Study On Fast Speed Optical System Of Uncooled MWIR Imager

Low-cost infrared imager based on uncooled focal plane arrays is one of the most important developments in thermal imaging techniques. It has wide application prospect for remote sensing high temperature events. The uncooled detector has many advantages such as lightweight, compactness, low power, and excellent reliability, but its radiation responsitivity is low at present. To compensate for this drawback, it is necessary to use midwave IR optical system with large relative aperture. At the same time, the developed high speed objective, which is expected to use as a spaceborne imaging system of microsatellite, should have high quality image, good adaptability to environment temperature fluctuation, lightweight and compact structure.The issue studied in this dissertation is put forward at first. The development and state of art of IR thermal imager with uncooled detector is introduced and its significance is pointed out. In chapter two, according to the requirements of detecting terrestrial high temperature events and the performance of current MWIR uncooled detector, the basic parameters of its used optical system, such as the focal length, relative aperture and field of view, are determined. Through comparing and analyzing various typical objectives, both refractive and refractive-diffractive hybrid Petzval lens are chosen as its candidate structure.In chapter three, the effect of environmental temperature fluctuation on the imaging quality of optical system is analyzed. Since its thermal defocus, due to the change of its enviromental temperature, extends far beyond the focus depth, thermal aberrations must be cancelled out. Compared with other various athermalization methods, optical passive method is preferred and studed for its light weight, good reliability and no power dissipation. The general equation for optical passive athermalization is deduced.In chapter four, the design of athermalized refractive optical system is introduced. Based on the conditons of optical passive athermalization and achromatism, its optical material and power distribution is determined through athermal chart. It is pointed out that at least four refractive lenses are necessary for Petzval refractive lens to meet these conditions. Its initial structure parameters are obtained though its primary aberration analysis. Among the obtained initial structures, the structure which high order spherical aberration is relatively low is chosen and further optimized.In the fifth chapter, the athermalized refractive-diffractive hybrid optical system design is detailed. The thermal and chromatic characteristics of diffractive optical element are introduced. For its unique dispersion and thermal properties, it can be regarded to make from a special kind of infrared material. Use of diffractive optical element nearly does not increase any system weight. Through introducing a diffractive optical element in the front group of the Petzval objective, a piece of large size lens can be saved so that its weight decreases considerable. The optical material and power distribution of the hybrid system is determined also through the athermal chart. Its initial structure is solved according to its primary aberration. The optimized refractive-diffractive hybrid system has distinctive advantage in weight over the designed refractive system. Then the former's manufacture and assembly tolerances are distributed optimally so that its implementation is faciliated.In the sixth chapter, how to fabricate aspheric and diffractive surfaces with single point diamond turning technique is introduced. And the hybrid germanium lens comprising of an aspherical and a diffractive surface is successively machined. The opto-mechanical structure and the alignment and assembly scheme of the objective are designed. The performance test and results of our developed objective are reported. Then, its modulation transfer function MTF is measured. It is demonstrated that the manufactured lens has high imaging quality under room temperature and also can maintain its stability within certain temperature range.Finally, the main work and the innovative points in this dissertation are summarized, and the further work is prospected.