The abbe number of diffractive optical element is negative, which is opposite to that of traditional refraction optical element. So diffractive optical element has unique advantages on achromatic and thermal compensation. Therefore, it can be applied to diffractive and refraction hybrid optical systems to simplify the optical structure and improve the imaging quality. However, the diffraction efficiency of the single-layer diffractive optical element depends on the wavelength mightily , However, the diffraction efficiency of the design order of the double-layer diffractive optical element in the whole working band is coincident basically and more than 90%, which can significantly suppress the stray light of the non-design order and advance the image contrast. Therefore, the double-layer diffractive optical elements are adopted in this long-wave infrared wide waveband optical system, and the design idea of superposing the diffractive surface on the aspheric surface is proposed. Only two lenses and two materials were used to realize the optimal design. By matching the microstructure parameters of the double-layer diffractive elements ,the diffraction efficiency of the whole working band reached more than 97%. Imaging quality analysis shows that the chromatic aberration of 8μm-14μm is well corrected. the secondary spectrum is well controlled also, and the MTF is close to the diffraction limit, which meets the design requirements.
The infrared continuous zoom optical system can realize the continuous change of focal distance in a certain range and keep the image surface stable and clear in the whole process of field of view change. Further integration and coordination of optical and mechanical design are required as infrared thermal imaging systems are increasingly used in high-precision applications of military and civil fields. However, the mid-wave infrared zoom system contain a large number of mirrors, a complex mechanism and higher design difficulty compared with other types of infrared zoom system. This paper introduced a mid-wave infrared zoom system, considering the design of optics and structures as well as the simplicity of assembly and adjustment. A stable and reliable small-F-number zoom infrared camera system which can meet the constraint condition was designed aiming at the key indicators such as imaging quality, optical axis shaking and adjusting operation. The operating wavelength band of the system was from 3.7 μm to 4.8 μm, the focal length was 30mm to 300mm, and the medium-wave refrigeration focal plane detector of pixels of 1024×768 was adopted. The optical imaging performance was close to diffraction limit, besides, image plane had a high consistency during the whole zooming process.
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