| Infrared technology has been widely used in military,remote sensing and other fields because of its passive operation,full-time observation and impregnability.Continuous zoom optical systems with short focal length search and high resolution of long focal length are increasingly used in airborne platforms.With the development of photoelectric technology,the optical system needs to adapt to the longer and longer distance,and the more and more demand for small target observation.This puts forward new requirements for the design of refrigerated infrared continuous zoom optical system with large aperture,long focal length and high variable ratio.Through the selection analysis of the structure of the optical system,the off-axis catadioptric zoom system is selected to achieve the characteristics of large aperture,long focal length,high variable ratio and so on.In the aviation environment,image motion compensation mechanisms such as fast steering mirrors(FSMs)installed on pupil planes are commonly used to improve the imaging quality and the image resolution due to the movement of the air vehicle relative to the targets,as well as owing to the vibration of the platform and the swing of the camera.Besides,the infrared system needs to meet the 100 percent cold shield matching to improve the observation ability of the system.Therefore,in the process of zoom,keeping the consistency of pupil position of infrared zoom subsystem is the main problem that airborne infrared zoom optical system is actually faced with.The constraints of volume and weight of aviation platform also increase the difficulty of optical system design.Therefore,it is of great significance to study airborne infrared continuous zoom optical system.Aiming at the special design requirements of infrared continuous zoom system,this paper studies some key technologies of airborne catadioptric infrared continuous zoom optical system,then proposes the design method of dual conjugate infrared zoom system based on particle swarm optimization algorithm,the following aspects are studied in this paper:(1)According to the design requirements of directional laser countermeasure system,the selection of optical system is studied,the advantages and disadvantages of various types are compared,and the design idea of off-axis folding trans is proposed.The infrared continuous zoom system is divided into two parts:off-axis reflectionfree system and rear group transmission continuous zoom system.The two parts are combined according to pupil matching and cold diaphragm matching for unified optimization.(2)In order to solve the problem that the traditional zoom optical system can not keep the pupil plane and image plane stable at the same time,the dual conjugate zoom theory is studied.Based on the Gaussian imaging theory and matrix optics theory,the calculation formula of the first order parameters of the initial structure of the dual conjugate zoom system is derived.According to the dual conjugate zoom theory,three groups of independent moving lenses are used to obtain the stable pupil plane position in the zoom system.By analyzing the realizability of the optical system,the zoom part is determined as "positive-negtive-positive" structure.(3)Based on particle swarm optimization(PSO)algorithm,an optimization model based on Gauss theory was established to solve the optimal solution of the first order parameters,so as to quickly obtain the initial optical focal distribution and optical spacing at long and short focal points.Firstly,the parameters of the optimization algorithm are discussed,and the factors affecting the global optimization and convergence rate are analyzed.Secondly,an evaluation function was established,including the volume and aberration of the system.The global optimal solution of the first order parameters was obtained through iteration,and the initial structure of the zoom system meeting the requirements was finally obtained.(4)In order to improve the reflectivity of the mirror in the laser spectrum and avoid the failure of the adhesive layer caused by the coating temperature,the flexible support structure of the microstress mirror was studied on the basis of the traditional adhesive method to reduce the influence of the support structure on the mirror profile.The mirror was coated before and then bonded,so that the surface profile of the mirror could meet the optical requirements before and after the support.(5)The common aperture laser infrared refraction type infrared zoom optical system is designed and completed.The laser beam expansion ratio is 5 times,the focal length range of the infrared system is 200mm～800mm,the detector is Stirling refrigeration type 320×256 medium wave focal plane array,F number is 4.Pixel size 30μm×30μm.An aspheric optical design with high steepness and fast focal ratio was used to compress the volume of the optical system,and the initial structure of the double conjugate zoom optical system was obtained by an optimization algorithm.Then,the optical design software was used to optimize the structure,and the dimensions and weight of the zoom system were reduced by combining the folding mode of the plane mirror.Through the optimization design,the modulation transfer function of each field of view at Nyquist spatial frequency 161p/mm is greater than 0.4.After the system is assembled,the focusing function of the laser transmitting system is verified by ground experiments,and the image motion compensation imaging experiment of the zoom system is carried out,and the image quality is excellent.Based on the theory of double conjugate zoom system and the solution of the first order initial structure,the design method of pupil plane stabilization is presented to solve the problem of image motion compensation in large aperture zoom system.At the same time,a new bonding method for laser lighting mirror is proposed.Complete theoretical analysis and proof are carried out,and the zoom system is verified by simulation and experiment.The final research results show that the large aperture long focal length medium-wave infrared continuous zoom system designed in this paper has good image motion compensation effect,compact volume,and meets the requirements of aerial zoom optical system. |
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