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通过该制备工艺制备的三种不同尺寸的4×4微透镜阵列如图2所示。从图2(a)~(c)中可以看出,微透镜阵列的接触角随着微透镜直径的增加而减小。这是由于在转速和旋涂时间相同的情况下,微柱阵列的直径越大,残留在微柱上的PDMS越薄,形成的微透镜阵列即越平滑。图2(d)~(f)分别为微柱直径为300、500、700 μm的微透镜阵列,微透镜阵列中的每个微透镜都具有相同的形状特征且彼此相连。从水平方向观测并计算得到这三种微透镜的曲率半径依次为319、809、2 016 μm。微透镜的焦距f计算公式如下:
图 2 显微镜成像的微透镜阵列。(a)~(c)三种微柱尺寸(300、500、700 μm)下的4×4 微透镜阵列侧视图;(d)~(f)三种微柱尺寸(300、500、700 μm)下的4×4 微透镜阵列俯视图
Figure 2. Microlens arrays through a microscope. (a)-(c) Side view of the 4×4 microlens arrays in three size (300,500 and 700 μm) of the arrays of micro-posts; (d)-(f) Top view of the 4×4 microlens arrays in three size (300,500 and 700 μm) of the arrays of micro-posts
$$f = \frac{{{d^2} + 4{h^2}}}{{8h\left( {n - 1} \right)}}$$ (1) 式中:d为球形顶部的直径;h为球形顶部的高度;n为PDMS的折射率,即n=1.403。通过计算得出微透镜的焦距依次为0.8、2、5 mm。
为了评估微透镜阵列的成像性能,搭建了如图3所示的成像系统。用白光LED照射物体,微透镜阵列置于物体后方以捕捉图像,实验中采用了USAF 1951标准的分辨率精度板作为观测目标对象。微透镜阵列的焦平面与物镜的焦平面对齐,使物镜从微透镜上获取清晰准确的图像。最后,利用电荷耦合器件(CCD)摄像机记录图像。
图4为通过尺寸300 μm和500 μm微柱上形成的微透镜阵列成像所获得的照片。从图4(a)中可以看出在USAF 1951年分辨率精度板上,通过微透镜阵列成像后的照片可以观察到第6组第1号元素的线对,由此得出尺寸为300 μm微柱上形成的微透镜阵列的分辨率可以达到8 μm。同样,从图4(e)中可以看出在USAF 1951年分辨率精度板上,通过微透镜阵列成像后的照片可以观察到到第4组第2号元素的线对,由此得出尺寸为500 μm微柱上形成的微透镜阵列的分辨率可以达到28 μm。
图 4 微柱尺寸为300 μm和500 μm的4×4微透镜阵列成像图。(a) 单一微透镜USAF 1951年分辨率精度板成像图;(b)~(d) 通过微透镜成像的十字图像
Figure 4. Images observed by using the 4×4 microlens array in 300 μm and 500 μm of the array of micro-posts. (a) Microscopic image of USAF 1951 resolution target obtained from a single microlens; (b)-(d) Images of the cross-shaped pattern obtained from the microlens array
此外,还测试了一组十字图案通过微透镜阵列成像的效果图,从图中可以看出一组十字形图案可以被明显观测到。其中通过尺寸为300 μm微柱上形成的微透镜阵列成像的十字图案自定义的线宽为240 μm,通过微透镜阵列成像的图片如图4(b)所示。通过尺寸为500 μm微柱上形成的微透镜阵列成像的十字图案自定义的线宽为280 μm,通过微透镜阵列成像的图片如图4(f)所示。由于待成像图案与微透镜阵列相对位置的限制,无法获取通过尺寸为700 μm微柱上形成的微透镜阵列的十字成像,但依然可以得出结论,通过这种工艺制备出的具有高填充因子的微透镜阵列满足不同待观测物体的观测要求。
分别任意取两组微透镜阵列中的两个微透镜所成的十字物像,如图4(c)、(d)、(g)、(h)所示。从图中可以分别看出两组微透镜阵列中的十字像均在每个微透镜中的位置略有不同,这是由于十字图案与微透镜的相对位置不相同。尽管如此,无论是从尺寸为300 μm微柱上形成的微透镜阵列观察到的十字图案还是从尺寸为500 μm微柱上形成的微透镜阵列观察到的十字图案在形状上具有高度的均一性,没有明显的失真现象。成像结果表明,实验中制备的微透镜阵列具有良好的成像性能,且可以通过调节制备过程中微透镜阵列的尺寸实现不同精度要求的物像观测。
Rapid fabrication and characteristics analysis of high-filling-factor microlens array
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摘要: 微透镜阵列是一种被广泛应用于光信息处理、光传感、光计算、光通信和高灵敏度成像等领域的精密光学元器件之一。通过一些先进的制造技术已经可以制造出不同几何形状、轮廓和光学特性的微透镜阵列。然而,由于三维微制造工艺的难度,使得高填充因子微透镜阵列中的微透镜很难实现紧密排列。提出了一种快速、低成本的微流体操纵技术,用于制备高填充因子微透镜阵列,且对其制备工艺进行了初步的演示。这种易于操作的制造技术适用于微透镜阵列的大批量生产,极大地提高了生产效率。通过预先制备出的三种不同尺寸(微柱直径分别为300、500、700 μm)的微柱,实现了与其对应不同形状和尺寸的微透镜阵列的制备,并搭建了一套光学成像系统以对这些微透镜阵列进行成像性能的评估。主要对微透镜阵列的焦距、成像精度和每个微透镜阵列中各个微透镜子单元成像的均一性进行测试,利用所提出的微流体操控技术制备的微透镜阵列具有良好的成像性能,有望能够被应用到三维成像、光均匀化等诸多应用中。Abstract: As a precise optical component, microlens array has applications in fields as optical information processing, optical sensing, optical computing, optical communications and high sensitivity imaging. Researchers have developed many advanced fabrication techniques, some of which already realized the preparation of the microlens array with required geometries, profile and optical properties. However, it would be extremely difficult to achieve a compact packing as such 3D micro-manufacturing techniques are hard to control. A novel rapid and low-cost microfluidic-manipulation based technique was proposed to fabricate high-filling-factor microlens array. A brief demonstration of the fabrication was given, which had excellence of suited to volume production and significant productivity boost. Meanwhile, the microlens arrays of three different properties were produced, which were realized by adjusting the size of the array of micro-posts whose sizes were 300, 500, 700 μm in diameter, respectively. The imaging system was set up to demonstrate the imaging performance of each of the microlens array, evaluating the precision of each microlens array and imaging uniformity of the microlens array. The results show that the fabricated microlens arrays have good imaging performance and have a promising prospect in the use of 3D imaging and optical uniformity.
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图 2 显微镜成像的微透镜阵列。(a)~(c)三种微柱尺寸(300、500、700 μm)下的4×4 微透镜阵列侧视图;(d)~(f)三种微柱尺寸(300、500、700 μm)下的4×4 微透镜阵列俯视图
Figure 2. Microlens arrays through a microscope. (a)-(c) Side view of the 4×4 microlens arrays in three size (300,500 and 700 μm) of the arrays of micro-posts; (d)-(f) Top view of the 4×4 microlens arrays in three size (300,500 and 700 μm) of the arrays of micro-posts
图 4 微柱尺寸为300 μm和500 μm的4×4微透镜阵列成像图。(a) 单一微透镜USAF 1951年分辨率精度板成像图;(b)~(d) 通过微透镜成像的十字图像
Figure 4. Images observed by using the 4×4 microlens array in 300 μm and 500 μm of the array of micro-posts. (a) Microscopic image of USAF 1951 resolution target obtained from a single microlens; (b)-(d) Images of the cross-shaped pattern obtained from the microlens array
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