This paper reviews on the latest advances in microwave photonics applied to radar systems, tracing the evolutions of functionalities in the photonic radar toward a new generation of enhanced-performance systems. Photonics is demonstrated to enable a new generation of miniaturized, heterogeneous, and distributed radars, i.e., future radars on chip with different features, working in different radio spectral regions, and organized in spatially distributed sensors for the enhanced detection of a wider range of target properties. In these systems, the use of photonics assures benefits in terms of frequency flexibility, accuracy, and computational load reduction. Innovative capabilities supported by the photonic approach are presented, such as the use of coherent sparse bands for the synthesis of ultrawide bands, the use of distributed sensors for multiple-input multiple-output detections, and the use of an ultrawide band photonics-based receiver for radio frequency spectrum scanning. The presented novel concepts will open the way to new research activities both in photonic technology and radar systems, contributing to the development of a new generation of remote sensing systems. This expanding cross fertilization will lead to exciting and challenging research activities in the years to come.

Toward a New Generation of Radar Systems Based on Microwave Photonic Technologies

Cognitive radio is considered as a possible disruptive force to improve the spectral resource efficiency through sensing and interacting with the environment. Traditional electrical technologies to implement the cognitive radio face challenges in terms of bandwidth, resolution, and speed. In this paper, the concept and architecture of broadband cognitive radio systems enabled by photonics are proposed. Key microwave photonic techniques for the architecture are reviewed, including the photonics-based spectrum sensing, the photonic arbitrary waveform generation, the photonics-based self-interference cancellation processing, and the microwave photonic dechirp processing and radar imaging. A preliminary demonstration of a cognitive radar system enabled by photonics is performed. The future possible research directions on this topic are discussed.

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Broadband Cognitive Radio Enabled by Photonics

Recent developments in photonics-based microwave image-reject mixers (IRMs) are reviewed with an emphasis on the pre-filtering method, which applies an optical or electrical filter to remove the undesired image, and the phase cancellation method, which is realized by introducing an additional phase to the converted image and cancelling it through coherent combination without phase shift. Applications of photonics-based microwave IRM in electronic warfare, radar systems and satellite payloads are described. The inherent challenges of implementing photonics-based microwave IRM to meet specific requirements of the radio frequency (RF) system are discussed. Developmental trends of the photonics-based microwave IRM are also discussed.

Photonics-Based Microwave Image-Reject Mixer

Electronic warfare receivers must satisfy particularly demanding requirements, and current electronic solutions must find a tradeoff between the request for high performance (in particular, very wide bandwidth, very high sensitivity, and very linear response) and the need for light and small systems. Photonics has been often proposed as a potential enabler of performance improvements, thanks to its wide tunability and precision, and recently the advancements of the photonic integration technologies are also promising strong reductions of size and weight. In this paper, we present an overview of the photonics-based solutions proposed so far in the field of electronic warfare. In particular, we focus on the specific implementation of a spectrum scanner we have recently proposed. Besides the performance in terms of bandwidth, linearity, and sensitivity, which are on par with the state-of-the-art electronic commercial systems, here, we report on our last advancements in terms of tuning speed. Further improvements are also discussed, in terms of both tuning speed and integrability.

Photonics for Ultrawideband RF Spectral Analysis in Electronic Warfare Applications

Conventional image-reject mixing based on Hartley and Weaver architectures cannot deal with the mixing spurs generated by the nonlinearity of the mixer, leading to a fundamental restriction on the instantaneous bandwidth and frequency tuning range. This paper proposes and demonstrates a new architecture for broadband image-reject mixing enabled uniquely by photonics. The RF signal and the electrical LO signal are converted into the optical domain and are then launched into the signal and LO ports of a 90-degree optical hybrid, respectively. The optical hybrid coupler introduces 0, π, π/2 and 3π/2 phase shifts to the input signals, so a new dimension for additional balanced detection is enabled, which can dramatically remove the undesirable mixing spurs and the common-mode noises. As a result, the proposed architecture suppresses the downconverted image and the nonlinear mixing spurs simultaneously, enabling a truly wideband microwave frequency mixer. A theoretical and experimental investigation is performed. More than 60-dB image rejection and mixing spur suppression is achieved over a 40-GHz working frequency range.

Photonics-enabled balanced Hartley architecture for broadband image-reject microwave mixing.

An innovative and effective photonics-based radio frequency (RF) spectrum scanner is presented and demonstrated, enabling the detection in the 0.5–28.5 GHz range. The proposed receiver scans the whole frequency range at discrete steps, down-converting at baseband single portions of the detected spectrum, where they can be precisely acquired, without using RF bandpass filters nor optical filters. The wideband scansion is achieved by simply tuning an optical carrier only, avoiding the use of an RF synthesizer or of multiple parallel local oscillators. In more detail, the optical carrier is modulated by the detected RF signal through an electro-optical modulator, and photomixed with an optical local oscillator to down-convert the modulation sideband to baseband. The optical carrier also feeds an optical frequency comb generator—driven by a fixed RF oscillator at 1 GHz—whose output injects the local oscillator. This way, the optical carrier and oscillator are phase locked and can be tuned at steps of 1 GHz. The developed demonstrator achieves an input range up to 28.5 GHz, and a simulative analysis suggests the capability of exceeding the 0.5–40 GHz range employing commercially available devices. Moreover, it shows a linear dynamic range of 160 dB/Hz and a spuriousfree dynamic range of 109 dB/Hz2/3 , aligned with the state-of-the-art broadband RF receivers. The proposed solution appears to fulfill the performance requirements of the demanding electronic support measures applications, with the potentials for largely reducing the receiver size.

A Photonically Enabled Compact 0.5–28.5 GHz RF Scanning Receiver

Disclosed herein is a photonic-assisted radio frequency spectrum scanning device for use in a receiver (100), including: a first optical waveguide arm (110) comprising, in cascade, an input end, a first electro-optical modulator (111), a tunable optical filter (112) and an output end, wherein said first electro-optical modulator (111) is designed to be connected to an antenna to receive therefrom an incoming radio frequency signal; a second optical waveguide arm (120) comprising, in cascade, an input end, a second electro-optical modulator (121), an optical delay line (122) and an output end; a mode-locked laser (101) connected, through an optical splitter (102), to the input ends of the first and second optical waveguide arms (110,120) to supply the latter with optical pulses; an optical hybrid coupler (105) connected to the output ends of the first and second optical waveguide arms (110,120) and operable to combine optical signals received from the latter to produce corresponding output optical signals; and photodetection means (131,132) connected to the optical hybrid coupler (105) to receive the output optical signals and configured to convert the latter into corresponding baseband electrical analog signals. The first electro- optical modulator (111) is configured to modulate the optical pulses supplied by the mode-locked laser (101) by means of the incoming radio frequency signal so as to carry out an optical sampling of the latter, whereby a modulated optical signal is produced, which is indicative of said optical sampling. The tunable optical filter (112) is operable to filter the modulated optical signal so as to select a portion of spectrum of the latter. The second electro-optical modulator (121) is operable to decimate the optical pulses supplied by the mode-locked laser (101). The optical delay line (122) is operable to delay the decimated optical pulses.

Photonic-assisted rf spectrum scanner for ultra-wide band receivers

We present the design and simulative results of an innovative RF receiver based on photonic techniques that quickly scans the RF band of interest in warfare applications, avoiding the spectrum channelization and thus reducing the receiver SWaP. The scheme is based on a stable mode-locked laser and a specifically designed tunable optical filter, and it simultaneously samples, filters, and down-converts the detected spectrum. The analysis confirms the scheme capability for detecting signals beyond 18GHz, with instantaneous bandwidth >1GHz and dynamic range >40dB. The scheme implementation with integrated photonics technologies will ensure fast scanning (filter repositioning in <100ns), high environmental stability, and reduced SWaP, meeting the increasingly stringent requirements of electronic warfare applications.

An RF scanning receiver based on photonics for electronic warfare applications

This paper presents an innovative photonics-based ultra wideband RF receiver for spectrum sensing applications. The scheme scans the RF spectrum at discrete steps, down-converting at baseband single portions of the detected spectrum, where they can be precisely acquired. The down-conversion is performed modulating a tunable optical carrier with the RF signal, and beating it with an optical local oscillator. The selection of the desired spectrum portion is achieved tuning the wavelength of the optical carrier, avoiding tunable optical filters and allowing a fast scanning of the detected spectrum. The phase locking between the carrier and the local oscillator by means of a coherent optical comb generator ensures very low phase noise. An experimental validation shows an RF input range of 0.5÷9.5 GHz, with high sensitivity and dynamic range. Increasing the number of comb lines by enhancing the comb generation, the architecture has the potential to detect RF signals up to 100GHz. Moreover, implementing the scheme through integrated photonics technologies will ensure high environmental stability and reduced size weight and power consumption, meeting the increasingly stringent requirements of RF spectrum sensing applications.

A photonics-based ultra wideband scanning RF receiver with high sensitivity and dynamic range

A demonstrator of DC offset-free direct conversion receiver is presented. The innovative architecture exploits optical domain up-conversion and photonic I/Q demodulation to perform a wideband scanning of the RF input spectrum avoiding bulky multiple crystal oscillators and bandpass filters. Moreover, the scheme provides a total immunity to local oscillator self-mixing and to RF-to-baseband feedthrough. The demonstrator achieves a measured DC offset lower than 50 μν and a second-order input-intercept point of 57 dBm. It also shows an operating and instantaneous bandwidth of 0.5÷10.5 GHz and 1 GHz, respectively, with linear dynamic range of 160 dB/Hz and spurious-free dynamic range of 109 dB/Hz2/3, outperforming current pure electronic direct conversion receivers. Further improvements are proposed to enhance the operating bandwidth and to reduce the size and power consumption, making the proposed direct conversion receiver suitable for practical electronic warfare applications.

A. Albertoni

Papers

A Photonically Enabled Compact 0.5–28.5 GHz RF Scanning Receiver

Photonic-assisted rf spectrum scanner for ultra-wide band receivers

An RF scanning receiver based on photonics for electronic warfare applications

A photonics-based ultra wideband scanning RF receiver with high sensitivity and dynamic range

A DC offset-free ultra-wideband direct conversion receiver based on photonics