Zhaoyang Sun

Terahertz Aperture Synthesized Imaging With Fan-Beam Scanning for Personnel Screening

A novel active terahertz imaging scheme based on the combination of fan-beam scanning and aperture synthesized reconstruction techniques is presented. In the horizontal direction, high resolution is obtained by the narrow side of the fan-beam based on real aperture focusing with special pillbox-like quasi-optics. In the vertical direction, high resolution is achieved by the aperture synthesized focusing techniques based on the broad side of the fan-beam. Appropriate focusing algorithms are proposed both for the single-frequency 2-D case and wideband 3-D imaging. With a 0.2-THz heterodyne transceiver, a prototype imager is developed and proof-of-concept experiments are performed. The experimental results on the resolution, specular reflection, and concealed threat objects detection are given in detail, which demonstrate the performance of the proposed imaging scheme and its potential application for fast personnel screening.

Three-Dimensional Image Reconstruction of Targets Under the Illumination of Terahertz Gaussian Beam—Theory and Experiment

In this paper, the wave equation based on phase shift migration technique is extended for terahertz 3-D imaging with quasi-optical transceivers. An analytical expression of the reconstructed 3-D point-spread function for targets under the illumination of a terahertz Gaussian beam was derived with this reconstruction technique. The quantitative relationship between the imaging quality and the parameters of the transmitted Gaussian beam was obtained, which provides a good criterion to be followed when designing the terahertz quasi-optical transceivers in the imaging systems. Moreover, the spatial sampling criterion was derived strictly which is also quantitatively related to the parameters of the transmitted Gaussian beam. Simulation results with fairly good agreement were given to verify the theoretical results derived in this paper. Finally, a monostatic prototype imager with a Gaussian beam transceiver was designed for the proof-of-principle experiments in 0.2-THz band. The 3-D imaging results of different targets and a mannequin with concealed threat objects were given to demonstrate the theoretical results obtained in this paper and the effectiveness of the 3-D terahertz image reconstruction for security applications.

Minimum-Entropy-Based Adaptive Focusing Algorithm for Image Reconstruction of Terahertz Single-Frequency Holography With Improved Depth of Focus

In this paper, the defocus of a 2-D image reconstruction of targets illuminated by Gaussian beams in terahertz (THz) single-frequency holography was studied. An analytical point-spread function was derived to quantitatively evaluate the defocus effect, considering the deviation of the restoration distance from the real distance of the target. It is found that the cross-range image defocuses in a similar way to the diffusion of a Gaussian beam from its beam waist, which limits the depth of focus for image restoration. To improve the restored images for targets with varying range distances, an adaptive focusing algorithm based on the minimum-entropy method was proposed for single-frequency holography. Simulation results with fairly good agreement were performed to verify the theoretical results and the proposed algorithm. Proof-of-principle experiments in the 0.2-THz band were also performed based on a monostatic prototype imager with a Gaussian beam transceiver. The experimental results confirm the effectiveness of the adaptive focusing reconstruction algorithm proposed in this paper.

Fast Three-Dimensional Image Reconstruction of Targets Under the Illumination of Terahertz Gaussian Beams With Enhanced Phase-Shift Migration to Improve Computation Efficiency

In this paper, an enhanced phase-shift migration-based image reconstruction (EPSM-IR) algorithm was investigated to improve the computation efficiency for three-dimensional (3-D) terahertz (THz) holographic imaging. In the proposed algorithm, an appropriate decomposition of the phase-shift operator was performed and a fast algorithm based on FFT was introduced to solve the integral transformation which is the most time-consuming step in the conventional phase-shift migration. The analytical expression of the reconstructed 3-D point-spread function (PSF) was derived based on the EPSM-IR algorithm. The reduction of the computation cost was evaluated quantitatively as compared with the conventional phase-shift migration. Simulation results with fairly good agreement were given to verify the proposed algorithm. Finally, a monostatic prototype imager with a Gaussian beam transceiver was designed for the proof-of-principle experiments in the 0.2-THz band. The 3-D image results of a target with metal strips and a mannequin with concealed threat objects were given to demonstrate the effectiveness and the efficiency of the 3-D EPSM-IR algorithm.

Study of Terahertz Superresolution Imaging Scheme With Real-Time Capability Based on Frequency Scanning Antenna

In this paper, a terahertz (THz) imaging scheme with real-time capability is proposed based on frequency- controlled beam scanning. The scheme holds a concise system architecture comprising a frequency scanning reflector antenna (FSRA) as the transmitting antenna, an omnidirectional antenna as the receiving antenna, and a vector network analyzer as the transceiver. By employing the diffraction enhancement mechanism to improve the diffraction efficiency and to suppress the specular beam, a high-directivity FSRA with periodic planar binary structure is proposed and developed at 0.235-0.33 THz band for imaging applications. A superresolution algorithm is proposed and developed by combining the direction of arrival (DoA) estimation and the matched filter technique, in which the tradeoff between available range and azimuth information can be successfully overcome. To achieve DoA estimation, the MUltiple SIgnal Classification (MUSIC) algorithm which is commonly used in phased-array antennas is extended to the novel imaging scheme based on FSRA, which is referred to as FS-MUSIC algorithm. The model of FSRA manifold vector is constructed for FS-MUSIC with a modified spatial smoothing method specifically developed to decorrelate the coherent back-scattering signals and to resolve the targets located within a same half-power beamwidth (HPBW). The resolution of the FS-MUSIC algorithm is derived based on the Bayesian approach. Superresolution imaging with real-time capability was demonstrated by both the simulation and the proof-of-principle experiments in the 0.3-THz band. An FSRA operating on 0.3-THz frequency band was developed.

Three-dimensional sparse image reconstruction for terahertz surface layer holography with random step frequency

In this Letter, a sparse image reconstruction approach is proposed for three-dimensional (3D) terahertz (THz) surface layer holography by a sharply dwindled amount of frequency samples, without reducing the high quality of the final reconstructed 3D THz images. To avoid the range ambiguity resulting from the reduction of frequency samples, a random step frequency method is adopted to evaluate the rough range profile of the 3D surface layer. With the obtained range profile, a de-ambiguity procedure is proposed to demodulate the sparse echoed data to greatly compress the maximum nonambiguous range and recover all the information for 3D holography image reconstruction. Proof-of-state experiments are performed in 0.2-THz band. The results verify the effectiveness and the efficiency of the sparse imaging scheme for THz surface layer 3D holography.

Implementation of Step-Frequency Continuous-Wave Scheme in Millimeter-Wave Inline Holography for Interferences Elimination

A millimeter-wave (MMW) inline holographic imag ing method based on step-frequency continuous-wave (SFCW) scheme is presented in this letter. By analyzing and decomposing the received SFCW power in range domain, it is found that the background and twin-image interferences can be well separated from the image-related term, and therefore can be removed with appropriate range windows. Moreover, a corresponding mathematical algorithm is developed to reconstruct the image from the inline hologram. To validate the effectiveness of the presented approach, a 200-GHz prototype system is designed and fabricated. Experimental results demonstrate the good performance of the imaging system and its potential application in high-resolution target detection.

Compensation of body shake errors in terahertz beam scanning single frequency holography for standoff personnel screening

In the terahertz (THz) band, the inherent shake of the human body may strongly impair the image quality of a beam scanning single frequency holography system for personnel screening. To realize accurate shake compensation in imaging processing, it is quite necessary to develop a high-precision measure system. However, in many cases, different parts of a human body may shake to different extents, resulting in greatly increasing the difficulty in conducting a reasonable measurement of body shake errors for image reconstruction. In this paper, a body shake error compensation algorithm based on the raw data is proposed. To analyze the effect of the body shake on the raw data, a model of echoed signal is rebuilt with considering both the beam scanning mode and the body shake. According to the rebuilt signal model, we derive the body shake error estimated method to compensate for the phase error. Simulation on the reconstruction of point targets with shake errors and proof-of-principle experiments on the human body in the 0.2-THz band are both performed to confirm the effectiveness of the body shake compensation algorithm proposed.

Terahertz image reconstruction based on sparse information

Terahertz (THz) wave has unique properties due to its special position in the electromagnetic spectrum. In recent years, imaging and sensing with THz radiation has aroused considerable interests and is found to be promising for plenty of applications. In this paper, several THz imaging concepts based on sparse information were introduced, such as THz surface layer reconstruction with sparse random step frequency, and the single frequency adaptive focusing with minimum-entropy (ME) method, which may find applications in reducing the complexity and the cost of imaging systems in THz band.

A Fast Three-Dimensional Image Reconstruction With Large Depth of Focus Under the Illumination of Terahertz Gaussian Beams by Using Wavenumber Scaling Algorithm

In this paper, a fast three-dimensional wavenumber scaling algorithm (3-D WSA) was proposed to reconstruct the wideband terahertz (THz) holographic image under the illumination of THz Gaussian beams. By taking the cross-range range coupling term into account, which is usually neglected in some fast algorithm, such as the enhanced phase shift migration (EPSM), the focusing depth for image reconstruction is greatly improved in the proposed fast algorithm. To deal with the cross-range range coupling term, a wavenumber scaling operator (WSO) was developed in the proposed algorithm which only uses chirp multiplications and FFTs with high computation efficiency and free of interpolation. Theoretical analysis on the relationship of the WSO parameters to the resulted maximum non-ambiguous range in holography image reconstruction was carried out with a quantitative criterion derived to appropriately determine the WSO parameters. Simulation results validate the proposed focusing algorithm and indicate that 3D WSA offers almost the same focusing capabilities as the most rigid phase shift migration (PSM) algorithm, meanwhile with computation efficiency greatly improved. Proof-of-principle experiments in 0.2 THz band were also performed based on a monostatic prototype imager. The experimental results demonstrate the effectiveness and the efficiency of the 3D WSA proposed in this paper.