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根据哈特曼波前传感器的动态范围定义,其与微透镜子孔径尺寸成正比,与焦距成反比,与检测精度成相互制约的关系。因此,为满足系统对传感器的大量程检测的指标要求,低阶哈特曼传感器的总体方案采用负微透镜阵列和凸透镜组合的方式,其结构示意图如图1所示。该传感器可实现绕过红外热像仪冷光阑的功能,以达到充分利用相机靶面进行成像的目的。与此同时,不同于传统哈特曼波前传感器微透镜阵列子孔径呈正方形分布,文中传感器的微透镜阵列采用圆对称环形分布,如图2所示,以此来更好地匹配高能激光领域中常见的环形激光束。为满足大量程检测要求,微透镜阵列子孔径数目设计为6单元,其尺寸为Φ3.3 mm。
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在哈特曼波前传感器应用中,目前最常见的微透镜阵列材料有两种,分别为紫外熔融石英和硅材料,其透射光谱范围如表1所示。而高能化学激光器的波长更多的处于2.6~4.2 μm的中红外波段范围内,因此,基于紫外熔融石英材料的微透镜阵列无法应用于中红外波段的检测。另一方面,基于硅材料的微透镜阵列虽然能满足中红外透射要求,却不透可见光波段,因此增加了传感器在实际应用时的光路调试复杂度。
表 1 常用光学材料的透射范围
Table 1. Transmission range of usual optical materials
Material Transmission range UV fused silica 185 nm-2.1 μm CaF2 180 nm-8.0 μm Silicon 1.2-8.0 μm BaF2 0.2-11 μm 根据表1所示,CaF2材料的透射光谱范围极宽,为180 nm~8.0 μm,并且具有高损伤阈值、机械稳定和环境稳定等特性,但非常遗憾的是其质地非常脆,无法加工百微米量级的微透镜阵列,这也是目前市面上还未出现基于CaF2材料的微透镜阵列的原因。而文中所设计的微透镜阵列子孔径尺寸为Φ3.3 mm,可采用传统冷加工的方式制作基于CaF2材料的微透镜阵列。因此,所设计的基于CaF2材料的低阶哈特曼波前传感器在高能激光器应用中,不仅能大大降低光路调试复杂度,而且还可同时应用于可见、近红外和中红外波段的光束像差检测。另外,在表1中,虽然BaF2材料的透射光谱范围比CaF2更宽,但由于其属于有毒材料,对眼睛、皮肤和上呼吸道均有强烈刺激作用,而且对环境有害,因此不适用于制作微透镜阵列。
表2为该传感器的光学参数。根据参考文献[15]中的几何光学理论,经仿真计算,传感器的光斑衍射极限半径为276 μm,约为18个像素。另外,传感器透镜组的等效焦距为110.5 mm,当输入光束为平行光束时其在相机靶面上的艾里斑分布图如图3所示,可知光斑最大间距为4 mm,约为相机靶面尺寸的一半左右。
表 2 低阶哈特曼传感器的光学参数
Table 2. Optical parameters of low-order Hartmann-Shack sensor
Part Item Parameter Negative micro-
lens arrayMaterial CaF2 Sub-aperture diameter/mm 3.3 Curvature radius/mm −72.7 Center thickness/mm 3 Convex lens Material CaF2 Front surface curvature radius/mm 36.2 Back surface curvature radius/mm 135.7 Center thickness/mm 5 Camera Pixel size/μm 15 Sensor size/mm2 9.6×7.7 -
针对文中所设计的6单元低阶哈特曼波前传感器,在实验室搭建了哈特曼波前传感器的性能测试系统,如图4所示。在测试系统中,输入光束为3.39 μm的平行光束,其孔径尺寸为Φ140 mm,并经过一个100 mm×35 mm的矩形光阑。另外,测试系统中的凹柱面镜安装在电动平移台上,通过计算机可实现凸柱面镜和凹柱面镜间距的自动控制。系统的测试原理主要是通过计算机控制凸柱面镜和凹柱面镜的间距变化来产生低阶像差(90°像散和离焦),同时利用文中所设计的低阶哈特曼波前传感器监测像差变化情况。由于该哈特曼波前传感器的可检测光束直径为Φ10 mm,测试系统光路中还需加入缩束器,其输入口径为Φ100 mm,缩束倍数为10。图5为实际制作的基于CaF2材料的低阶哈特曼波前传感器实物图。
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表3所示的为测试系统中的凸柱面镜和凹柱面镜的镜面参数。根据几何光学理论,测试系统中的凸柱面镜与凹柱面镜的共焦距离为435.17 mm,此时可实现光束平行进、平行出的功能,在此基础上距离每增加(或减小) 0.1 mm所对应的输出光束PV值变化为0.28 μm。图6所示的是系统中凸柱面镜和凹柱面镜的间距满足共焦条件时低阶哈特曼波前传感器所采集到的图像,该图像的焦斑分布情况与仿真计算的图3基本一致。另外在图6中,有个别的焦斑形态不规则,其原因可能有两方面:一方面是测试系统输入光束局部存在着高阶像差,另一方面是微透镜加工具有一定的误差所造成的。
表 3 测试系统柱面镜参数
Table 3. Parameters of the cylindrical mirror in the measurement system
Type Curvature radius in the X-direction Curvature radius in the Y-direction/mm Size in the X-direction/mm Size in the Y-direction/mm Convex cylindrical mirror +∞ −453.1 100 35 Concave cylindrical mirror +∞ 1293.37 100 100 为测试低阶哈特曼波前传感器的检测精度和量程,通过计算机以0.01 mm的步长逐步改变凸柱面镜与凹柱面镜的间距,获得了不同间距处哈特曼波前传感器测得的PV值,测试结果如图7所示。在图7中,红线为实测的PV值曲线,蓝线为根据几何光学理论计算的理论PV值曲线,两者的标准偏差为0.1λ,而且低阶哈特曼波前传感器的量程达到±8λ,两个指标均满足前面所述的系统设计要求。由图7可知,实测PV值与理论PV值存在一定的偏差,经分析,其误差来源主要有以下三个方面:(1)低阶哈特曼中的负微透镜阵列和凸透镜存在一定的加工偏差;(2)测试系统中各光学元件之间的间距存在测量误差;(3)低阶哈特曼波前传感器阵列光斑的质心计算存在误差。
Design of low-order Hartmann-Shack wavefront sensor for annular laser beam
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摘要: 针对高能化学激光器出光过程中存在着大比例、大PV值低阶像差这一现象,设计了专用于前5项Zernike像差检测的低阶哈特曼波前传感器。该传感器的透镜部分采用呈环形分布的6单元微透镜阵列与凸透镜组合的方法,且均用CaF2材料制作。该方法不仅可以同时用于可见光和红外光的低阶像差测量,还具有成本低、光路结构简单、探测范围大等优点,适用于环形激光束的大PV值低阶像差检测。之后还搭建了测试光路系统,测试结果表明,该低阶哈特曼波前传感器的波面测量PV值量程为±8λ (λ=3.39 μm),测量精度优于λ/10 (λ=3.39 μm)。Abstract: Based on the fact that there were a large percentage of low-order aberrations which had large PV values in high-energy laser beam, a Hartmann-Shack wavefront sensor for measuring first five-order Zernike aberrations was presented and designed. The lens part of the sensor adopted a method with combination of a 6-units micro-lens array and a convex lens, and the micro-lens array was distributed in annular. Since the lens material of this sensor was GaF2, the designed sensor could be applied to the low-order aberrations measurement of visible and infrared laser beam. This method had the advantages of low cost, simple structure and large detection range, which was suitable for the measurement of large PV-value low-order aberrations of the annular laser beam. Afterwards, a measurement system for the low-order Hartmann-Shack wavefront sensor had been set up, and the results show that the measuring range was ±8λ (λ=3.39 μm) and the accuracy was less than λ/10 (λ=3.39 μm).
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Key words:
- H-S wavefront sensor /
- low-order aberration /
- high-energy laser /
- wavefront aberration
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表 1 常用光学材料的透射范围
Table 1. Transmission range of usual optical materials
Material Transmission range UV fused silica 185 nm-2.1 μm CaF2 180 nm-8.0 μm Silicon 1.2-8.0 μm BaF2 0.2-11 μm 表 2 低阶哈特曼传感器的光学参数
Table 2. Optical parameters of low-order Hartmann-Shack sensor
Part Item Parameter Negative micro-
lens arrayMaterial CaF2 Sub-aperture diameter/mm 3.3 Curvature radius/mm −72.7 Center thickness/mm 3 Convex lens Material CaF2 Front surface curvature radius/mm 36.2 Back surface curvature radius/mm 135.7 Center thickness/mm 5 Camera Pixel size/μm 15 Sensor size/mm2 9.6×7.7 表 3 测试系统柱面镜参数
Table 3. Parameters of the cylindrical mirror in the measurement system
Type Curvature radius in the X-direction Curvature radius in the Y-direction/mm Size in the X-direction/mm Size in the Y-direction/mm Convex cylindrical mirror +∞ −453.1 100 35 Concave cylindrical mirror +∞ 1293.37 100 100 -
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