became larger,the uniformity of material removal of the element surface would became worse.In terms of temperature field,the heat transfer model of the polishing interface in the continuous polishing process was established.To reveal the cause of the non-uniform temperature distribution in the element,and the simulation model of the temperature distribution in the optic was established.The simulation results showed that the bottom surface temperature of the optic was larger than the top surface,and the center temperature of the optic was larger than the edge.Then the non-uniform temperature distribution in the element during continuous polishing is measured by blind holes at specific positions on the back of the element,which verifies the correctness of the simulation model of temperature distribution in the element.The influence of polishing parameters such as rotating speed,eccentricity,ambient temperature and polishing fluid temperature on the temperature distribution in the element was also analyzed.The thermal deformation law of optical elements under the influence of temperature field was analyzed.The polishing experiment revealed the essence of the influence of the temperature distribution in the element on the final surface shape accuracy of the element.It is because the temperature distribution in the element causes the thermal deformation of the element,and the actual processing is the deformed element.Therefore,after the processing,the shape is also changing with the internal temperature cooling of the element,and finally the surface shape is continuously changing.The influence of different polishing parameters on the shape change after machining was also studied.Then the shape control technology of large aperture planar optical elements based on the integrated control of temperature-fluid field was proposed.Based on the theory of contact mechanics,the pressure distribution and material removal distribution models of thermal deformation optical elements were established.The influence of temperature-fluid field on the surface shape of optical elements was verified by polishing experiments.The surface shape control method based on the polishing parameters control such as the rotating speed and changing the temperature distribution in the optic was proposed.And the method of controlling the temperature distribution in the element based on the temperature control cover and polishing liquid was proposed.The application effect of the active control method of the temperature distribution in the element is also verified by experiment.Through polishing experiments,the application effect of the surface accuracy method of active control elements was verified.For different initial surface shapes of optical elements of different materials,different process parameters were selected to realize the rapid convergence of surface accuracy of elements.To quickly converge the medium frequency error of large aperture planar optical elements and improve the material removal rate,a vibration assisted polishing method was proposed,and a short-range vibration processing platform for optical elements was set up.The kinematic model,pressure distribution model and material rate model of optical elements were established successively.The vibration assisted polishing experiment verified that the vibration continuous polishing could effectively suppress the medium frequency waviness error.The convergence efficiency was increased,and the material removal efficiency was increased by four times.The genetic algorithm was used to optimize the process of vibration polishing,so that the medium frequency error of large aperture optical elements was quickly converged in the continuous polishing process,and the medium frequency PSD is better than 2 nm.