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Phase contrast imaging based on continuous-wave terahertz digital holography
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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.-
Keywords:
- terahertz imaging /
- digital holography /
- resolution /
- phase contrast imaging
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References
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Overview
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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Figure 1.
Schematic of recording process of CW terahertz off-axis digital holography[28]
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Figure 2.
(a) Experimental setup for the creation of terahertz digital holograms based on Gunn diode oscillator; (b) Recontructed intensity image of engraved metal alphabet[31]
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Figure 3.
(a) The setup of Mach-Zender off-axis terahertz digital holography; (b) Unwrapped phase reconstruction of TPX lens[32]
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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]
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Figure 5.
(a) The experimental set-up of the off-axis terahertz digital holography based on the QCL source; (b) Experimental results: object (left), reconstructed intensity (center) and phase map (right) distributions[23]
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Figure 6.
(a) Schematic diagram of the experimental setup; (b) Three-dimensional plot of the relative depth profile of the plastic sheet[40]
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Figure 7.
The schematic layout of the terahertz digital holography based on the FIR laser and micro-bolometer
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Figure 8.
Schematic of recording process of CW terahertz in-line digital holography[28]
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Figure 9.
(a) The schematic layout of the in-line CW terahertz digital holography; (b) Imaging result of "HIT"; (c) Reconstructed image of (b) [47]
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Figure 10.
(a) Reconstructed phase distribution of dragonfly hindwing with extrapolation; (b) Reconstructed phase distribution of human liver cancer tissue[49-51]
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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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Figure 12.
(a) Reconstructed intensity distribution and (b) reconstructed phase distribution[26]
