Design Of Infrared Dual-band Athermal Optical System Based On Diffraction Imaging

Airborne aerial camera plays an important role in national defense security,hydrologic exploration.Because of its advantages of high timeliness,reliable accuracy and low cost,it plays an important role in ground target search and location.In particular,dual-band infrared cameras can realize round-the-clock surveillance.Traditional optical design relies on a large number of refraction lenses to correct various aberrations.Because of its special dispersion and thermal difference coefficient,diffractive optics technology can meet the design requirements of traditional airborne camera's lightweight and athermalization.In this paper,the development history of diffraction technology and the latest diffraction optical projects at home and abroad are introduced.Based on the basic theory of scalar diffraction optics,the technical advantages of diffraction imaging and the primary aberration properties of diffraction elements are analyzed.The variable factors affecting the diffraction efficiency of optical system are studied,including the number of micro steps,spectral band,processing error,light incidence angle,high and low temperature environment,etc.By comparing the difference among single-layer diffraction element,harmonic diffraction element and multi-layer diffraction element,MATLAB was used to fit the applicable band range of different components and the relevant characteristics of diffraction efficiency.Secondly,aiming at the non-thermal design method of optical system,the non-thermal principle based on hybrid refractive-diffractive technology is analyzed,and the advantages and disadvantages of this technology and other three non-thermal technologies are compared.Based on this design,a three separation medium wave infrared simulation system is designed.Two kinds of non-thermal T-C diagrams are established according to the passive non-thermal theory of optics.Two kinds of non-thermal simulation systems are designed according to the principle of optical focal distribution of the initial structure.The environmental temperature adaptability of different structures is studied,and the feasibility of applying diffraction technology to the non-thermal design of the system is proved.Finally,according to the actual application environment,the design index of the optical system is determined,the key parameters of the system design are calculated,the preliminary selection and structure calculation of the optical system are carried out,and the SNR and MRTD of the optical system under the condition of long-distance detection are solved.The specific design scheme is divided into the following two parts.(1)The structure of the first scheme is two optical path systems with a total aperture,which uses dichroic mirror for spectral beam splitting.Combining with the non-thermal T-C diagram and the theory of heat dissipation,the initial system is established.The single-layer diffraction element is introduced into each beam splitting system for lightweight improvement.Based on this basis,the non-thermal simulation and optimization are carried out respectively.On the premise of ensuring diffraction efficiency,the mass of the correction mirror group is reduced by about 30%,and all indexes meet the design requirements.(2)The structural form of the second scheme is a common aperture and common optical path system.The double-layer diffractive optical elements are introduced to develop the design.The influence of the factors such as the base material of the diffractive surface,high and low temperature environment,optical band,incident angle on the diffractive efficiency of the optical system is analyzed.According to the diffractive theory,the optimal solution of the material combination is obtained.According to the MATLAB fitting analysis,the double-layer diffractive elements are adopted when the incident angle is within3°.The combination of IRG26 and ZNS can achieve a diffraction efficiency of more than 98%.Based on this,the initial structure of the common optical path system is established,and the non-thermal design of the system is further realized by combining the software.The distribution of the secondary ghost image and the cold reflection energy is analyzed with the help of the macro program,and the corresponding measures are proposed to suppress the stray light effect of the system.