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Editor Work2018 Vol. 7, No. 1
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2018, 7(1): 1-21.
Abstract
4945KB
Terahertz radar has unique advantages, including large bandwidth, high resolution, Doppler sensitivity, and anti-interference; it is a significant development in the field of target detection. Herein, the history of electronic and optical terahertz radar systems is introduced, and the current situation and latest progress pertaining to these systems are reviewed. The target characteristics of terahertz radar are summarized based on its mechanism, calculation, and measurement. Moreover, the current research status of terahertz SAR, ISAR, array, and aperture encoding imaging are discussed, and the applications of terahertz radar, such as early warning detection and security anti-terrorism systems, are briefly introduced. Finally, the development direction of terahertz radar technology is forecast.
Terahertz radar has unique advantages, including large bandwidth, high resolution, Doppler sensitivity, and anti-interference; it is a significant development in the field of target detection. Herein, the history of electronic and optical terahertz radar systems is introduced, and the current situation and latest progress pertaining to these systems are reviewed. The target characteristics of terahertz radar are summarized based on its mechanism, calculation, and measurement. Moreover, the current research status of terahertz SAR, ISAR, array, and aperture encoding imaging are discussed, and the applications of terahertz radar, such as early warning detection and security anti-terrorism systems, are briefly introduced. Finally, the development direction of terahertz radar technology is forecast.
2018, 7(1): 22-45.
With years of development and accumulation, a considerable amount of research has focused on micro-motion, an important auxiliary feature in radar target detection and recognition. With the recent rise of terahertz, micro-motion feature extraction in the terahertz region has increasingly highlighted its advantages. Herein, we systematically surveyed the recent research on terahertz radar micro-motion feature extraction and discussed micro-motion feature analysis, micro-motion feature extraction, and micro-motion target imaging. And then we emphatically introduced the work of our research team, including the theoretical and experimental research on micro-motion feature analysis, micro-motion feature extraction and high-resolution/high-frame micro-motion target imaging. Furthermore, we analyzed the growing trend of micro-motion feature extraction in the terahertz region, and pointed out the new technology directions worth to be studied further and the technical challenges to be solved.
With years of development and accumulation, a considerable amount of research has focused on micro-motion, an important auxiliary feature in radar target detection and recognition. With the recent rise of terahertz, micro-motion feature extraction in the terahertz region has increasingly highlighted its advantages. Herein, we systematically surveyed the recent research on terahertz radar micro-motion feature extraction and discussed micro-motion feature analysis, micro-motion feature extraction, and micro-motion target imaging. And then we emphatically introduced the work of our research team, including the theoretical and experimental research on micro-motion feature analysis, micro-motion feature extraction and high-resolution/high-frame micro-motion target imaging. Furthermore, we analyzed the growing trend of micro-motion feature extraction in the terahertz region, and pointed out the new technology directions worth to be studied further and the technical challenges to be solved.
2018, 7(1): 46-55.
In this paper, we review the recent developments on information metamaterials, including digital metamaterials, coding metamaterials, and programmable metamaterials; furthermore, we discuss their applications in the terahertz (THz)-frequency region. In addition their flexibility to manipulate the electromagnetic waves, the physical principle, numerical simulation, fabrication, and application of information metamaterial are discussed in detail. Moreover, we developed and applied a coding metasurface that works in the THz band. Furthermore, the principle of real-time programmable metamaterials and their application in novel imaging systems and radar systems are illustrated. Information metamaterials and metasurfaces can be used for various functional devices such as beam splitting and low radar cross section, which open up a novel route to manipulate THz radiations.
In this paper, we review the recent developments on information metamaterials, including digital metamaterials, coding metamaterials, and programmable metamaterials; furthermore, we discuss their applications in the terahertz (THz)-frequency region. In addition their flexibility to manipulate the electromagnetic waves, the physical principle, numerical simulation, fabrication, and application of information metamaterial are discussed in detail. Moreover, we developed and applied a coding metasurface that works in the THz band. Furthermore, the principle of real-time programmable metamaterials and their application in novel imaging systems and radar systems are illustrated. Information metamaterials and metasurfaces can be used for various functional devices such as beam splitting and low radar cross section, which open up a novel route to manipulate THz radiations.
2018, 7(1): 56-66.
To fulfill the requirements of the dielectric property measurement in the terahertz band, herein, a broadband quasi-optical system was designed and verified utilizing a planar scanning system. Additionally, the method of retrieving the dielectric parameters was discussed. Our experimental findings indicated that the measurement results were in good agreement with the theoretical results. Boron silicon, and deionized water were used for verifying the measurement, and the permittivity was obtained using a numerical method. We found that the dielectric properties were in good agreement with the typical values. This indicated that the proposed quasi-optical method effectively characterized the permittivity.
To fulfill the requirements of the dielectric property measurement in the terahertz band, herein, a broadband quasi-optical system was designed and verified utilizing a planar scanning system. Additionally, the method of retrieving the dielectric parameters was discussed. Our experimental findings indicated that the measurement results were in good agreement with the theoretical results. Boron silicon, and deionized water were used for verifying the measurement, and the permittivity was obtained using a numerical method. We found that the dielectric properties were in good agreement with the typical values. This indicated that the proposed quasi-optical method effectively characterized the permittivity.
2018, 7(1): 75-82.
An efficient hybrid algorithm is proposed to analyze the electromagnetic scattering properties of an infinitely thin metal plate in the lower terahertz (THz) frequency region. In this region, the metal plate can be viewed as a perfect electrically conductive object with a marginally rough surface. Hence, the THz scattered field from the metal plate can be divided into coherent and incoherent parts. The physical optics and truncated-wedge incremental-length diffraction coefficients methods are used to compute the coherent part, whereas the small perturbation method is used to compute the incoherent part. Then, the radar cross section of the rough metal plate surface is computed by the multilevel fast multipole and proposed hybrid algorithms. The numerical results show that the proposed algorithm has a good accuracy when rapidly simulating the scattering properties in the lower THz region.
An efficient hybrid algorithm is proposed to analyze the electromagnetic scattering properties of an infinitely thin metal plate in the lower terahertz (THz) frequency region. In this region, the metal plate can be viewed as a perfect electrically conductive object with a marginally rough surface. Hence, the THz scattered field from the metal plate can be divided into coherent and incoherent parts. The physical optics and truncated-wedge incremental-length diffraction coefficients methods are used to compute the coherent part, whereas the small perturbation method is used to compute the incoherent part. Then, the radar cross section of the rough metal plate surface is computed by the multilevel fast multipole and proposed hybrid algorithms. The numerical results show that the proposed algorithm has a good accuracy when rapidly simulating the scattering properties in the lower THz region.
2018, 7(1): 83-90.
The terahertz scattering characteristics of metallic and dielectric rough targets is important for the investigation of the terahertz radar targets properties. According to the stationary phase theory and scalar approximation, if the radius of curvature at any point of the surface is much larger than the incident wavelength, and the wavelength is also much longer than the surface height function and Root-Mean-Square (RMS) surface slope, the coherent and incoherent scattering Radar Cross Section (RCS) of rough metallic and dielectric targets can be obtained. Based on the stationary phase approximation, the coherent RCS of rough conductors, smooth dielectric targets and rough dielectric targets can be easily deputed. The scattering characteristics of electrically large smooth Al and painted spheres are investigated in this paper, and the calculated RCS are verified by Mie scattering theory, the error is less than 0.1 dBm2. Based on lambert theory, it is demonstrated that the incoherent RCS is analyzed with better precision if the rough surfaces are divided into much more facets. In this paper, the coherent and incoherent scattering of rough Al and painted spheres are numerically observed, and the effects of surface roughness and materials are analyzed. The conclusions provide theoretical foundation for the terahertz scattering characteristics of electrically large rough targets.
The terahertz scattering characteristics of metallic and dielectric rough targets is important for the investigation of the terahertz radar targets properties. According to the stationary phase theory and scalar approximation, if the radius of curvature at any point of the surface is much larger than the incident wavelength, and the wavelength is also much longer than the surface height function and Root-Mean-Square (RMS) surface slope, the coherent and incoherent scattering Radar Cross Section (RCS) of rough metallic and dielectric targets can be obtained. Based on the stationary phase approximation, the coherent RCS of rough conductors, smooth dielectric targets and rough dielectric targets can be easily deputed. The scattering characteristics of electrically large smooth Al and painted spheres are investigated in this paper, and the calculated RCS are verified by Mie scattering theory, the error is less than 0.1 dBm2. Based on lambert theory, it is demonstrated that the incoherent RCS is analyzed with better precision if the rough surfaces are divided into much more facets. In this paper, the coherent and incoherent scattering of rough Al and painted spheres are numerically observed, and the effects of surface roughness and materials are analyzed. The conclusions provide theoretical foundation for the terahertz scattering characteristics of electrically large rough targets.
2018, 7(1): 91-96.
Based on the physical optics method, the scattering characteristics of fractal rough surface coated objects are studied in the terahertz (THz) range herein. A blunt model based on fractal rough surfaces is built. The surface current is calculated according to the Fresnel reflection coefficient, and the Radar Cross Section (RCS) of the rough coated target is obtained. The RCS of rough and smooth surface targets are compared. Numerical results for a rough coated blunt cone model are provided, and discussed from the perspective of different frequencies and coating thickness values. The results show that the surface roughness of the target has a significant effect on scattering in the terahertz range.
Based on the physical optics method, the scattering characteristics of fractal rough surface coated objects are studied in the terahertz (THz) range herein. A blunt model based on fractal rough surfaces is built. The surface current is calculated according to the Fresnel reflection coefficient, and the Radar Cross Section (RCS) of the rough coated target is obtained. The RCS of rough and smooth surface targets are compared. Numerical results for a rough coated blunt cone model are provided, and discussed from the perspective of different frequencies and coating thickness values. The results show that the surface roughness of the target has a significant effect on scattering in the terahertz range.
2018, 7(1): 97-107.
Echo simulation is a precondition for developing radar imaging systems, algorithms, and subsequent applications. Electromagnetic scattering modeling of the target is key to echo simulation. At terahertz (THz) frequencies, targets are usually of ultra-large electrical size that makes applying classical electromagnetic calculation methods unpractical. In contrast, the short wavelength makes the surface roughness of targets a factor that cannot be ignored, and this makes the traditional echo simulation methods based on point scattering hypothesis in applicable. Modeling the scattering characteristics of targets and efficiently generating its radar echoes in THz bands has become a problem that must be solved. In this paper, a hierarchical semi-deterministic modeling method is proposed. A full-wave algorithm of rough surfaces is used to calculate the scattered field of facets. Then, the scattered fields of all facets are transformed into the target coordinate system and coherently summed. Finally, the radar echo containing phase information can be obtained. Using small-scale rough models, our method is compared with the standard high-frequency numerical method, which verifies the effectiveness of the proposed method. Imaging results of a full-scale cone-shape target is presented, and the scattering model and echo generation problem of the full-scale convex targets with rough surfaces in THz bands are preliminary solved; this lays the foundation for future research on imaging regimes and algorithms.
Echo simulation is a precondition for developing radar imaging systems, algorithms, and subsequent applications. Electromagnetic scattering modeling of the target is key to echo simulation. At terahertz (THz) frequencies, targets are usually of ultra-large electrical size that makes applying classical electromagnetic calculation methods unpractical. In contrast, the short wavelength makes the surface roughness of targets a factor that cannot be ignored, and this makes the traditional echo simulation methods based on point scattering hypothesis in applicable. Modeling the scattering characteristics of targets and efficiently generating its radar echoes in THz bands has become a problem that must be solved. In this paper, a hierarchical semi-deterministic modeling method is proposed. A full-wave algorithm of rough surfaces is used to calculate the scattered field of facets. Then, the scattered fields of all facets are transformed into the target coordinate system and coherently summed. Finally, the radar echo containing phase information can be obtained. Using small-scale rough models, our method is compared with the standard high-frequency numerical method, which verifies the effectiveness of the proposed method. Imaging results of a full-scale cone-shape target is presented, and the scattering model and echo generation problem of the full-scale convex targets with rough surfaces in THz bands are preliminary solved; this lays the foundation for future research on imaging regimes and algorithms.
2018, 7(1): 108-118.
Two important issues must be considered when modeling the terahertz (THz) wave scattering behavior using microwave EM methods. The first is the material response characteristics, including the metallic materials that may be beyond the scope of the Drude model’s description and the dielectric materials that may lack appropriate description models. The second is the modeling method for investigating the THz scattering behavior of the surface random roughness and the complex fine structures. Several theoretical endeavors are presented in this paper to elucidate the surface- and volume-scattering phenomenon observed in the experiment data. First, we employ the Integral Equation Method (IEM) to fit the measured data of an aluminum plate. Good agreements confirm the superior applicability of IEM to the metallic materials. Nevertheless, for dielectric materials, the volume-scattering contributions of the inner microstructures or particles whose sizes are comparable to the THz wavelength are required to be considered. Furthermore, our findings state that, for dielectric materials, it can be fit to the experimental data with the help of the Vector Radiative Transfer (VRT) theory. Finally, this research proposes a semi-deterministic description-based ray-tracing high-frequency algorithm to realize the rapid modeling of the coherent and incoherent scattering of electrically large complex targets at THz bands.
Two important issues must be considered when modeling the terahertz (THz) wave scattering behavior using microwave EM methods. The first is the material response characteristics, including the metallic materials that may be beyond the scope of the Drude model’s description and the dielectric materials that may lack appropriate description models. The second is the modeling method for investigating the THz scattering behavior of the surface random roughness and the complex fine structures. Several theoretical endeavors are presented in this paper to elucidate the surface- and volume-scattering phenomenon observed in the experiment data. First, we employ the Integral Equation Method (IEM) to fit the measured data of an aluminum plate. Good agreements confirm the superior applicability of IEM to the metallic materials. Nevertheless, for dielectric materials, the volume-scattering contributions of the inner microstructures or particles whose sizes are comparable to the THz wavelength are required to be considered. Furthermore, our findings state that, for dielectric materials, it can be fit to the experimental data with the help of the Vector Radiative Transfer (VRT) theory. Finally, this research proposes a semi-deterministic description-based ray-tracing high-frequency algorithm to realize the rapid modeling of the coherent and incoherent scattering of electrically large complex targets at THz bands.
2018, 7(1): 119-126.
This research proposes a novel terahertz frequency scanning antenna based on slotted waveguide arrays, and the antenna consisted of 31 elements in a linear array. Usually, the mechanical steering scheme is employed to realize two-dimensional terahertz imaging for most of the systems that inherently have a low frame rate limitation. As an attractive scheme to obtain high frame rate, electrical beam steering by frequency scanning antennas is the preferable approach. Previous scanning waveguide arrays operating on terahertz band cannot suppress sidelobe levels effectively. Thus, to fulfill the requirement of high radiation efficiency, low waveguide loss, and low sidelobe levels, arranging the slots to conform to Taylor distribution was considered. The distributions effectively become frequency-dependent for the beam-steering concept. Compared with the uniform slot distribution, a Taylor distribution ensures broadband radiation patterns with low sidelobe levels. Furthermore, the offset of the slots has been optimized through a power transmission method in combination with full wave simulation. The frequency-controlled beam steering concept and the sidelobe suppression effect were verified by the quasi-optical measurements in the 0.2 THz band. The fabricated slot-array antenna has large angle scanning ability and a low sidelobe property. In addition, the measured scanning range is larger than 50° with a moderate gain of 15 dB over the frequency band 165～215 GHz. The sidelobe levels are remarkably inhibited over 20 dB normalized to the mainlobe level, and the measured radiation patterns and sidelobe suppression effects agree well with HFSS full-wave simulation. The proposed beam-steering antenna has potential applications for THz imaging with a high frame rate.
This research proposes a novel terahertz frequency scanning antenna based on slotted waveguide arrays, and the antenna consisted of 31 elements in a linear array. Usually, the mechanical steering scheme is employed to realize two-dimensional terahertz imaging for most of the systems that inherently have a low frame rate limitation. As an attractive scheme to obtain high frame rate, electrical beam steering by frequency scanning antennas is the preferable approach. Previous scanning waveguide arrays operating on terahertz band cannot suppress sidelobe levels effectively. Thus, to fulfill the requirement of high radiation efficiency, low waveguide loss, and low sidelobe levels, arranging the slots to conform to Taylor distribution was considered. The distributions effectively become frequency-dependent for the beam-steering concept. Compared with the uniform slot distribution, a Taylor distribution ensures broadband radiation patterns with low sidelobe levels. Furthermore, the offset of the slots has been optimized through a power transmission method in combination with full wave simulation. The frequency-controlled beam steering concept and the sidelobe suppression effect were verified by the quasi-optical measurements in the 0.2 THz band. The fabricated slot-array antenna has large angle scanning ability and a low sidelobe property. In addition, the measured scanning range is larger than 50° with a moderate gain of 15 dB over the frequency band 165～215 GHz. The sidelobe levels are remarkably inhibited over 20 dB normalized to the mainlobe level, and the measured radiation patterns and sidelobe suppression effects agree well with HFSS full-wave simulation. The proposed beam-steering antenna has potential applications for THz imaging with a high frame rate.
2018, 7(1): 127-138.
Terahertz coded-aperture imaging follows the basic principles of optical coded-aperture imaging and microwave coincidence imaging and is a novel imaging technique. Herein, the wave spatial distribution or illumination pattern is usually obtained by a sub-reflector antenna. Terahertz coded-aperture imaging has some significant advantages such as a high frame rate, high resolution, and ability of forward-looking and staring imaging. To achieve simultaneous functions of aperture coding and beam scanning, we designed a terahertz coded-aperture imaging system that utilizes digital sub-reflector antenna and quasi-optical techniques. Based on this system, we deduce and simulate the influencing factors on its resolution. Then, different algorithms are applied to the imaging model in order to verify the superiority of sparse reconstruction for coded-aperture imaging. Finally, we compare the imaging results of our imaging system and that of a traditional real aperture imaging structure for the same simulation parameters. The results prove that our imaging system performs better with high resolution, small volume, and low cost. This new imaging technique can be applied to areas such as battlefield reconnaissance, security checks, anti-terrorism, and terminal guidance.
Terahertz coded-aperture imaging follows the basic principles of optical coded-aperture imaging and microwave coincidence imaging and is a novel imaging technique. Herein, the wave spatial distribution or illumination pattern is usually obtained by a sub-reflector antenna. Terahertz coded-aperture imaging has some significant advantages such as a high frame rate, high resolution, and ability of forward-looking and staring imaging. To achieve simultaneous functions of aperture coding and beam scanning, we designed a terahertz coded-aperture imaging system that utilizes digital sub-reflector antenna and quasi-optical techniques. Based on this system, we deduce and simulate the influencing factors on its resolution. Then, different algorithms are applied to the imaging model in order to verify the superiority of sparse reconstruction for coded-aperture imaging. Finally, we compare the imaging results of our imaging system and that of a traditional real aperture imaging structure for the same simulation parameters. The results prove that our imaging system performs better with high resolution, small volume, and low cost. This new imaging technique can be applied to areas such as battlefield reconnaissance, security checks, anti-terrorism, and terminal guidance.
2018, 7(1): 139-146.
To improve the imaging accuracy of Interferometry Inverse Synthetic Aperture Radar (InISAR), in this paper, we propose a novel method based on the joint processing of dual-frequency data. First, we divide the radar echo into two parts in the fast-time domain, then we apply the conventional InISAR method to each dataset. By comparing and analyzing the imaging results of the two parts, we can identify and remove redundant and bad pixels. Using the proposed joint processing method, we obtained the InISAR imaging results of a scatter model of an airplane and calculated the Root Mean Square Error (RMSE). The simulation results show that the proposed method can effectively improve imaging accuracy with different Signal-to-Noise Ratio (SNR).
To improve the imaging accuracy of Interferometry Inverse Synthetic Aperture Radar (InISAR), in this paper, we propose a novel method based on the joint processing of dual-frequency data. First, we divide the radar echo into two parts in the fast-time domain, then we apply the conventional InISAR method to each dataset. By comparing and analyzing the imaging results of the two parts, we can identify and remove redundant and bad pixels. Using the proposed joint processing method, we obtained the InISAR imaging results of a scatter model of an airplane and calculated the Root Mean Square Error (RMSE). The simulation results show that the proposed method can effectively improve imaging accuracy with different Signal-to-Noise Ratio (SNR).
2018, 7(1): 67-74.
The phonon dispersion spectrum, eigenvector, and lattice vibration frequency of cadmium telluride with a zinc blende structure have been investigated using the density functional theory, and the permittivity of cadmium telluride crystal is numerically calculated. The permittivity of the crystal is measured using the terahertz time-domain spectroscopy system. The experimental results are consistent with the theoretical calculations on the modified local density approximation, the general gradient approximation, and the modified general gradient approximation. Finally, the differences among the three approximate exchange correlation potentials indicate that in the terahertz region, the permittivity of cadmium telluride is dominantly contributed by the coupling between electron and phonon; however, the phonon frequencies of transverse wave and longitudinal wave were sensitive to electron density distribution.
The phonon dispersion spectrum, eigenvector, and lattice vibration frequency of cadmium telluride with a zinc blende structure have been investigated using the density functional theory, and the permittivity of cadmium telluride crystal is numerically calculated. The permittivity of the crystal is measured using the terahertz time-domain spectroscopy system. The experimental results are consistent with the theoretical calculations on the modified local density approximation, the general gradient approximation, and the modified general gradient approximation. Finally, the differences among the three approximate exchange correlation potentials indicate that in the terahertz region, the permittivity of cadmium telluride is dominantly contributed by the coupling between electron and phonon; however, the phonon frequencies of transverse wave and longitudinal wave were sensitive to electron density distribution.
2018, 7(1): 147-152.
The measurement system has been built with Stepped Frequency Continuous Wave (SFCW) signal for Radar Cross Section (RCS) measurements in free-space at 220～330 GHz. The system consists of vector network analyzer, Millimeter-Wave Converters and target support structure. The measurement techniques which are used in scattering measurements are verified by RCS measurement data. The system measurement accuracy is control within about ±3 dB at –23.6 dBsm.
The measurement system has been built with Stepped Frequency Continuous Wave (SFCW) signal for Radar Cross Section (RCS) measurements in free-space at 220～330 GHz. The system consists of vector network analyzer, Millimeter-Wave Converters and target support structure. The measurement techniques which are used in scattering measurements are verified by RCS measurement data. The system measurement accuracy is control within about ±3 dB at –23.6 dBsm.
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