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摘要:
传统的小孔球面波数字同轴全息受小孔的不确定度影响，成像质量并不理想。本文提出一种产生均匀球面波得到宽视场高分辨的显微成像方法。激光经过扩束镜、显微物镜后聚焦成一个极小的光斑，调节针孔阵列与焦点的距离，针孔直径与焦斑相配形成理想球面波。照明被测物后，透射球面波和物体散射的物光波形成干涉条纹，由大靶面图像传感器采集。载物与不载物的图像相减去掉脏点和杂光干扰。菲涅耳逆变换重构算法恢复物体信息。生物实验证明，均匀球面波数字同轴全息能够获得高质量显微成像，视场范围3.22 mm×3.22 mm，分辨率5.09 μm，其快速、非接触、灵活的放大倍率可广泛应用于光学元件检测、材料识别、生物医学领域。
Abstract:Traditional pinhole spherical wave digital in-line holography has proved to be powerful imaging tools. Image quality is affected by uncertain round of pinhole. Here, we propose a well-distributed sphere wave generation method and it demonstrates wide field of view and high resolution microscopy. The laser focuses into an infinitesimal spot through laser beam expander and microscope objective. Pinhole permutation with different sizes is utilized to match the focal point, and emerges an ideal spherical wave. Interference fringes pattern, formed by reference sphere wave and scattered sphere wave of object, is collected by large area image sensor. The influence of dirty in image sensor and parasitic light is eliminated through subtraction with and without object. Fresnel inverse transformation reconstruction algorithm presents the object information. Biology microscopy experiments demonstrate that the proposed techniques increase the flexibility in producing well-distributed point light source and improve the image quality. Field of view is 3.22 mm×3.22 mm and resolution is 5.09 μm. Furthermore, adjustable field of view with magnification, fast, no-contact make it to be a promising tool in optical element measurement, material identification, biology and medicine.
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Key words:
- digital in-line holography /
- well-distributed sphere /
- biology microscopy
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Overview: Digital in-line holography (DIH) with spherical wave, originally proposed by Gabor, is the simplest way in realizing holographic. The object light and reference light are coaxial, and interference fringes pattern are recorded digitally by image sensor. Complex amplitude distribution of object are displayed through reconstruction algorithm. In visible light range, although the resolution is micron, it provides wide field of view. Moreover, the characters of fast, real-time, non-contact make it be a promising tool in material identification, biology microscopy, lab-on-a-chip applications and particle track.
The quality of point source spherical wave, emerged from pinhole, has an important impact on the imaging. However, the size and uncertain round of pinhole cannot be eliminated refer to fabrication error. Although researchers have been capable of manufacturing nanometer accuracy pinhole, the cost is expensive extremely far more than optical elements. On the other hand, wetting films, pixel super-resolution, differential-interference-contrast are applied efficiently to improve image quality, field-of-view, and resolution, but they require sophisticated operation steps far beyond the simplicity of the spherical wave digital in-line geometry.
Diffraction is much better in condition of pinhole diameter matching for incident light wavelength. And the actual size of pinhole is determined by parameters and distance of image sensor. It makes heavy demands on manufacturing accuracy. We first consider obtaining the well-distributed spherical wave. Laser focuses into an infinitesimal spot through laser beam expander and microscope objective in turn. Altering axis distance between pinhole array and microscope can obtain suitable focal spot. Matching with the pinhole, an ideal spherical wave is generated. The influence of uncertain round of pinhole can be shifted to the edge of image sensor that is negligible. Then reconstruction algorithm simplify the computational process, which presents the object information. Finally, as a proof-of-concept, biology experiments demonstrate the proposed techniques.
As shown in mosquito eggs microscopy. Figure (a) is reconstruction result without any image processing, the field of view is 3.22 mm×3.22 mm and the resolution is 5.09 μm. Figure (b) and (c) are magnified digitally four times to display single egg. The difference of reconstruction distance, the result is changing. It is called digital focusing. Besides, the whole measurement process is fairly high efficiency, because only three steps: placing object, exposure, and reconstruction. Large area and high resolution recovery image is our target, and also the characteristic of this microscopy system. It can be used in detection of micro optical element, biological recognition, path tracking of plankton etc. Especially in biology and medicine research, high efficient, flexible working distance and field of view characters are much suitable.
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