Research On Key Techniques Of Thermo-Optical Stability For Refractive Infrared System

With the development of space and military technology, the refractive infrared (IR) optical systems are widely applied, the tough working conditions put forward higher requirement to the capability of optical system. As the infrared materials are sensitive to the temperature variation, thermal environment is the major factor which impacts the performance of refractive infrared optical system. Particularly, for missile borne infrared optical system, aerodynamic heating may result larger temperature difference in optical system. It has great significance on the development of military technology to improve the stability of IR optical system under thermal environment.In this dissertation, the correlation theories of the thermal effects of refractive infrared optical system, key techniques in developing the thermo-optical stability of infrared system, simulation and test methods of thermal stability are mainly discussed. The dissertation includes the following several parts:(1)Athermalization of optical system under uniform temperature.The thermal effects of optical system and the mechanism of thermal aberration are studied. Based on the thermal aberration complementary principle, a mid-wave (3.7μm~4.8μm) infrared athermalized optical system is designed, which has stable image quality in the uniform temperature environment during -40℃~60℃range, the modulation transfer function (MTF) value of optical system reaches to 0.5 at the spatial frenqucy of 17lp/mm.(2)The characteristics of optical system under axial temperature gradient. Lens surface deformation and variation of refractive index under axial temperature gradient are studied. Firstly, by ignoring the lens interior axial temperature gradient, the athermalization principle and design method for optical system are studied, the generalized athermalization equations of optical system under axial temperature gradient are proposed An athermalization chart (T-C-Δt chart) is built by introducing axial temperature gradientΔt to the stastic athermalization chart (T-C chart). An athermalized optical system under axial temperature gradient is designed, which can keep the image quality stable and good in the temperature range of -20℃~60℃, regardless of whether the axial temperature gradient exists or not. Secondly, based on ray tracing, the impact of lens internal axial temperature gradient is discussed, and the variation tendencies of ray tracing with the structure parameters are studied. Then the modification principles of structure parameters are put forward to improve the image quality of optical system under aixal temperature gradient, by which the original system is optimized to keep good image quality when lens internal axial temperature gradient is considered.(3)The characteristics of optical system under radial temperature gradient. The thermal effect models of lens deformation and refractive index distribution are built, which are used to research the variation tendencies of optical system performance with the structure parameters. An athermalization method of optical system under radial temperature gradient is proposed based on the models. Using this method, an athermalized optical system under radial temperature gradient is designed, which can keep the image quality stable and good, in the temperature range of -20℃~60℃, regardless of whether the radial temperature gradient exists or not.(4)Lens athermal mounting technique. A new assumption of bond layer constrain is put forward for full circle bond lens. Based on this assumption, the athermal bond thickness equation is derived; a simplified model and a modified model of athermal bond thickness are built. Through finite element analysis, the simulation results of athermal bond thicknesses are solved for a series of bond lens. Through comparing the simulation solutions and the analytic solutions calculated with athermal bond thickness equations, the application range of the analytic equations are derived.(5)Thermo-optical stability simulation and experiment. As the thermal environments of gradient temperature distribution are difficult to realize in the experiment, the performance of optical system under axial and radial temperature gradient is analyzed by simulation method, the performance of optical system under uniform temperature analyzed by experiment. The results of simulation and experiment indicated that, the optical system designed by athermalization have stable optical performace under thermal environment, the athermalization methods put forward in this dissertation can improve the stability of optical system effectively.