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图 1为典型共形光学头罩的2维结构示意图。共形光学头罩表面为椭球形,其表面几何参量由以下3个方程确定:
$ k=1 /\left(4 F^{2}\right)-1 $
(1) $ r=D /(4 F) $
(2) $ F=L / D $
(3) 式中,F为长径比,L和D分别为共形整流罩的长度和口径, k是二次曲面常数, r为顶点曲率半径,tc是中心厚度,te是边缘厚度。共形光学头罩的优点在于能够有效降低系统的空气阻力,长径比越大,头罩的空气阻力系数越小。
在共形成像光学系统设计中,把动态校正器设计为移动的相位板,当这些光学元件调整到不同的观测角时,光束通过头罩的不同区域。对于普通的椭圆形头罩而言,光束通过头罩的中心时,会看到方向对称的光学元件,而光束通过整个头罩的边缘时,会看到一个不同曲率半径的光学元件,不同的曲率使光的特性发生变化,同时视角的变化引入了大量像差。作者研究了共形成像系统中的轴向移动相位板对像差的校正作用,图 2为在系统中加入轴向移动相位板时共形成像系统在0°, 5°, 10°和15°扫描视场的示意图。
轴向平移相位板实质上是两个平面平行相位板,它是两个独立的非球面平板的组合,通过转动两个相位板彼此的相位,可以产生不同量的像差,通过这种组合产生特定组合的波前像差,可以校正可变像差量。通过计算给定光束穿过一对板相对于某一参考光束的光程,可以确定相位板面引入的波前像差。两个相位板的内表面用同一个表达式表示,这样相位板本身就匹配成一对板,当它们靠在一起时,就具有一个独立的平行平面板的特点。
本设计中,采用相位板的制冷型中红外共形光学系统采用F/2中波红外制冷型焦平面阵列探测器,像元尺寸为30μm,光学系统设计指标如下:工作波段为3.7μm~4.8μm;焦距为40mm;长径比为1.0;瞬时视场为2°;扫描视场为±15°。
因为长径比F=1.0, 根据上述公式计算可得,窗口的二次曲面常数k=-0.75,顶点的曲率半径为30mm,窗口的长度为120mm,根据上述参数建立了光学窗口的模型结构。
然后在系统中加入两块相位板和后面的透镜组,两块相位板校正光学窗口的一部分像差,残余像差由两块相位板后面的透镜组校正。考虑到加工因素,两块相位板的基底都采用锗材料。第1块相位板的1次相位系数为6.8942×10-4,2次相位系数为-2.4086×10-2,3次相位系数为-4.1051×10-4,4次相位系数为7.7921×10-5,5次相位系数为-2.3295×10-3,6次相位系数为1.7720×10-8。第2块相位板的1次相位系数为-6.8942×10-4,2次相位系数为2.4086×10-2,3次相位系数为4.1051×10-4,4次相位系数为-7.7921×10-5,5次相位系数为2.3295×10-3,6次相位系数为-1.7720×10-8。相位板的参数和光学窗口的参数、后组透镜以及系统总体的成像性能有关,因此其校正补偿范围由光学窗口的参数、后组透镜以及系统总体的成像性能决定,不同的参数和要求,相位板的校正补偿范围也不同。
相位板制冷型的中红外共形光学系统采用一次成像的透射式结构,没有遮拦,光学透镜的参数如表 1所示。光学系统总长可以很短,实现了100%冷光阑效率,两个相位板后的校正透镜组由为4片式结构,光学材料分别为硅、锗、锗、硒化锌,其后是红外探测器组件。
Table 1. Optical data
No. radius of curvafure/mm interval/mm material 1 51.74 3.82 Si 2 -1015.29 3.21 — 3 -78.07 3.00 Ge 4 75.86 3.00 — 5 -34.08 4.00 Ge 6 -26.11 3.00 — 7 22.62 3.00 ZnSe 8 17.64 3.00 — 9 11.34 3.00 Si 10 9.97 3.00 — 11 ∞ 19.80 — 相位板制冷型的中红外共形光学系统的光路图如图 3所示。系统选用的是制冷型中红外探测器,因此需要考虑冷光阑效率,冷光阑效率是指来自于目标到指定像素的总立体角与整个冷屏开口到同一像素的总立体角之比,采用相位板的制冷型中红外共形光学系统要满足100%冷光阑效率。
该系统采用一次成像的光学结构形式,设计时将采用相位板的制冷型中红外共形光学系统的出瞳和探测器的冷光阑重合,最终设计时, 该光学系统出瞳大小为10.5mm,探测器的冷光阑的大小为10.5mm,出瞳在探测器像面前19.8mm,和探测器的冷光阑重合。可知采用相位板的制冷型中红外共形光学系统满足100%冷光阑效率。
采用相位板的中波红外共形光学系统设计
Design of a middle-wavelength infrared conformal optical system using phase plate
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摘要: 为了满足现代军事装备中的侦查需求,提出了一种新型的红外共形成像光学系统,在消除光学头罩引入的复杂像差过程中,采用一对轴向移动的相位板来实现像差动态校正。给出了一个红外共形成像光学系统设计实例,其工作谱段为3.7μm~4.8μm,焦距为40mm,长径比为1.0,瞬时视场为2°,扫描视场为±15°。结果表明,在奈奎斯特频率17lp/mm处,各个扫描视场的调制传递函数均大于0.6,系统的光学传递函数接近衍射极限,各个扫描视场的弥散斑都小于30μm,适用于像元尺寸为30μm×30μm的中红外制冷型焦平面阵列探测器。该研究对于促进新型成像光学装备在军事领域中的进一步的发展、应用是有帮助的。Abstract: To meet the demand of reconnaissance in modern military equipment, a novel conformal infrared optical system was presented. Two axial translation phase plate were used to correct the dynamic aberrations which were caused by the conformal structure.For the designed optical system, the working band was 3.7μm~4.8μm, the focal length was 40mm, the ratio of length to aperture was 1.0, the instantaneous angle was 2°, and the scan angle is ±15°, respectively. The results show that the modulation transfer function of each scan fields of view is greater than 0.6 with the nyquist frequency of 17lp/mm, and the optical transfer function is comparable to the diffraction limit. The spot of several scan fields is less than 30μm and falls within the area of one pixel. The designed optical system with well optical performance can be a suitable candidate for the application of the mid-infrared refrigerated focal plane array detector with the pixel size of 30μm×30μm. This research is potentially helpful for the further development and application of new imaging optical equipment in the military field.
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Key words:
- imaging systems /
- phase plate /
- aberration correction /
- conformal optics system /
- mid-wave infrared
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Table 1. Optical data
No. radius of curvafure/mm interval/mm material 1 51.74 3.82 Si 2 -1015.29 3.21 — 3 -78.07 3.00 Ge 4 75.86 3.00 — 5 -34.08 4.00 Ge 6 -26.11 3.00 — 7 22.62 3.00 ZnSe 8 17.64 3.00 — 9 11.34 3.00 Si 10 9.97 3.00 — 11 ∞ 19.80 — -
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