Study of a middle-wavelength infrared athermalized optical system

Abstract

For cooled 320 240 detector with staring focal plane array, a novel middle infrared athermal optical system is presented. The system is composed of 5 spherical lenses. The materials of lenses are silicon and germanium. The optical parameters and modulation transfer function (MTF) are investigated. The system has the diffraction limited image quality and stable image plane from -30 o C to 70 o C. The characteristic parameters of the system are as follows: f/number of 4, cold shield efficiency of 100%, spectrum region of 3.7 4.8 m and transmissivity of 80%. The system has the merits of simple structure, low price, and it is easy to machining.In spatial and military application, the infrared optical systems need to have diffraction limited image quality and stable image plane from -30 o C to 70 o C. Compared with visible light material, the refractive index change coefficient dn/dT of infrared material is much bigger. So the focus shift with temperature is a significant problem in infrared optical systems. At present, aspherical and diffraction surfaces are used in athermalized optical systems to obtain the best thermal compensation, which results in the disadvantages of high price and difficulty of machining [1][2][3][4] . In order to solve the above problem, a novel optical passive athermal system is proposed in this letter. The system is designed for cooled 320 240 detector with staring focal plane array. The system, which is composed of 5 spherical lenses, is a re-imaging optical system. The materials of lenses are silicon and germanium, and the material of mechanical structure is Aluminum. The optical design parameters, schematic diagram, and MTF curves at different temperatures are given. Because the optical system is designed for a 320×240 staring focal plane array sensor, based on practical application requirement and feature parameters of the detector, it has the following characteristic parameters, as shown in Tab.1. According to the characteristics of cooled infrared optical system, it has the merits of small lens aperture and 100% cold shield efficiency [5][6] . The system adopts the optical pas-