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基于多角度无透镜傅里叶变换数字全息的散斑噪声抑制成像研究*
Research on sp eckle denoising by lensless Fourier transform holographic imaging with angular diversity
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摘要: 在数字全息成像中，散斑噪声严重影响了再现像的信噪比和成像分辨率，因此为了提高数字全息成像质量，迫切需要抑制再现像的散斑噪声.分析并给出了矩形散射光斑的强度协方差，定量计算了特定实验条件下产生退相关散斑图样的最小角度差.结合透镜的性质设计并搭建了多角度无透镜傅里叶变换数字全息成像系统，利用光纤端面在透镜焦平面的二维机械移动代替传统反射镜的旋转，使照明光束在不改变照明位置和大小的同时，可任意改变光束方向.移动光纤端面使多角度照明满足最小角度差，获取了81幅数字全息图.利用单次快速傅里叶逆变换实现数值再现，对多幅再现像的强度像求平均，实验结果表明散斑对比度降低为单幅再现像的14.6%，使图像信噪比提高了6.9倍.该抑制散斑的多角度数字全息成像方法有效的抑制了散斑噪声，且成像光路结构简单，可操作性强.Abstract: The signal-to-noise ratio and resolution of the reconstructed image is seriously influenced by speckle noise in digital holography, so it is essential to reduce the speckle noise and improve the image quality. Intensity correlation between two speckle patterns with the rectangle speckle spot is analyzed and deduced, and the minimal angular difference of illumination beams is given quantitatively under a special situation. A lensless Fourier transform holographic imaging system with angular diversity is designed based on the lens property, in which the direction of the wave can be changed by shifting the fiber instead of conventionally rotating the mirror, and the formor can change the direction of illumination with a fixed illumination size and location. Eighty one holograms with uncorrelated speckle patterns are recorded at different illumination angles by shifting the fiber. Then a digital reconstruction is achieved only by a fast Fourier transform, and the multiple reconstructed images are averaged. Experimental results show that the speckle contrast of the averaged reconstructed image can be reduced to 14.6%that from a single reconstructed image, and the signal-to-noise ratio is improved 6.9 times. This proposed digital holographic imaging method can suppress the speckle noise greatly, has a simple setup, and is easy to operate.
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