Study And Fabrication Of Athermalized Infrared Refractive/Diffractive Hybrid Optical System

Most of optical instruments for military and aerospace application are expected to perform over a wide temperature range. The change in temperature will cause not only physically expand or contract the lens curvature as well as the thickness of the lens, which will furthermore cause the defocus of the image surface and therefore the performance degradation. Because of the large temperature coefficient of refractive index, infrared optical systems often suffer the performance degradation seriously. For this reason, several active or passive compensation mechanisms have to be appended to thermal imaging systems to compensate the performance degradation with temperature changes. There are usually three compensating methods: mechanical, electro-mechanical and optical. Among them, the optical compensation method which is frequently called athermalization is highly valued for its simple mechanism, small volume, light weight and high reliability. Some theories and methods of designing athermalized optical systems have been developed. In recent years, the progress in the theory and fabrication of diffractive optical elements (or binary optical elements) makes it possible for a hybrid optical system which employs both refractive and diffractive optical element. The general condition for athermalizing an optical system when the object surface is at finite or infinite distance is deduced in this paper, and the principle and design method of employing a diffractive optical element to athermalize and achromatize an optical system with normalized T-C chart are expanded. An infrared diffractive-refractive hybrid optical system in 8~12 μm with 16°field of view was designed. In this system, two materials of ZnSe and Germanium are used, the image quality of the system achieves diffractive limit at the temperature ranging from 20℃to 50℃. Additionally, the main fabrication process of the designed rotationally symmetric diffractive optical element on planar Germanium substrate by single point diamond turning utilizing NANOFORM 250 lathe is described. In the end, the measurement results of the system performance by OPTIKOS infrared MTF equipment at different temperatures are given and the results show that the performance of the system is steady in the designed temperature range.