微波光子雷达利用光子学方法实现雷达信号的产生与处理，具有突出的宽带工作能力，能显著提升雷达距离分辨率。为了提升雷达角度分辨能力并实现灵活波束控制，将微波光子雷达技术与阵列技术相结合是必然的发展趋势。目前研究较多的宽带阵列雷达采用光真延时技术克服宽带波束倾斜问题，通常面临复杂度高、灵活性差、延时精度有限等问题。近年来，基于微波光子倍频与去斜接收的宽带雷达收发架构得到了广泛关注，基于此技术构建的阵列雷达，在实现宽带工作的同时具备实时数字补偿与处理功能，为宽带阵列雷达的发展提供了新的思路。文中针对作者在此方面的最新研究进展进行了综述，在阐明基于微波光子倍频与去斜接收实现宽带雷达收发机理的基础上，介绍了构建宽带相控阵雷达的方法以及实现数字波束扫描与成像的性能。然后，将阵列形式扩展至多输入多输出（MIMO）形式，介绍了基于光波分复用技术实现宽带微波光子MIMO雷达的方法，并分析了微波光子MIMO雷达在目标探测与成像方面的性能。

We propose a photonics-assisted equivalent frequency sampling (EFS) method to analyze the instantaneous frequency of broadband linearly frequency modulated (LFM) microwave signals. The proposed EFS method is implemented by a photonic scanning receiver, which is operated with a frequency scanning rate slightly different from the repetition rate of the LFM signals. Compared with the broadband LFM signal analysis based on temporal sampling, the proposed method avoids the use of high-speed analog to digital converters, and the instantaneous frequency acquisition realized by frequency-to-time mapping is also simplified since real-time Fourier transformation is not required. Feasibility of the proposed method is verified through an experiment, in which frequency analysis of Kα-band LFM signals with a bandwidth up to 3 GHz is demonstrated with a moderate sampling rate of 100 MSa/s. The proposed method is highly demanded for analyzing the instantaneous frequency of broadband LFM signals used in radar and electronic warfare systems.

We propose a photonic-assisted single system for measuring the frequency and phase noise of microwave signals in a large spectral range. Both the frequency and phase noise to be measured are extracted from the phase difference between the signal under testing and its replica delayed by a span of fiber and a variable optical delay line (VODL). The system calibration, frequency measurement, and phase noise measurement are performed by adjusting the VODL at different working modes. Accurate frequency and phase noise measurement for microwave signals in a large frequency range from 5 to 50 GHz is experimentally demonstrated.

A switchable microwave photonic filter (MPF) using a phase modulator (PM) and a silicon-on-insulator micro-ring resonator (MRR) is proposed and demonstrated. By adjusting the polarization controller between the PM and the MRR, the filtering function of the MPF can be switched between a band-stop filter and a band-pass filter. In a proof-of-concept experiment, an MPF with a rejection ratio of 30 dB (or 15 dB) for the band-stop (or band-pass) response and a frequency tuning range from 9.6 to 20.5 GHz is achieved.

Photonics-based radar with a photonic de-chirp receiver has the advantages of broadband operation and real-time signal processing, but it suffers from interference from image frequencies and other undesired frequency-mixing components, due to single-channel real-valued photonic frequency mixing. In this paper, we propose a photonics-based radar with a photonic frequency-doubling transmitter and a balanced in-phase and quadrature (I/Q) de-chirp receiver. This radar transmits broadband linearly frequency-modulated signals generated by photonic frequency doubling and performs I/Q de-chirping of the radar echoes based on a balanced photonic I/Q frequency mixer, which is realized by applying a 90° optical hybrid followed by balanced photodetectors. The proposed radar has a high range resolution because of the large operation bandwidth and achieves interference-free detection by suppressing the image frequencies and other undesired frequency-mixing components. In the experiment, a photonics-based K-band radar with a bandwidth of 8 GHz is demonstrated. The balanced I/Q de-chirping receiver achieves an image-rejection ratio of over 30 dB and successfully eliminates the interference due to the baseband envelope and the frequency mixing between radar echoes of different targets. In addition, the desired de-chirped signal power is also enhanced with balanced detection. Based on the established photonics-based radar, inverse synthetic aperture radar imaging is also implemented, through which the advantages of the proposed radar are verified.

Real-time and high-resolution imaging is demonstrated based on field trial detection of a non-cooperative target using a photonics-based inverse synthetic aperture radar (ISAR). By photonic generation and de-chirping of broadband linear frequency modulation signals, the radar can achieve a high range resolution thanks to the large instantaneous bandwidth (8 GHz at the K band), as well as real-time ISAR imaging using low-speed analog-to-digital conversion (25 MSa/s). A small-size unmanned aerial vehicle is employed as the non-cooperative target, and ISAR imaging is realized with a resolution far better than those achieved by the previously reported photonics-based ISARs. The capability for real-time ISAR imaging is also verified with an imaging frame rate of 25 fps. These results validate that the photonics-based radar is feasible in practical real-time and high-resolution ISAR imaging applications.

An approach to implementing optical single sideband (OSSB) polarization modulation, which is a combination of two orthogonally polarized OSSB modulations with complementary phase differences between the optical carrier and the sideband, is demonstrated based on two cascaded polarization modulators (PolMs). The two PolMs are driven by two RF signals that are 90° out of phase. By properly adjusting the polarization state between the two PolMs, OSSB polarization modulation with large operation bandwidth can be realized. An experiment is performed. OSSB polarization modulation with an operation bandwidth from 2 to 35 GHz is successfully demonstrated. The spectral profile of the OSSB polarization-modulated signal is observed through an optical spectrum analyzer, and its complementary phase properties are analyzed by sending the signal to a photodetector (PD) for square-law detection. Due to the complementary phase differences between the optical carrier and the sideband along the two polarization directions, no microwave frequency component is generated after the PD. The generated OSSB polarization-modulated signal is transmitted through 25 and 50 km single-mode fiber with 50 Mbaud 16 quadrature amplitude modulation baseband data to investigate the transmission performance of the proposed system in radio-over-fiber applications, and very small error vector magnitude degradation is observed. OSSB polarization modulation is also employed to realize a microwave photonic phase shifter. A full-range tunable phase shift is obtained for 2 and 35 GHz microwave signals.

A novel scheme for the generation of background-free pulsed microwave signals is proposed and experimentally demonstrated based on spectral shaping, frequency-to-time mapping, and balanced photodetection. In the proposed scheme, the optical spectral shaper, which consists of a differential group delay (DGD) element, two polarization controllers, and a polarization beam splitter, has two outputs with complementary power transfer functions. By passing a short optical pulse through the spectral shaper and a dispersive element (DE), a pulsed microwave signal is obtained after balanced photodetection. Thanks to the balanced photodetection, the lowfrequency components (i.e., the background signal) in the electrical spectrum is suppressed, leading to the generation of a background-free pulsed microwave signal. Meanwhile, the spectral power of the obtained microwave signal is enhanced compared to that obtained by single-end detection. Experimental results for the generation of a pulsed microwave signal centered at 12.46 GHz show that the background signal can be suppressed by more than 30 dB, and the spectral power is increased by 5.5 dB. In addition, the central frequency of the obtained background-free pulsed microwave signal can be tuned by changing the DGD introduced by the DGD element, and/or by changing the dispersion of the DE.