Aero-Optical Effect And Suppression Techniques Of Large Aperture Optical System

Aero-optical effect is an important factor to reduce the image quality of large aperture optical systems or optical systems in complex airflow environment.The image quality can be improved effectively by studying the aero-optical effects and proposing corresponding suppression measures.Therefore,the study of aero-optical effect and its suppression technology have a very highly value in scientific research and engineering significance.Aero-optical effect induced by fluid is mainly caused by the uneven distribution of refractive index.Flow fields of a meter-level aperture beam expander system are analyzed.Ambient wind level is 3,4 or 5(4 m/s,6 m/s and 9 m/s),and its inlet angle is 0 °,30 °,45 ° or 60 °.It is found that the ambient air flow entered to the beam expander system interacts with the complex structure inside the system.It can produce low-speed turbulence,so the flow field contains many eddy current groups which are not easy to dissipate.It can lead the change of flow density and finally cause the change of medium refractive index.The distribution of density field is related to the shape,scale and location of vortex.Density changes become more complex with the increase of the ambient airflow wind grade and the diameter of beam expanding system.The influence of ambient air inlet angle on the flow field is restricted by the lens barrel of the beam expanding system.When the inlet angle is between 0 ° and 45 °,most of the air can enter the system to influent the flow field.As the inlet angle is about 60 °,the lens barrel blocks the airflow into the internal flow field,so the influence reduces.The temperature difference between the ambient airflow and the beam expanding syatem will cause gas flow,which could lead to more eddy currents in the flow field.After mapping the density distribution of the flow field to the refractive index field,the transmitted ray can be traced.Solving the ray differential equation is a common ray tracing method.This paper presents a proposed method for solving ray differential equations.In this method,the refractive index distribution of discrete points is obtained by fitting formula.Then the refractive index gradient is calculated by analytical method.A custom transition function is used to directly correspond the position of points(x,y,z)in Cartesian coordinates with arc length s.The solution expression of the ray differential equation is obtained.This expression can be used to calculate the ray transmission in any refractive index distribution flow field.The optical path length(OPL)and optical path difference(OPD)of ray transmission can be obtained through the calculation of ray differential equation.Based on the optical path difference obtained by optical tracing,the optical image quality of the beam expanding system was evaluated.The used evaluation methodology is wavefront shape,Zernike polynomial and Strehl ratio.The wave aberration generated by the turbulence increases with the increase of the ambient air velocity and the diameter of the beam expanding system.Meanwhile,the change of the inlet angle will cause the wave aberration to alter.Among them,under the wind angle within the range of 0 ° to 45 °,OPD increased significantly.When the inlet angle reaches 60 °,the lens barrel blocks the ambient flow.The amount of air enterd the beam expander system decrease and the turbulence kinetic energy is reduced,thus the wave aberration declines.The temperature difference between the ambient airflow and the beam expanding syatem will not only cause low order aberration,but also bring high order averation.The aim of studying the mechanism and producing conditions of aero-optics is to design reasonable measures to restrain aero-optics effects.In this paper,the aero-optical effects caused by ambient air flow are restrained by suppression technology.Thses technologies includ lengthen the mirror ballel(passive),optimizing the inlet of the mirrors(active)and optimizing the inlet of the cylinder wall(active).Through above measures,the position of the eddy current in the flow field is pressed at the edge of the main ray aperture.In this paper,the aero-optical effects caused by ambient airflow are also studied experimentally.Based on the scale model of a ?520 mm lens barrel,the influence tendency of the inlet angle and wind level of the environmental air has been studied.The trend is consistent with that the environmental air impact on a meter-level aperture expander beam system.The flow field analysis,ray tracing and optical path difference(OPD)algorithm involved in this paper were used to carry out numerical calculation on the scale model.The results of the flow field analysis can fully explain the turbulent motion caused by the ambient air flow in the cylinder.At the same time,the calculated OPD is compared with the measured one,and the error of the two is between 7.43% and 20.01%.