Design of a Refractive/Diffractive Hybrid Middle Infrared Continuous Zoom Optical System

The application of diffractive surface in the continuous zoom system is investigated by taking advantage of the negative dispersion characteristics of binary elements. A sample design is presented in this paper using cool 320×240 detector with staring focal plane array and secondary imaging, and a mid-wave optical system using mechanical-compensated with refractive/diffractive hybrid technique is designed. The Pixel Dimensions of the detector is 30μm, and the wavelength between 3.7μm ~4.8μm.The system adopts negative group variable times and positive group of compensation which can realize 18mm～180mm continuous zoom and FOV =36.8°~3.8° , F-number is 2, it consists of 7 lenses including 2 aspheric surface and 1 diffractive surface. The length of this system is 230mm with the reflection mirror to shorten optical path. The results show that the modulation transfer function(MTF) is above 0.6 within the whole focal range at spatial frequency of 17 lp/mm, and Root Mean Square (RMS) value of spot diameter were smaller than the Pixel Dimensions. After the image quality being optimized, the monotonic and smooth cam curve is given and 100% cold shield efficiency is obtained. The curve shows that the imaging plane is stable and the cam is easy to process. The system has advantages of simple structure, high image quality and short zoom path etc.