For real-time millimeter-wave (MMW) imaging applications, spatially 2-D sparse sampling is one of the most promising techniques to reduce the heavy cost of signal acquisition. Therefore, in this article, we develop a new MMW synthetic aperture radar (SAR) sparse imaging method by establishing a 2-D pseudorandom spiral-sampling pattern. The method consists of two phases: sparse sampling-pattern design and sparse-imaging processing. The sparse sampling-pattern design is modeled as an optimization programming problem, in which the initial sampling pattern is a classical Archimedes spiral, and then, the number of samples per cycle is adjusted by coming up with a modified sigmoid function. This function is employed to increase the angular aperiodicity and randomness over the whole planar aperture. For sparse-imaging processing, a singular value thresholding (SVT) algorithm is applied to reconstruct the complete echo from partial samples observed by sparse sampling, and then, the inversion can be performed via the traditional imaging algorithms. In particular, a performance-guarantee condition of SVT is introduced to control the minimum number of samples, which bridges the relationship between pattern optimization and reconstruction performance. Our motivation is to answer what the undersampling pattern is and to estimate the extent to which the number of samples can be reduced by the current sparse imaging method. Finally, simulations and experiments validate the feasibility and effectiveness of the proposed sampling pattern, and the imaging results further demonstrate that our method with partially observed samples can offer the performance comparable to dense sampling.

High-frequency vibration of motion platform leads to paired echo for synthetic aperture radar (SAR) imaging, especially in terahertz band due to its shorter wave length. Different from most existing parameters estimation methods only considering single component high-frequency vibration, in this paper a novel method considering multi-components vibration model is proposed based on fractional Fourier transform (FrFT) with combination of quasi-maximum likelihood (QML) and random sample consensus (RANSAC). Based on the model establishment of high-frequency vibration error in the echo, its instantaneous chirp rate (ICR) is firstly estimated by FrFT in sliding sub-aperture, followed which the vibration parameters are coarsely obtained through spectrum analysis and least square (LS) regression. To further refine the parameters estimates, QML is developed for compensating the deviation both caused by the frequency spectrum leakage and the error propagation effects by one-dimensional search over the vibration frequency. Meanwhile, RANSAC is adopted for avoiding the outlier of the ICR estimates in LS regression, especially at low signal-to-noise ratio (SNR). Thus, the refinement strategy based on the combination of QML and RANSAC is developed, whose utilization improves the estimation accuracy of vibration parameters. Finally, the paired echo in terahertz SAR (THz-SAR) imaging is effectively suppressed by the proposed method, and the high-quality THz-SAR imaging results are achieved. Both simulations of single component and multi-components high-frequency vibration are used to verify the validity of the proposed method. The simulation results show that the proposed method has higher estimation accuracy even at low SNR.

Compared with microwave synthetic aperture radar (SAR), terahertz SAR (THz-SAR) is easier to achieve ultrahigh-resolution image due to its higher frequency and shorter wavelength. However, higher carrier frequency makes THz-SAR image quality very sensitive to high-frequency vibration error of motion platform. Therefore, this paper proposes a novel high-frequency vibration error estimation and compensation algorithm for THz-SAR imaging based on local fractional Fourier transform (LFrFT). Firstly, the high-frequency vibration error of the motion platform is modeled as a simple harmonic motion and THz-SAR echo signal received in each range pixel can be considered as a sinusoidal frequency modulation (SFM) signal. A novel algorithm for the parameter estimation of the SFM signal based on LFrFT is proposed. The instantaneous chirp rate of the SFM signal is estimated by determining the matched order of LFrFT in a sliding small-time window and the vibration acceleration is obtained. Hence, the vibration frequency can be estimated by the spectrum analysis of estimated vibration acceleration. With the estimated vibration acceleration and vibration frequency, the SFM signal is reconstructed. Then, the corresponding THz-SAR imaging algorithm is proposed to estimate and compensate the phase error caused by the high-frequency vibration error of the motion platform and realize high-frequency vibration error estimation and compensation for THz-SAR imaging. Finally, the effectiveness of the novel algorithm proposed in this paper is demonstrated by simulation results.

To relieve the system cost in densely sampling when applying the millimeter-wave (MMW) synthetic aperture radar (SAR) technique for imaging applications, an MMW sparse imaging method is presented by the combination of the under-sampling scheme in space and the following sparse imaging algorithm. Different from the requirement on sub-wavelength sampling interval by Nyquist law, the system only needs to collect a random subset of spatial samples. To transform the spatial samples into wavenumber domain, a sparse range migration algorithm (SRMA) is proposed to realize MMW sparse imaging by embedding the matrix completion (MC) technique into classical RMA. In light of the relationship of Fourier transform between the scattering coefficients and the wavenumber-domain echo, the proposed algorithm utilizes the atomic norm minimization to fulfill the MC from the small set of wavenumber-domain entries to their full data matrix. The experiments are conducted by a wideband MMW transceiver mounted on a linear trajectory for imaging two specimens with different kinds of cover. Their clear imaging contrast of azimuth-range profiles is obtained through SRMA under different under-sampling conditions and RMA with fully sampled data, and verify the effectiveness of the proposed method.

To avoid the difficult transceiver-isolation problem of a monostatic synthetic aperture radar (SAR) for terahertz (THz) near-field azimuth-range imaging, a compact bistatic SAR geometry is demonstrated in this paper. The system consists of one fixed transmitter, one moving echo receiver and one fixed direct-wave receiver. The transmitter emits wideband signals for range resolution, and the echo receiver moves along a designed trajectory for the cross-range resolution, and the direct-wave receiver is taken as a reference for synchronization. Since the wavelength of THz is in the order of millimeter or submillimeter, the requirement on subwavelength interval of spatial sampling by the Nyquist theory aggravates the measurement difficulty. To break this limitation, a compressed sensing-based imaging algorithm is presented. Allowing the undersampling both in spatial domain and frequency domain, the proposed algorithm can provide high-resolution performance with few measurements. Thus, the balance between the resolution and the amount of measurements can be made. The system geometry and imaging performance are then demonstrated by both the simulations and the experiments in the 0.178-0.188 THz band.

Active controlling the phase of terahertz wave has paid promising attentions. Here, we present a complete theoretical treatment by using coupled-mode theory to active control the reflective wave phase in attenuated total reflection based metasurface. Controlling is achieved by variations in the air gap, enabling the radiative loss (radiative Q factor) to be continuously adjusted over a narrowband frequency range. A singularity can be found in the phase slope spectrum when the radiation loss is equal to the intrinsic loss determined by material properties. The theoretical results have been verified by simulation and the experimental results. The physical mechanism of this phase transition in attenuated total reflection configuration would pave the way for potential applications such as polarization modulation, and guide further researches for terahertz physics.