With the development of 5G and other technologies, radar imaging techniques in high-frequency bands up to the terahertz (THz) range have attracted considerable attention. In particular, the photonic radar technique benefits from the unique advantages of optical devices, including an ultrawide and flexible bandwidth of more than 100 GHz, leading to submm-order range resolution, reduced phase noise, and flexible optical links. This article reviews recent progress in photonic radar systems from the millimeter wave to THz bands, including the hardware components, imaging technology, and radar imaging applications, and discusses the potential high-resolution imaging techniques enabled by photonic radar systems.

Terahertz (THz) imaging techniques have attracted significant attention and have developed rapidly in recent years. However, despite several advances, these techniques are still not mature, and their high cost and system complexity continue to limit their applications. In this article, the techniques for achieving a practical imaging system with a compact THz transceiver are addressed, while considering the limitations of the current technique. The aim is to provide a brief review of related topics, while also covering our recent progress, which can provide some general perspectives and contrasting approaches for realizing a practical THz imaging system. The continuous wave devices are mainly focused for their flexibility of balancing the imaging resolution and data acquisition time. The importance of transceiver integration is also discussed and illustrated by introducing a 600-GHz band micro-photonic interface for integrating a THz source and detector, with a single resonant tunneling diode as a transceiver. With regard to system issues, spatial sampling with mechanical beam-scanning is discussed as an intermediate approach for moving stage and array technology. The potential and limitations of this approach are evaluated, along with an elliptical reflector as an alternative to an f-theta lens owing to its low cost and simplicity. The combination of integrated devices, along with the mechanical beam-scanning, is also discussed for demonstrating our current concept of realizing a practical THz imaging system.

Three-dimensional (3D) imaging remains an expensive and challenging task in the THz band. In this study, a frequency-modulated continuous-wave photonic radar system was presented in the 300-GHz band, instead of using electronic-based devices. The proposed system can obtain a 6-dB bandwidth of 120 GHz to achieve a range resolution of ∼1.1 mm. The frequency sweep linearity of a laser source was calibrated with respect to the imaging distance and range resolution, through which a maximum detectable distance of ∼800 mm was achieved with the collimated beam. To obtain the desired 3D information, the synthetic aperture radar (SAR) technique was introduced to the proposed system to achieve 3D imaging, which was not bounded by the fixed focal distance of a focus lens. It can provide a spatial resolution of ∼1.5 mm within a 3D imaging range up to ∼300 mm. The potential and limitations of SAR imaging schemes are discussed based on the theoretical and experimental results.

We developed a silicon (Si) dielectric diplexer module for 600-GHz-band frequency-division multiplexing wireless communication. This diplexer was developed based on Si dielectric waveguide technology, which provides low-loss transmission characteristics compared with the conventional metallic terahertz-transmission platform, such as metallic hollow waveguides. It includes straight waveguides based on an effective medium composed of Si and air, and a coupler comprising of two adjacent unclad Si waveguides. Furthermore, for the communication system integration, we developed a practical and low-insertion loss module with WR1.5 metallic hollow waveguide input–output interfaces and modularized the diplexer. We performed transmission measurement experiments with the fabricated diplexer module and achieved 3-dB bandwidths of 101 and 37 GHz, and crosstalk of $-$ 34 dB at 680 GHz and $-$ 41 dB at 618 GHz in the cross and bar directions, respectively. Finally, we performed a two-channel wireless communication experiment using the developed diplexer module in the 600-GHz band. We achieved error-free communication at 10 and 6 Gbit/s on the cross and bar channels, respectively. In addition, we achieved a data rate of up to 12.5 Gbit/s at the forward error correction limits for both channels.

In this paper, we propose a novel processing model for compressive sensing (CS)-based stepped-frequency continuous-wave (SFCW) radar near range imaging, which takes the azimuth dependence of the reflection coefficients of targets into consideration. Based on the block sparse property of the received signal in the defined dictionary, 2-D images of the targets can be obtained at each spatial sampling point. A cross-correlation method is then employed to fuse these 2-D images to obtain the final result. Random undersamplings of frequencies and spatial sampling points are conducted to reduce the data acquisition time, the data size, and the computational complexity. Experimental results of an SFCW-based MIMO radar and a ground-based SAR system show that, compared with the conventional matched filtering-based methods, the proposed method can provide artifacts-reduced higher resolution images by using reduced frequencies and spatial sampling points. We also demonstrate that, compared to the conventional CS-based methods, due to the more suitable established observation model, the proposed method can achieve better imaging results with fewer artifacts for near range targets.