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 ultra-wide bands, the use of distributed sensors for multiple-input multiple-output detections, and the use of an ultra-wide band photonics-based receiver for radio frequency (RF) 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.

The cross-fertilization between photonics and microwave systems is setting new paradigms in radio technologies, promising improved performance and new applications, with strong benefits for communication systems broadly as well as for public safety specifically. In particular, the use of photonics in radar systems is leading to a new generation of multifunctional systems that can manage multiple simultaneous coherent radio signals at different frequencies, thus enabling multispectral imaging for advanced surveillance. In fact, thanks to its tenability and huge bandwidth, photonics matches the flexibility requirements urgently needed in future software-defined radar architectures. It also guarantees system compactness because a single shared transceiver can be used for multiband operations with the added potential of photonic integration.

This paper extends a recent demonstration of a hierarchical spatial division network architecture testbed for petabit per second throughput. A spatial cross-connect is implemented to handle 1.08 Pb/s per line side. The spatial crossconnect is based on a low loss 44x44 MEMS switch, which enables switching up to 22 spatial channels, each carrying 202-channel, 24.5 GBaud polarization-multiplexed 64-ary quadrature amplitude modulated signals. Transmission is performed using a 22-core homogeneous multicore fiber. The demonstration includes spatial bypass and spatial add and drop scenarios as well as optical bypass and optical add and drop scenarios. The latter uses wavelength cross-connects implemented using conventional wavelength selective switches. A network scenario implementing 1+1 spatial multiplex section protection is also demonstrated using three 7-core joint spatial switches to protect 21 spatial channels. Finally, the use of hybrid spatial division multiplexing networks with multi-core and few-mode fibers is addressed, showing that skew between spatial channels may be the most significant obstacle to the implementation of such networks. This work concludes with an analysis of the inter-core skew introduced by the 22-core multicore fiber and its impact on a potential implementation of a hierarchical spatial division network.

We study the phase and amplitude stability of RF signals, generated at different frequencies by heterodyning pairs of modes from the optical spectrum of an active mode-locked laser, and we focus on the dependence of noise on the RF frequency. A specific theoretical model is derived, and the amplitude and timing jitter behaviour of the RF signals are analyzed and experimentally validated. The timing jitter reveals to be constant at any RF generated frequency, making the considered RF generation method suitable for the realization of flexible and highly stable micro- and millimeter-wave coherent radar transceivers. The performance of the considered RF generation architecture are compared with a state-of-the-art RF synthesizer, proving that the optically generated RF signals meet the high stability requirements of the new generation of coherent radar systems, even at extremely high frequencies (EHF).

The Internet-of-Things paradigm is pushing the current fully-automated industry to Industry 4.0, laying on a high degree of end-to-end digitization of all physical assets and their integration into data-based services. A significant contribution to recent advances of digital technologies comes from microwave photonics, whose benefits for RF systems in terms of performance and flexibility have been largely demonstrated. In the radar domain, the high degree of coherence granted by photonics makes possible multiple input-multiple output systems, enabling an enhancement of surveillance and monitoring capabilities, thanks to the possibility of performing centralized processing and raw-data fusion. The advantages offered by photonics for surveillance converge with the Industry 4.0 revolution, helping in the actualization of the “Port of the Future” paradigm, to monitor the freight traffic inside ports, increasing goods distribution efficiency, at the same time reducing carbon footprint. Here, we present the first photonics-based widely distributed multiple input-multiple output dual-band radar network, designed to operate in a real port, showing the achieved enhanced potential detection and imaging capabilities. The integration of the targets radar tracks within the monitoring and control platform of the port is reported, enabling the fusion with other data managed by the multiservice monitoring platform.