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摘要:
太赫兹波具有独特的低能性、高穿透性、惧水性等成像特性，将其应用于相衬成像能够反映物体的内部结构和更加丰富全面的生物信息，在生物医学检测等领域具有重要的应用。其中，太赫兹波数字全息成像是一种可以给出定量的振幅和相位信息的非接触、全场相衬成像方法，是太赫兹成像技术领域的重要研究方向之一。本文基于连续太赫兹源，从离轴式和同轴式数字全息成像的相衬成像原理、光路系统和再现算法多个方面，介绍了相关技术的研究现状，分析了太赫兹源、再现算法等因素对成像分辨率的影响，并对太赫兹数字全息的发展趋势进行了展望。
Abstract:Terahertz (THz) radiation, due to its unique propagation characters of low-energy, high-penetration, water-absorption, provides internal structure of objects and comprehensive biological information in phase contrast imaging. It has been applied in biomedical imaging, non-destructive testing, and other fields. As an important part of THz imaging technology, continuous-wave (CW) THz digital holography (TDH) is qualified as a non-invasive and whole-field phase contrast imaging method. In this paper, we review the development and status of off-axis and in-line TDH, including the recording and reconstruction theory, experimental setup, and reconstruction algorithms. The influence of existing THz sources and the reconstruction algorithms on resolution and fidelity of imaging are analyzed. And the development trend of TDH is prospected in the end.
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Key words:
- terahertz imaging /
- digital holography /
- resolution /
- phase contrast imaging
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Overview: Terahertz (THz) radiation is characterized with low-energy, high-penetration, and water-absorption, which could provide internal structure of objects and comprehensive biological information by THz phase contrast imaging. Due to this unique feature, THz radiation has been applied in biomedical imaging, non-destructive testing, and other fields. As an important part of THz imaging technology, continuous-wave THz digital holography (TDH) by recording the complex amplitude in the hologram and numerically retrieving the corresponding phase-shift properties of object, is qualified as a non-invasive and whole-field phase contrast imaging method. With the development of continuous-wave THz sources, detectors, and imaging components, the continuous-wave TDH imaging technology is well developed.
The development and status of off-axis and in-line TDH are reviewed, including the recording and reconstruction theory, experimental setup, and reconstruction algorithms. For the off-axis TDH, the reflective and transmitted TDH has all been introduced. The firstly off-axis TDH configuration is attempted using a 100 GHz Gunn diode oscillator and Schottky-barrier diode. Complete "full-field" phase imaging with higher lateral resolution is achieved using an optically pumped FIR laser and a pyroelectrial array detector, which is then widely applied in continuous-wave TDH configuration later. And the effectiveness of Rayleigh-Sommerfeld convolution algorithm, Fresnel angular spectrum algorithm, and angular spectrum integral are evaluated for off-axis TDH. Miniaturized THz quantum cascade laser with high power and high frequency are also used in TDH full-field imaging system to improve the imaging resolution. For the in-line TDH, the scattered beam by sample interferes with the unscattered part to form the in-line hologram, which is quite suitable for isolated objects imaging. Compared with off-axis TDH, the recording geometry of in-line TDH is more compact, and reconstruction resolution is higher. The twin image is one of the most important problems for in-line TDH, which is solved by iterations with proper constraints in these two planes and phase retrieval algorithm. Constraints on the object plane and extrapolation algorithms for iterative reconstruction of in-line TDH are studied to achieve higher resolution. In addition, different methods are presented to improve the imaging quality of continuous-wave TDH, such as synthetic aperture, denoising method, auto-focusing algorithms, and multi-plane imaging.
In conclusion, the paper summarized the progress of continuous-wave THz digital holography, including the influence of existing THz imaging components and the reconstruction algorithms on resolution and fidelity of imaging are analyzed. And the future development trend of continuous-wave TDH is discussed in the end of the paper.
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图 4 (a) 基于面阵式热释电探测器的太赫兹波离轴成像系统；(b)间隔为0.4 mm的金属横靶条强度再现结果；(c)间隔为0.4 mm的金属竖靶条强度再现结果[33]
Figure 4. (a) The experimental set-up of the off-axis terahertz digital holography based on pyroelectric array camera; (b) Reconstructed intensity image of horizontal strips of 0.4 mm resolution chart; (c) Reconstructed intensity image of vertical strips of 0.4 mm resolution chart[33]
图 7 基于光泵太赫兹源和微测热辐射计的太赫兹数字全息成像光路[24]
Figure 7. The schematic layout of the terahertz digital holography based on the FIR laser and micro-bolometer
图 11 (a), (d)分辨率为50 μm和40 μm物体的光学显微图像；(b), (c)分辨率为50 μm物体的再现复振幅分布；(e), (f)分辨率为40 μm物体的再现复振幅分布[26]
Figure 11. (a), (d) Optical microscopic images of 50 μm and 40 μm resolution targets; (b), (c) Reconstructed complex amplitude distributions of 50 μm resolution targets; (e), (f) Reconstructed complex amplitude distributions of 40 μm resolution targets[26]
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