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摘要:
为了进一步提高合成孔径雷达(synthetic aperture radar，SAR)面对海量回波数据的实时成像处理能力，基于4f光学结构对SAR实时成像光学处理器进行了光机系统设计和分析。首先，设计了适用于滤波算法的入瞳直径21 mm、视场角7°、焦距172 mm的傅里叶变换透镜，并对4f光学系统采取紧凑化设计。然后，利用集成优化方法优化了4f光机结构中的柔性镜座，并对整体结构进行了模块化设计和分析。分析结果表明：4f光学系统成像质量趋于衍射极限，傅里叶变换透镜的MTF在55 lp/mm处优于0.57，4f光机系统在常温1
g 重力工况下透镜面形RMS值小于$$ \lambda /50 $$ ，整体结构基频大于100 Hz。4f光学处理器整体尺寸为405 mm×145 mm×92 mm，质量约为2.94 kg，其体积、质量分别仅是同等SAR数据处理水平的斜平面光学处理器的30%、48%。通过数据的模拟仿真，表明系统设计满足星载或机载的实时成像使用需求。Abstract:In order to further improve the real-time imaging processing ability of synthetic aperture radar (SAR) in the face of massive echo data, the optical and mechanical system of SAR real-time imaging optical processor is designed and analyzed based on 4f optical structure. Firstly, a Fourier transform lens with an entrance pupil diameter of 21 mm, a field angle of 7°, and a focal length of 172 mm is designed for the filtering algorithm, and a compact design is adopted for the 4f optical system. Then, the flexible mirror base in 4f optical mechanical structure is optimized by using the integrated optimization method, and the overall structure is modularized designed and analyzed. The results show that the imaging quality of 4f optical system tends to the diffraction limit, and the MTF of Fourier transform lens is better than 0.57 at 55 lp/mm. The RMS value of lens surface shape of 4f optical mechanical system under normal temperature 1
g gravity condition is less thanλ /50. The fundamental frequency of the overall structure is greater than 100 Hz. The overall size of 4f optical processor is 405 mm×145 mm×92 mm, the mass is about 2.94 kg, and its volume and mass are only 30% and 48% of those of oblique plane optical processors with the same SAR data processing level. Through data simulation, it shows that the system design meets the needs of real-time imaging on satellite or airborne.-
Key words:
- synthetic aperture radar /
- real-time imaging processing /
- compact design /
- modular design
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Overview: This paper is devoted to the research of synthetic aperture radar (SAR) real-time imaging processor. As the number of SAR imaging channels increases, the number of SAR imaging channels also presents new challenges. The optical processor not only has strong parallel processing ability, but also has the advantages of low power consumption, small volume, fast processing speed and programmability. Therefore, this paper designs and analyzes the SAR real-time imaging optical processor from the perspective of optical mechanical system design. Firstly, the system scheme principle of optical processor based on 4f optical structure is proposed, and the filtering algorithm is described in detail according to the principle. Secondly, according to the algorithm requirements, the relevant Fourier transform lens design is completed, and the compactness of 4f optical system is further strengthened. Then, the flexible design of the lens base is carried out, and the optimal parameter model is found by using the integrated optimization method. At the same time, it meets the modular design idea, completes the corresponding optical mechanical structure design, and obtains the optical mechanical system model of the overall scheme. The specific design results obtained based on the above research methods are as follows: in the optical design process, a Fourier transform lens with an entry pupil diameter of 21 mm, a field angle of 7°, and a focal length of 172 mm is obtained, and its MTF is better than 0.57 at 55 lp/mm. And the 4f optical system whose imaging quality tends to the diffraction limit meets the Rayleigh criterion. In the process of optical mechanical structure design, the overall size of 4f optical mechanical system is 405 mm×145 mm× 92 mm, with a mass of about 2.94 kg, and its volume and mass are only 30% and 48% of that of the inclined plane optical processor with the same SAR data processing level; At the same time, the RMS value of lens surface under normal temperature 1g gravity condition is less than λ/50(λ= 532 nm), the fundamental frequency of the overall structure is greater than 100 Hz, which can fully meet the expected design goal of the processor optical mechanical system. Finally, the simulation processing of SAR data is carried out on the optical platform. According to the simulation results, it shows that the system can be suitable for airborne or spaceborne real-time processing scenes. To sum up, the 4f optical processor designed in this paper can provide a certain reference value for improving the real-time imaging processing ability of SAR.
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图 3 傅里叶变换透镜结构图和像质评价图。(a) 结构图；(b) 点列图；(c) MTF图；(d) 波前图
Figure 3. Structure diagram and image quality evaluation diagram of Fourier transform lens. (a) Structure chart; (b) Spot diagram; (c) MTF figure; (d) Wavefront figure
图 4 光学系统结构图和像质评价图。(a) 结构图；(b) MTF图
Figure 4. Optical system structure diagram and image quality evaluation diagram. (a) Structure chart; (b) MTF chart
图 5 傅里叶变换镜组结构图。(a) 前2f镜组；(b) 后2f镜组
Figure 5. Structure of Fourier transform. (a) 2f lens group in the front; (b) 2f lens group in the back
图 7 前2f镜组变形云图。(a) X向；(b) Y向；(c) Z向
Figure 7. Cloud diagram of front 2f lens group deformation. (a) X direction; (b) Y direction; (c) Z direction
图 9 整体结构前两阶模态云图。(a)一阶模态云图；(b)二阶模态云图
Figure 9. First two modal cloud images of the whole structure. (a) First modal cloud image; (b) Second modal cloud image
图 10 4f系统装置图和模拟结果图。(a) 装置图；(b) 模拟结果图
Figure 10. 4f system device diagram and simulation result diagram. (a) Device diagram; (b) Simulation result diagram
表 1 SLM规格参数
Table 1. SLM specifications
规格参数 具体数值 调制类型 纯相位型/纯振幅型 像素数 1920$ \times $1152 像素大小 9.2 μm 有效像面大小 17.6 mm$\times$10.7 mm 工作波长范围 400 nm~1650 nm 填充因子 95.7% 帧频 76.8/93.7/169/211.1 Hz 
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表 2 傅里叶变换透镜光学参数
Table 2. Optical parameters of Fourier transform lens
# 光学元件 半径/mm 厚度/mm 材料 半口径/mm 1 透镜1 −171.909 15.748 SILICA 16.932 2 −74.994 14.257 17.984 3 透镜2 −28.628 5.682 N-KZFS11 17.919 4 −31.483 35.986 19.444 5 透镜3 31.483 5.682 N-KZFS11 19.623 6 28.628 14.257 18.098 7 透镜4 74.994 15.748 SILICA 18.245 8 171.909 126.3 17.258
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表 3 常用光机结构材料属性表
Table 3. Properties of common opto-mechanical structural materials
材料名称 密度$ \rho / $(g/cm3) 弹性模量E/(GPa) 比刚度(E/$ \rho $)/(GN·(m/g)) 线胀系数$ \alpha $/(10−6/K) 导热系数$ \lambda $/(W/(m·K)) TC4 4.4 114 25.9 9.10 7.40 7A09 2.8 71 25.4 23.6 142.00 铟钢 8.9 141 15.8 2.60 13.70 CFRP 1.8 纵向95 52.8 0~1 70.00 高体分SiC/Al 3.0 180 60.0 8.0 225.00
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