部分相干光照明的双波长轻离轴数字全息显微研究

Digital holographic microscopy (DHM) is a full field and non-destructive test means in diverse disciplines such as microscale topography, biological cell phase contrast imaging and so on. The adoption of partial coherence light in DHM has advantages of lower coherent noise. However, off axis hologram can’t be obtained when two beams of partial coherence light interference directly due to its short temporal coherence length. In addition, unambiguous phase range is limited to a wavelength in single wavelength interferometry. Dual wavelength slightly off-axis DHM, which combines dual wave interferometry and slightly off-axis configuration, has advantages of extended unambiguous phase range and lower carrier frequency of the hologram. This research focuses on partial coherence light illuminated slightly off-axis DHM with advantages of low noise, extended unambiguous phase range, real time measurement capability and super resolution. A longer“beat wavelength”is synthesized by two LEDs with shorter wavelengths, and hence larger step height can be measured. Using carrier frequency multiplication technique, a new reconstruction algorithm for dual wavelength interferometry is deduced with two slightly off-axis holograms I1, I2 which corresponding to two waves, respectively. A hologram with orthogonal slightly off-axis fringes is formed by two LEDs with different wavelength based on a Linnik interferometer and can be acquired through one shot via a CCD camera, thus the real time measurement of the specimen is possible, in addition the amplified phase noise in dual wavelength interferometry can be reduced with total variation regularization. A grating based structured illuminaion is implemented to enhance spatial resolution. This work will build a solid foundation for the development and application of lower noise and higher performance dual wavelength digital holographic microscope.

数字全息显微为微观形貌测量和细胞相衬成像等领域提供了一种全场无损的检测技术。部分相干光照明时相干噪声低，但是干涉时不能形成离轴全息图; 另外，单波长干涉时相位无包裹范围只有一个波长。双波长轻离轴数字全息显微，结合了双波长干涉和轻离轴记录方式，有相位无包裹范围大和载频低的优点。本课题旨在研究噪声低、相位无包裹范围大、可实时测量及横向分辨率高的部分相干光照明的双波长轻离轴数字全息显微方法。采用两个短波长的LED照明，生成长的“合成波长”，实现深台阶状样品的测量；利用双波长对应的轻离轴干涉图I1、I2，采用全息图倍频技术，构建新的重建算法；在Linnik干涉仪基础上，利用CCD同时记录载频正交的双波长轻离轴全息图，实现测量的实时性，并采用全变分技术降低放大的相位噪声；采用低频光栅产生的结构光照明样品，提高测量的横向分辨率。本研究将为低噪声、高性能的双波长数字全息显微镜的研发打下良好技术基础。

数字全息显微技术是一种全场的定量检测技术，具有非接触、分辨率高、无需标记等优点，在微观形貌测量和细胞相衬成像等领域获得了广泛应用。部分相干光照明时相干噪声低，但是光波干涉不能形成离轴全息图; 另外，单波长干涉时相位无包裹范围小，只有一个波长。针对这些问题，本项目设计了一种双波长照明的实时轻离轴数字全息显微装置。该装置基于长工作距离物镜，通过在物镜和样品之间加入分光棱镜的方法，构建了一种紧凑的迈克尔逊干涉仪。它结合了双波长干涉和轻离轴记录方式，有相位无包裹范围大和分辨率高的优点。另外该方案具有准物参共路的特点，比传统的装置具有更高的时间稳定性。在测量中，两路对应不同波长的参考光传播方向可独立调整，因此可以同时得到两幅条纹相互正交的干涉图。实验表明，装置测量的时间稳定性达到了2.04 nm。为了解决双波长测量中的噪声放大问题，我们提出了两种去噪方法。第一种是基于全变分的双波长去噪算法，它把去噪问题转化为最优化问题，并利用Split Bregman算法实现了有效求解。算法具有鲁棒性高的优点，重建相位的精度可达到单波长数字全息显微中的相位精度；第二种算法是基于区域分割的方法，把相位图分割为不同的区域，同一区域有相同的偏移量，根据此特点实现去噪。算法运行速度快，简单有效。实现了一种针对轻离轴干涉图的重建算法。利用倍频技术，把双波长对应的轻离轴干涉图I1、 I2转化为空间频率高的离轴干涉图，然后用频域滤波法实现相位重建。该算法可以从一幅轻离轴干涉图完成重建，实时性高。对结构光照明的数字全息显微进行了探索。在马赫-曾德光路中，实现了一种LED照明的离轴干涉显微技术。分别选取光栅衍射的0级照明物体作为物光波， +1级作为参考光波。光栅G所在的平面、样品S所在平面和CCD记录面三者互为共轭，物光和参考光在CCD平面相遇发生离轴干涉，实现实时测量。另外，实现了一种LED照明的、基于长工作距离物镜相移轻离轴干涉显微技术。具有低成本、精度高的优点。