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Study of low-noise phase-shifting digital holographic microscopy using a long working distance objective
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摘要:
在数字全息显微技术中，为了提高测量精度，提出了一种利用长工作距离物镜的相移数字全息显微的测量装置和方法。该装置采用LED作为照明光源，可以有效地抑制相位噪声，提高了重建精度。通过在长工作距离物镜和样品之间加入分光棱镜的方法，构建了一种准物参共路的迈克尔逊干涉仪。该装置结构简单，调整方便，在部分相干光照明时，容易实现干涉。重建时，采用盲相移干涉技术，结合两步盲相移算法，重建出物体的表面相位分布。实验中，分别采用LED照明和He-Ne激光照明，测量了一个反射式USAF1951分辨率板的高度分布。结果表明，两者的测量结果相互吻合，但是LED照明时的噪声与激光照明时相比降低了70%。此外，为了进一步验证装置的有效性，使用该装置对刻于硅基底的微纳矩形台阶进行测量，测量结果与标称值具有良好的一致性，表明该装置在微结构的形貌测量方面有广阔的应用前景。
Abstract:In order to improve the measurement accuracy, a kind of phase shifting digital holographic microscopy based on a long working distance microscopic objective is proposed. In the setup, an LED is adopted as the illumination light source, which can suppress coherent noise effectively and hence improve the measurement accuracy. A michelson quasi-common-path interferometer is constructed by adding a beam-splitter between the long working distance objective and the sample. The layout of the setup is simple and it can be easily adjusted, and thus the interference can be come into being conveniently especially when the sample is illuminated with a partial light source. The blind phase-shifting interferometry is adopted in the reconstruction procedure, and the two-step blind phase-shifting algorithm is used to reconstruct the phase map of the measured sample. In the experiments, the height maps of a reflective USAF 1951 resolution target are measured under LED illumination and He-Ne laser illumination, respectively. The measurement results show that both coincide with each other; the phase noise under LED illumination is, however, reduced by 70% when compared with that under laser illumination. In addition, in order to further verify the effectiveness of the device, the device is used to measure a micro-nano rectangular step engraved on the silicon substrate. The measurement results are in good agreement with the nominal values. This technique can be potentially used in the topographic measurement of micro-structures.
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Overview: Digital holographic microscopy (DHM) has been widely applied in biological sample imaging, three-dimensional morphology detection, flow field measurement, and other fields, because it has the advantages of non-contact, non-destructive, high resolution, and so on.
In DHM, the laser is usually used as the illumination source. However, the speckle noise induced by laser is large, and hence it degrades the quality of reconstructed phase image of objects. In order to reduce speckle noise, we use partially coherent light source light emitting diode (LED) as the illumination source. With the adoption of an LED, we proposed a phase-shifting digital holographic microscopic device based on a long working distance objective. By taking advantage of characteristic of long working distance of the objective, a micro-Michelson interferometer is constructed by inserting a non-polarizing beam splitter (NPBS) between the objective and the sample. In the system, as both object wave and reference wave pass through almost the same path, thus the interferometer is a quasi-common-path one. In addition, the system has a compact structure and it is convenient to adjustment. When it is illuminated by an LED, the interference between object wave and reference wave can be easily realized. To reconstruct the phase maps from the holograms, two-step blind phase-shifting algorithm is adopted. To achieve this, a low-cost mechanical micro-displacement platform is used to introduce phase shift between the holograms in the system to replace the usually expensive piezoelectric ceramic transducer (PZT).
Several experiments were conducted to verify the correctness and effectiveness of the system. In the experiments, we measured samples respectively under LED illumination and laser illumination and compared the phase noise level of phase images under different illuminating sources. In the first experiment, the sample is a reflective USAF 1951 resolution target and the experimental results demonstrate that the noise in the phase map with LED illumination is reduced by 70% when compared with that of laser illumination. At the same time, there is no significant difference in system resolution between two reconstructed phase maps corresponding to LED and laser illumination, respectively. Consequently, the proposed scheme can effectively reduce the phase noise and improve the quality of reconstructed phase image. In the second experiment, the device was used to measure a micro-nano step. The measured height was in good agreement with its nominal value. The experimental results show that the system can be potentially applied in the precise measurement of the three-dimensional topography of the micro-structure surface and other related fields.
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图 1 基于长工作距离物镜的相移干涉实验装置
Figure 1. Experimental setup of phase-shifting interferometry based on a long working distance objectiv
图 2 (a) 全息图 1；(b)全息图 2；(c)物光强度图；(d)参考光强度图
Figure 2. (a) Hologram 1; (b) Hologram 2; (c) Object intensity; (d) Reference intensity
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