An optical true time delay (TTD) unit capable of adding independent time delays to multiple RF signals is proposed, which can be used for multi-beamforming in both transmit and receive modes. In the proposed unit, N RF signals with different center frequencies are modulated on an optical frequency comb (OFC). After transmission through a dispersive element, the RF-modulated OFC is split into N paths. In each path, a comb line is selected by a tunable optical filter. Thanks to the chromatic dispersion of the dispersive element, independently-controllable TTDs can be obtained in all paths. Then, a microwave photonic filter (MPF) is incorporated in each path, allowing a designated RF signal to undergo the TTD in that path. A proof-of-concept experiment is carried out. A two-path unit with a low-pass MPF in one path and a high-pass MPF in the other path is built. Controllable TTDs up to ~1.4 ns with a step of ~69 ps are demonstrated based on a 25-GHz-spacing OFC. In addition, a wideband multi-beam phased-array antenna system that can work in both transmit and receive modes is designed using the proposed TTD unit.

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Radar is the only sensor that can realize target imaging at all time and all weather, which would be a key technical enabler for future intelligent society. Poor resolution and large size are the two critical issues for radar to gain ground in civil applications. Conventional electronic radars are difficult to address due to both issues, especially in the Ka band or lower. In this work, a chip-based microwave-photonic radar based on silicon photonic platform, which can implement high-resolution imaging with very small footprint, is proposed and experimentally demonstrated. Both the wideband signal generator and the de-chirp receiver are integrated on the chip. A broadband microwave-photonic imaging radar occupying the full Ku band is experimentally established. A high-precision range measurement with a resolution of 2.7 cm and an error of less than 2.75 mm is obtained. Inverse synthetic aperture imaging of multiple targets with complex profiles is also implemented.

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A photonics-based radar for high-resolution 3D microwave imaging is proposed and demonstrated, in which high range resolution is achieved by employing a wideband linearly frequency modulated signal, and high cross-range resolution in both azimuth and elevation is realized by rotating the target around two orthogonal axes to compose a 2D inverse synthetic aperture. The proposed radar performs photonics-based in-phase/quadrature up-conversion and de-chirp processing in the transmitter and receiver, respectively, which features a compact structure with low-sampling-rate electronics in the receiver. An experiment is carried out. The established radar works in the K-band, with bandwidth as large as 8 GHz. Captured echo signals at different rotation angles of the target are processed to a 3D image, in which the voxel values are scattering intensities at different spatial positions. INDEX TERMS Radar, inverse synthetic aperture radar, microwave photonics, 3D imaging, de-chirp processing.

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