IEEE JSTQE 28, 6000310 (2022)

Automatic driving, intelligent manufacturing, and smart city demand high-resolution sensors. Optical cameras and lidars would not operate properly when working in severe weather, such as rain, snow, and haze. Microwave radars can work in all-weather and all-day conditions but are challenging to acquire images of the target with high resolution, which is vital for high-accuracy target recognition. To address this problem, researchers from the Microwave Photonics Research Laboratory of Nanjing University of Aeronautics and Astronautics (NUAA) in China developed a new kind of radar, i.e., photonics-based radar (IEEE JSTQE 28, 6000310 (2022)), which can remarkably improve the radar imaging resolution.

The spatial resolution of radar, including the range resolution along the radar line-of-sight direction and the angular resolution along the perpendicular direction, determines the ability to distinguish different targets. To achieve a high range resolution, the radar should work with large bandwidth, which is challenging for state-of-the-art electrical radars because the performance of electrical devices degrades as the bandwidth increases. To realize a high angular resolution, the radar needs a large antenna size. However, when the radar volume is the primary concern, improving radar angular resolution by enlarging the antenna is not feasible.

Photonics-based radar uses photonic techniques to generate and process radar signals. Compared with conventional radars, a photonics-based radar expands its bandwidth by leveraging the broadband operation capability of optoelectronic devices. For the proposed photonics-based radar, nonlinear electro-optical modulation is applied to implement microwave frequency multiplication, through which high-frequency and broadband frequency-modulated radar signals are generated using low-speed electrical devices. Meanwhile, photonic frequency mixing is adopted to de-chirp the broadband radar echoes and thus transfer the target information to a low-frequency signal. This not only lowers the sampling speed of analog-to-digital converters but also enables real-time signal processing for fast construction of the target image. Thanks to the photonics-based radar processing, the radar bandwidth can be enlarged from lower than several gigahertz to tens of gigahertz. Correspondingly, the radar range resolution is improved from several decimeters to several centimeters or millimeters.

To improve the angular resolution, multiple-input-multiple-output (MIMO) technique is incorporated into the photonics-based radar. The photonics-based MIMO radar can form a large virtual aperture that is far beyond the real physical array. Supposing the photonics-based MIMO radar is composed of ten transmit antennas and ten receive antennas, its equivalent number of antenna elements reaches a hundred. This way, the radar aperture size is boosted, and the angular resolution is improved several times or dozens of times. To construct a broadband photonics-based MIMO radar, time-division multiplexing realized in the optical domain is particularly suitable to achieve high spectral efficiency and complete orthogonality between different channels.

As a new kind of radar, photonics-based radar combines photonic, microwave, and radar technologies. This photonics-based MIMO radar can have a broad operation bandwidth and a large aperture size, enabling radar imaging with resolutions far exceeding traditional electrical radars. Currently, photonics-based radar has attracted the attention of both academia and industry. It is attractive in various applications such as automatic driving, security check, environmental monitoring, etc. While, for practical applications of photonics-based radars, there is still a long way to go. The key to further developing the photonics-based radar is the photonic integration, which should aim to dramatically lower the cost, reduce the volume, and enhance the reliability.

The research was published in IEEE Journal of Selected Topics in Quantum Electronics (https://ieeexplore.ieee.org/document/9696366).