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In this paper, we demonstrate a method to modulate the intensity and central frequency of THz generation from two-color laser-induced air plasmas, via transforming the pump Gaussian beams into vortex beams. Under the same input laser energy, the outputs of THz emission generated by the vortex beam can be enhanced, showing the potential of producing stronger THz radiation. Meanwhile, the vortex pump-beams also provide an unique possibility to modulate the central frequency of THz outputs, implying an all-optical method of THz modulation based on the shaped plasmas.

Dielectric lenses are widely used in terahertz imaging and communication systems to focus or collimate the Gaussian beam by adjusting the phase distribution of wave front. However, due to the high frequency and short wavelength of the terahertz band, the focusing lenses used in the application systems are usually electrically large, which bring the extremely difficulty to carry out efficient electromagnetic (EM) simulation and optimization design. In this paper, a dimensionality reduction concept was introduced to achieve efficient design and optimization of electrically-large terahertz lenses, which have symmetric structures and are commonly used in quasi-optical systems, such as circular lenses and cylindrical lenses. To precisely solve the EM problem with reduced dimension, a two-dimensional moment method (2D-MOM) for homogeneous dielectric targets was studied and successfully developed by solving the surface coupled integral equation discretized with appropriate basis and test functions. Then, the dimensionality reduction approach with the combination of the ray-tracing method (RTM) and the 2D-MOM was developed for the shaped design of terahertz lens with high efficiency. A 0.3THz lens with diameter 10cm was designed with the proposed approach as an example, with its pattern measured by a terahertz field scanning platform. It's found that, with the dimensional reduction, the unknowns can be reduced more than 1500 times and the memory required can be reduced more than 2.5 million times, as compared to the traditional 3D-MOM simulation. And the simulation results agree well with the experiments, which both demonstrate the greatly improved performance of the lens designed by the proposed approach, as compared to the standard lens.

In this paper, the enhanced phase shift migration (PSM) algorithm for multi-input-multi-output (MIMO) side-looking (SL) scheme was proposed for the 2-dimension (2D) fast image reconstruction in the terahertz (THz) band. By decomposing the MIMO imaging problem into a superposition of several single-input-multiple-output (SIMO) sub-arrays, the proposed algorithm can be applied to systems with randomly distributed transmitting array. By deriving and applying the modified phase shift factor and necessary phase compensation, zero-padding in the transmitting array and data reorganization in the spatial-frequency domain of the conventional wavenumber domain MIMO algorithm can be totally avoided, which will hopefully further reduce the amount of calculation. A comprehensive MIMO-SL-EPSM algorithm was established in the wavenumber domain. Proof-of-principle experiments in the 0.1-THz band were performed based on a MIMO-SL prototype system. The reconstructed images for different targets with high efficiency and quality demonstrate the theoretical results and the effectiveness of the proposed algorithm.

In this paper, the extended phase shift migration (PSM) for three-dimensional (3-D) multi-input-multi-output synthetic aperture radar (MIMO-SAR) imaging in terahertz (THz) band was studied. Based on one-dimensional MIMO arrays combined with synthetic aperture scan along another dimension, MIMO-SAR imaging scheme allows the number of array elements to be greatly reduced compared with the two-dimensional (2-D) MIMO arrays. By analyzing the derived analytical expression of the scattered waves in the frequency-wavenumber domain, the MIMO-SAR data in a certain frequency is mapped to another frequency of the `explode fields' of the monostatic form in accordance with a newly defined 3-D dispersion relations. By multiplying the modified phase shift terms in each frequency of `explode fields', the `explode fields' at different range planes can be reconstructed. Finally, the `explode fields' at time t = 0 can be successfully derived to realize fast imaging reconstruction for MIMO-SAR, with great reduction on the time consuming as compared with the BP algorithm and better accuracy compared with the Stolt migration. Additionally, due to its iterative nature in the range direction, the proposed algorithm is more flexible in treating more complicated scenarios, such as the image reconstruction in the multi-layer medium. A bistatic prototype imager was designed for the proof-of-principle experiments in THz band. The 3-D imaging results of different targets and computational complexity were also given to demonstrate the good performance of the proposed algorithm for THz MIMO-SAR imaging.

Near-filed (NF) scanning measurement is an effective way to evaluate the antenna radiation characteristics, among which antenna directivity is an important parameter for radar systems. In this paper, an accurate and fast method is proposed to determine the antenna directivity based on NF scanning measurement. The antenna far-field (FF) patterns are firstly produced by extrapolating the planar NF data obtained in NF scanning, then the antenna directivity can be estimated using the pattern integral method. Different from the existing methods, an integral transformation method is proposed to calculate the pattern integral, which deals with the FF responses in the wavenumber domain directly, avoiding the complex interpolation operation in angle domain. The proposed method is verified with both simulated data and experimental data. The results indicate that the proposed method can significantly promote the computation efficiency and estimation accuracy, which is superior to the existing methods.

Coherent integration is a conventional approach to improve detection ability in radar applications, whereas its performance is decreased by migration through range cell due to the motion of target, limiting the time of integration. This problem becomes more severe in terahertz band. This paper introduces Keystone transform to terahertz radar in order to realize long-time coherent integration, aiming to get a successful detection of targets. Moreover, doppler ambiguity is resolved based on intra-pulse interference processing. A simulation is conducted to demonstrate the effective improvement for target detection using Keystone transform in terahertz radar.

In this paper, a fast back-projection (BP) algorithm based on time-delay surface correction (TSC) for two-dimensional arrays is proposed. For the range interpolation problem caused by the conventional BP algorithm, the echo data distance migration phase correction method is used to avoid range interpolation and improve the efficiency of BP algorithm while ensuring accurate target positioning. And the computational efficiency of the two methods is compared by the simulation. Then an array optimization design scheme using the Simulated Annealing (SA) algorithm and Genetic Algorithm (GA) is proposed for eliminating the grating lobes in the W-band sparse array. The feasibility of this method is verified by simulation.

The minimum entropy autofocusing for terahertz SAR motion compensation is proposed. Minimum entropy is a nonparametric autofocusing algorithm, which estimates the phase errors by minimizing image entropy. It works better in sparse scenes, such as a collection of point scatterers, which is often encountered in terahertz SAR processing. In this paper, the minimum entropy and Newton method are combined to compensate the platform motion errors. The experimental results show that the minimum entropy autofocusing method commonly used in microwave SAR is also applicable to the terahertz band.

Real-time imaging technology is receiving more and more attention in medical imaging, non-destructive testing, especially security screening. In this paper, a fast equivalent monostatic range migration algorithm (RMA) based on a standoff Multi-input multi-out (MIMO) screening system is proposed at terahertz band. The collected multistatic data is calibrated firstly to be transformed into the monostatic one. Then a high-efficiency RMA is adopted to realize the near real-time imaging. The performance and efficiency are validated in the simulation.

A novel terahertz (THz) imaging method based on vortex wave is presented. First, the analytical expression of field distribution of the THz vortex beam generated by spiral phase plate (SPP) is derived, and the propagation characteristics of vortex beams with different topological charges are analyzed. Subsequently, the THz vortex imaging model is established, and Fourier transform (FT) is used to reconstruct the target’s azimuthal image. Simulation results demonstrate that vortex beam has the prospect for acquiring the azimuthal information of target. The work can also promote the development of the novel THz imaging technology.

A new interferometry positioning and tracking method with short baseline for ultra-wideband (UWB) through-the-wall radar system is proposed in this paper. To improve the accuracy of two-dimensional positioning and tracking, the high-precision distance difference of the two receiving antennas is obtained by interferometry. The accurate two-dimensional position can be acquired by the trilateration with the distance difference and the distance of the first receiving channel obtained by the decentralized procedure. The algorithm performance has been experimentally validated in the through-the-wall environment and is much better than that of the decentralized procedure in terms of positioning accuracy and tracking errors.

Lunar Regolith Penetrating Radar (LRPR) is a high resolution penetrating and imaging radar, which is a main payload of Chang'E-5. The primary aim of LRPR is to survey the lunar regolith thickness and the subsurface structure of landing area. The aim is achieved by transmitting pulse and receiving echo signal. As the width of the transmitting pulse is picosecond level, it is difficult to achieve real-times sampling of echo signal on the circuit. Equivalent sampling is adopted. The commonly used methods of equivalent sampling are implemented by the delay chip or hardware circuit. This article points out the disadvantages of the two methods, then designs a method based on the principle of Vernier Caliper. The method with strong adaptability is implemented by PLL based on FPGA, which has high reliability and high precision. There is no extra expense in hardware circuit with this method, and the delay accuracy can be adjusted.

China's first Mars probe including an orbiter and a landing rover will be launched by 2020. A subsurface penetrating radar instrument has been selected to be a part of payload on the rover. The main scientific objective of the SPR is to characterize the thickness and sub-layer distribution of the Martian soil. The SPR consists of two channels. The low frequency channel of the SPR will provide a penetrating depth of 10 to 100 meters with a resolution of a few meters within the Martian soil. The higher frequency channel will penetrate to a depth of 3 to 10 meters with a resolution of a few centimeters within the Martian soil. The engineering model of the SPR was designed and manufactured. In order to illustrate and validate the equipment's performance, some laboratory field tests have been performed. The objectives of these tests were to evaluate the interaction between the radar and the rover and detect the internal objects buried in the pool filled with artificial volcanic ash.

Lunar Regolith Penetrating Radar (LRPR) is one payload of Lander for Chang'E-5. LRPR's scientific missions are: (1) providing information support for drill and sampling device; (2) sounding lunar regolith thickness and structure below the Lander area, which is covered by radar's antenna. In order to obtain detection data effectively, it is necessary to overcome the problem that the Lander cannot move. The LRPR uses an electric scanning method to traverse 12 antennas instead of the traditional detection mode. With the method, LRPR requires only one receiver and transmitter, thereby can effectively reduce weight and power consumption. LRPR is a time domain carrier-free pulse radar, which is compose of antenna array, electronic box and RF cable. It is shown that task requirements are met by confirmatory test of simulation Lunar Regolith on the round.

UWB radar is widely used ranging from tracing through the wall, anti-terrorist stabilization, earthquake and disaster relief to archaeological discoveries and geological surveys. In this paper, a m-sequence UWB radar is designed to solve the problem of detection range and resolution, and to meet the relatively simple structure and can be applied to a variety of application scenarios. Finally, the contrast field test with an impulse radar was carried out, and it confirmed that the m-sequence UWB radar has more detection capability and anti-noise performance than impulse radar

In this paper, a design of MIMO array for 0.2 THz imaging was studied. An imaging algorithm with high efficiency was developed by successfully implementing the phase shift migration (PSM) into MIMO reconstruction. To numerically verify the imaging quality and the proposed method, the scattered data of targets was obtained based on the near-field distribution of the transceiving antenna from full wave electromagnetic simulations. With the multistatic scattered data rearranged in spatial frequency domain, image with high quality was reconstructed based on MIMO-PSM algorithm, with greatly improved efficiency as compared to the conventional back-projection (BP) algorithm. The work in this paper provides a guideline for THz-MIMO array design and proposed a fast imaging algorithm for MIMO imaging.

Traditional gradient descent inversion for transient electromagnetic (TEM) data is time and memory comsuming because the derivative matrices need to be computed repeatly. In this paper, we apply the Supervised Descent Method (SDM) into pixel-based inversion for TEM data. This method is based on the concept of gradient learning. In an offline stage, the average descent direction can be learned from a set of training data; and in an online stage, data inversion can be achieved by the learned descent directions without computing the derivative matrices. Numerical tests verify that this algorithm converges faster and is also more efficient. Moreover, SDM offers a more convinient way to incorporate prior information into inversion that could improve the efficiency of data interpretation.

The high amplitude pulses played a crucial role and is of paramount importance for obtaining a good radar range, especially when performing the task of penetrating thick and lossy building walls. The outputs of monocycle pulse generator affect the performance directly. Aimed at increasing the output power of the transmitter and improving the present pulse generator, in this paper, we present a novel monocycle pulse generator. This generator can operate well at the pulse repetition frequency (PRF) up to 5 MHz and generate a monocycle pulse with good symmetry and very low ringing. It combines wideband bipolar junction transistors (BJTs), linear power amplifier transistor, step recovery diode (SRD), variable capacitance diodes (VCDs) as a unique pulse compressor, LC high-pass filter as a differentiator and short-circuited microstrip line to generate the monocycle pulse directly. Measurement results show that the output peak-to-peak amplitude of the monocycle pulse is 25V and the duration is 450ps. These characters has significant potential in applications where long detection range and a high resolution are concerned, especially when penetrating thick and lossy medium for applications in through-wall and ground penetrating radar sensors.

The ionosphere is an essential part of Earth's upper atmosphere. It can be explored by ground-based ionosonde or space-based topside sounder. The ionogram contains ionosphere's characteristics, and plenty of automatic scaling procedures for ionogram have been developed. Most of them can output correct results as the SNR is higher. This paper presents a novel method for automatic scaling of the F layer from ionograms based on improved filter and morphology method, which can eliminate the noises and interference of ionospheric ionograms. Digital ionogram data from the Chinese Academy of Science Digital Ionosonde (CAS-CADI), installed at Huailai (115°52'N, 40°40'E) of Hebei province, were used to demonstrate the data processing procedures.

A novel Ultra-Wideband (UWB) picoseconds pulse generator is proposed in this paper. It contains an avalanche transistor and a unique pulse shaping circuit. The avalanche transistor can produce drive pulses with the timing and amplitude parameters required by the step recovery diode (SRD), and the pulse shaping circuit utilizes the transition characteristics of SRD effectively to generate a couple of balanced fast leading edge pulses with high pulse repetition frequency (PRF) up to 1MHz. Measurement results show that the peak voltages of the output pulses are approximately ±30V at 500 kHz PRF and the leading edge of output pulses both are approximately 250 ps. The wide bandwidth of this generator ensures fine range resolution and high penetration capability for application in Trough-the-Wall radar sensors effectively.

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 step-frequency continuous-wave (SFCW) scheme was proposed for millimeter-wave (MMW) inline holography to eliminate the background and twin-image interferences with appropriate range windows. The image reconstruction algorithm is developed for the proposed the hologram approach. A 200GHz prototype experiment is performed to demonstrate the effectiveness of the imaging scheme.

Terahertz imaging scheme with simplified single homodyne channel was proposed. To realize the calibration and coherent imaging, a Hilbert transform receiver was employed to recover the quadrature signal and obtain the phase information of the return wave. Proof-of-State experiment at 0.22THz band was performed to verify the effectiveness of the proposed imaging scheme, which may find potential applications in non-destructive testing with comparatively low cost.

The sounding depth is one of the most important parameters of Ground Penetrating Radar (GPR), and it is also the main limitation of GPR's applications. A pseudo random coded GPR for deep investigation is described in this paper. It could achieve a sounding depth more than 130 meters. A long Golay sequences and TGA (time-gain amplifier) technology are utilized to increase the SNR of faint echoes from deep layer underground, and enhance the sounding depth. The platform could be either a vehicle or a large airship. Field tests were carried out on Kubuqi desert located in Inner Mongolia, north-west of china, to verify the detection ability of this GPR system. The sounding results agree well with the results from the drilling and resistivity logging.

Lunar Regolith Penetrating Radar (LRPR) is a high resolution penetrating and imaging radar, which is a main payload onboard the detector of Chang'E-5. It is mounted on the bottom of the detector of Chang'E-5 and its main task is to survey the lunar regolith thickness and subsurface structure of landing domain. Because the detector is static, LRPR has to work only in stationary condition and is designed as a multi-input and multi-output ground penetrating radar system, employing an antenna array including 12 off-ground Vivaldi antennas as transmitters or receivers. When one serves as the transmitter antenna, the others are as the receiver antennas, in turn, to complete all scan probe task. During the development of the equipment, the detection capability and the imaging method of processing the dataset must be verified on the ground. A multi-offset diffraction stack migration algorithm to process LRPR dataset for obtaining a high resolution image of detect domain is developed on this paper. The results of ground verification demonstrate that the multi-offset diffraction stack migration algorithm can be applied to process LRPR dataset, and LRPR meets the design specifications: detecting depth over 2m, and detecting resolution better than 5 cm in lunar regolith.

In this paper, according to the two design principles of uniform distribution and less element shadowing for effective aperture, a novel two-dimensional (2-D) sparse ultra-wideband (UWB) multiple-input-multiple-output (MIMO) antenna array topology is proposed for near-field high-resolution imaging. Compared with another different array having the same number of transceivers, the proposed optimal array has a better control over the sidelobe level (SL) both in the interference region (IR) and non-interference region (NIR) of the simulated radiation patterns, from which the peak sidelobe levels (PSLs) of the optimal array obtained are lower than the compared one. Experimental imaging results for a mannequin target also verify the outstanding imaging capability of the proposed array. Therefore, the proposed array topology has potential applications for high-quality UWB near-field imaging.

China's Mars probe including an orbiter and a landing rover will be launched by 2020. A subsurface penetrating radar (SPR) instrument has been selected to be a part of the payload on the rover. The main scientific objective of the SPR is to characterize the thickness and sub-layer distribution of the Martian soil. The SPR consists of two channels. The low frequency channel of the SPR will provide a penetration depth of 10 to 100 meters with a resolution of a few meters within the Martian soil. The higher frequency channel will penetrate to a depth of 3 to 10 meters with a resolution of a few centimeters within the Martian soil. The SPR first version prototype was designed and some field tests have been conducted with it.

A novel ellipse cavity-backed and slot-tuned bow-tie antenna was designed to facilitate the application of ultra-wide band (UWB) ground penetrating radar (GPR) in low frequency. Lumped resistor loading was applied at the end of antenna arms to broaden the impedance bandwidth. Taper tuned method was also used to optimize the current distribution. Antenna characteristic was analyzed by simulation. Finally, pairs of antenna were manufactured and measured. The measured results showed that the proposed antenna had an impedance bandwidth from 12 MHz to 143 MHz with VSWR smaller than 2. The antenna achieves a compact size with 1.52∗0.58∗0.41 m3 by ellipse-shape cavity. Furthermore, road detection experiment demonstrates that the antenna can be well suitable for relatively deep target detection.

In this paper, a vital sign feature (V SF) detection and search strategy using multiple UWB life-detection-radars with different locations is proposed. Firstly, the different VSFs from all single UWB life-detection radars placed on multiple radar observation points are categorized. Then, the proposed detection strategy is used for the different VSF categories. Finally, the target's spatial location is estimated according to the radar observation points and the VSFs in the proposed search strategy. Experiments are carried out to validate the effectiveness of the proposed strategy.

An adaptive focusing (AF) algorithm based on minimum-entropy (ME) method is proposed for THz single frequency holography to improve the depth of focus for practical image applications. Experiments in 0.2THz band were performed to confirm the effectiveness of the proposed algorithm.

A frequency-scanning grating-reflector antenna operating in 0.2 Terahertz band is proposed based on the diffraction enhancement mechanism. The antenna was fabricated and the proposed concept was verified based on the quasi-optical measurements.

In pavement survey engineering, ground penetrating radar (GPR) is an effective and non-destructive survey method, compared with traditional core drilling method. Dielectric constant and layer thickness can be acquired from inversion analysis of the GPR signals. In this paper, metal reflection method and a layer picking algorithm called trace correlation are combined to estimate pavement layer thickness rapidly and efficiently. Highway testing shows this survey approach gives small errors, and meets the pavement survey requirements.

Aperture synthesized focusing technique was extended for terahertz imaging with fan-beam scanning. Appropriate algorithms were developed both for the single-frequency two-dimensional case and wide-band three-dimensional case. Proof-of-concept experiments were performed at 0.2 THz band.