As the only method for all-weather, all-time and long-distance target detection and recognition, radar has been intensively studied since it was invented, and is considered as an essential sensor for future intelligent society. In the past few decades, great efforts were devoted to improving radar's functionality, precision, and response time, of which the key is to generate, control and process a wideband signal with high speed. Thanks to the broad bandwidth, flat response, low loss transmission, multidimensional multiplexing, ultrafast analog signal processing and electromagnetic interference immunity provided by modern photonics, implementation of the radar in the optical domain can achieve better performance in terms of resolution, coverage, and speed which would be difficult (if not impossible) to implement using traditional, even state-of-the-art electronics. In this tutorial, we overview the distinct features of microwave photonics and some key microwave photonic technologies that are currently known to be attractive for radars. System architectures and their performance that may interest the radar society are emphasized. Emerging technologies in this area and possible future research directions are discussed.

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Microwave Photonic Radars

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.

Photonics-based radar with balanced I/Q de-chirping for interference-suppressed high-resolution detection and imaging

Photonics-based microwave frequency mixing provides distinct features in terms of wide frequency coverage, broad instantaneous bandwidth, small frequency-dependent loss, and immunity to electromagnetic interference as compared with its electronic counterpart, which can be a key technical enabler for future broadband and multifunctional RF systems. Herein, all-optical and optoelectronic microwave frequency mixing techniques are reviewed, with an emphasis on the latest advances in photonics-based microwave frequency mixers with improved performance in terms of conversion efficiency, dynamic range, mixing-spur suppression, mixing functionality, and polarization independence. Innovative applications enabled by photonics-based microwave frequency mixers, such as radio-over-fiber communication systems, radar systems, satellite payloads and electronic warfare systems, are also reviewed. In addition, efforts in implementing integrated photonics-based microwave mixers that lead to a dramatic reduction in size, weight, and power consumption are also reviewed.

Photonics-Based Microwave Frequency Mixing: Methodology and Applications

Microwave photonic radars enable fast or even real-time high-resolution imaging thanks to its broad bandwidth. Nevertheless, the frequency range of the radars usually overlaps with other existed radio-frequency (RF) applications, and only a centimeter-level imaging resolution has been reported, making them insufficient for civilian applications. Here, we propose a microwave photonic imaging radar with a sub-centimeter-level resolution by introducing a frequency-stepped chirp signal based on an optical frequency shifting loop. As compared with the conventional linear-frequency modulated (LFM) signal, the frequency-stepped chirp signal can bring the system excellent capability of anti-interference. In an experiment, a frequency-stepped chirp signal with a total bandwidth of 18.2 GHz (16.9 to 35.1 GHz) is generated. Postprocessing the radar echo, radar imaging with a two-dimensional imaging resolution of ∼8.5 mm × ∼8.3 mm is achieved. An auto-regressive algorithm is used to reconstruct the disturbed signal when a frequency interference exists, and the high-resolution imaging is sustained.

Microwave Photonic Imaging Radar With a Sub-Centimeter-Level Resolution

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.

Chip-Based Microwave-Photonic Radar for High-Resolution Imaging

The signal-to-noise ratio (SNR) of the photonic time-stretching receiver in the photonic time-stretch coherent radar (PTS-CR) system is theoretically analyzed. According to the analysis based on the erbium-doped fiber amplifier (EDFA) characteristic, it is found that the SNR is dominantly determined by the input optical power of the EDFA. With the improvement of the SNR of the photonic time-stretching receiver, the radar detection sensitivity is consequently enhanced. Furthermore, a PTS-CR system operating at W band with the ultrabroad bandwidth of 12 GHz is experimentally enabled, leading to the range resolution of ∼1.48  cm in dual-target detection.

Signal-to-noise ratio improvement of photonic time-stretch coherent radar enabling high-sensitivity ultrabroad W-band operation.

The electronic aperture jitter effect originating from the electronic digitization is investigated in the channel-interleaved photonic analog-to-digital converter (PADC) system. The influence of the electronic aperture jitter to the effective number of bits (ENOB) of the PADC system is extracted and evaluated. According to the theoretical analysis, the electronic aperture jitter can be significantly suppressed by the channel-interleaving scheme. In the experiment, the effect of electronic aperture jitter is measured under different optical-electronic conversion (OEC) bandwidths and channel numbers. It is eventually found that the condition of the OEC bandwidth equaling to the Nyquist frequency of one single channel is critical to optimize the electronic aperture jitter and ENOB.

Investigation of electronic aperture jitter effect in channel-interleaved photonic analog-to-digital converter.

We have previously demonstrated a photonic time-stretched coherent radar (PTS-CR) with central-frequency programmable and bandwidth-tailorable agility. The sampling bandwidth in the reception was essentially compressed by the photonic time-stretched process. However, in the PTS-CR, the reception aperture suffering dead zone was hardly tuned by the fiber delay line and the measurement range was restricted by the pulse repetition rate (PRR) of the mode-locked laser. Here, we report a novel scheme to overcome the restrictions. The reception aperture can be simply tuned by adjusting the center wavelength of an optical filter and the PRR is reduced for enlargement of the measurement range by use of a photonic switch in the transmitter. In the experiment, a tenfold enlargement of measurement range is demonstrated to identify two neighboring targets separated by ~4 cm.

Enlarged Range and Filter-Tuned Reception in Photonic Time-Stretched Microwave Radar

A scheme of high-resolution inverse synthetic aperture radar (ISAR) imaging based on photonic receiving is demonstrated. In the scheme, the linear frequency modulated (LFM) pulse echoes with 8 GHz bandwidth at the center frequency of 36 GHz are directly sampled with the photonic analog-to-digital converter (PADC). The ISAR images of complex targets can be constructed without detection range swath limitation due to the fidelity of the sampled results. The images of two pyramids demonstrate that the two-dimension (2D) resolution is 3.3 cm × 1.9 cm. Furthermore, the automatic target recognition (ATR) is employed based on the high-resolution experimental dataset under the assistance of deep learning. Despite of the small training dataset containing only 50 samples for each model, the ATR accuracy of three complex targets is still validated to be 95% on a test dataset with the equal number of samples.

High-resolution ISAR imaging based on photonic receiving for high-accuracy automatic target recognition.

We propose a convolutional recurrent autoencoder (CRAE) to compensate for time mismatches in a photonic analog-to-digital converter (PADC). In contrast of other neural networks, the proposed CRAE is generalized to untrained mismatches and untrained category of signals while remaining robust to system states. We train the CRAE using mismatched linear frequency modulated (LFM) signals with mismatches of 35 ps and 57 ps under one system state. It can effectively compensate for mismatches of both LFM and Costas frequency modulated signals with mismatches ranging from 35 ps to 137 ps under another system state. When the spur-free dynamic range (SFDR) of the unpowered PADC decreases from 10.2 dBc to -3.0 dBc, the SFDR of the CRAE-powered PADC is over 31.6 dBc.

Na Qian

Papers

Signal-to-noise ratio improvement of photonic time-stretch coherent radar enabling high-sensitivity ultrabroad W-band operation.

Investigation of electronic aperture jitter effect in channel-interleaved photonic analog-to-digital converter.

Enlarged Range and Filter-Tuned Reception in Photonic Time-Stretched Microwave Radar

High-resolution ISAR imaging based on photonic receiving for high-accuracy automatic target recognition.

Photonic analog-to-digital converter powered by a generalized and robust convolutional recurrent autoencoder