数字像面全息显微技术的降噪方法

数字像面全息显微技术中，记录过程引入的噪声限制了轴向测量的精度和可靠性。目前对于抑噪方法的研究，主要针对高频噪声。因此提出了一种针对数字像面全息显微技术的降噪方法，不仅抑制高频噪声，对噪声的低频部分也有抑制作用，提高重构相位的信噪比。这种降噪方法对数字像面全息显微图进行优化，实现途径为利用二维经验模态分解中，第一层本征模态函数与全息图干涉条纹灰度信息契合的特点，提取第一层本征模态函数实现优化。应用优化后的像面全息图，实现重构，求解相位，测量样本表面形貌。应用优化方法对标准纳米台阶的表面形貌进行测量，对比并分析优化前后全息图的频谱以及测量的高度图，证明优化方法在保证对微结构表面形貌实现可靠测量的基础上，降低了数字像面全息显微技术的重构相位噪声。

Method to suppress noises in digital image-plane holographic microscopy

In digital image-plane holographic microscopy (DIPHM), the precision and reliability of measurement are limited by noises introduced in recording process. Until now, the noise reduction research was focused on noise with high frequency. Here, a method was proposed to reduce noises not only with high frequency, but also with low frequency. By using the denoising method, the signal-to-noise ratio (SNR) of reconstructed phase was improved at the same time. The denoising method was realized by optimizing the digital image-plane microscopic hologram. The characteristics of the first intrinsic mode function (IMF1) in bi-dimensional empirical mode decomposition (BEMD) were in good accordance with that of gray value information of interference fringes in digital image-plane hologram. Hence, the digital image-plane hologram was decomposed by BEMD to extract the IMF1, utilizing the IMF1 as the optimized digital image-plane hologram. After that, the optimized digital image-plane hologram was reconstructed to retrieve phase to measure the surface profile of tested samples. The surface profile of a standard nano-step was measured by DIPHM with BEMD. By comparing and analyzing the frequency spectrum and measured height profile with and without BEMD, the optimizing method was proved to realize the correctness in surface profile measurement and noise suppression of reconstructed phase.