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.

This work reports the first realization of a combined radar&lidar system based on silicon-on-insulator photonic integrated circuit (PIC). The software-defined architecture comprises a frequency-flexible and simultaneous multi-band radar operation and an advanced lidar with coherent detection for range and velocity measurements. Both systems are implemented within a single chip, allowing a coherent radar and lidar parallel data acquisition in order to take advantage of their complementary characteristics. This feature potentializes the study of complex environments providing a more complete cognition of the target or scene under observation. Here, we present the first sets of results of the radar and lidar subsystems characterization without any auxiliary frontend that confirms the large frequency flexibility of the radar system and the great potentiality for high resolution range and velocity measurements of the lidar system. These results open the way to an innovative generation of transceivers in the field of microwave photonics, potentially outperforming current systems thanks to its multiband and multifunction capability owing to a unique hybrid radar&lidar coherent architecture. Moreover, by operating in the photonic domain, radar and lidar subsystems are able to share the same optical source and digital electronics unit, allowing for an ultimate reduction of size, weight, and power consumption, thus making the proposed architecture suitable for the most stringent applications.

We report of a silicon photonics (SiP) flat-top bandpass microwave photonic (MWP) filter with large out-of-band power rejection (OBPR) of up to more than 40 dB and exhibiting central frequency tunability over a range as large as 70 GHz, while still guaranteeing a minimum full-span OBPR larger than 25 dB. Furthermore, the RF bandpass transfer function can be widely configured without sacrificing the filter performance. In the present realization, a continuous bandwidth tuning from 5 to 10 GHz (corresponding to a 100% of bandwidth variation) is demonstrated. The operation is based on optical-to-RF mapping the response of a tunable and bandwidth reconfigurable photonic integrated optical filter, realized with a high-order distributed feedback resonator (DFBR) waveguide Bragg grating structure. A high level of functional integration is provided in a compact footprint of less than 2 mm ² by embedding within the circuit a SiP phase modulator generating the modulation sideband scanning the DFBR filter, and a tunable optical splitter optimizing the MWP filter performance. Possibility of further extending system integration for monolithic on-chip operation is also investigated.

An integrated silicon-on-insulator microwave photonic phase shifter is demonstrated, based on an optical deinterleaver providing box-like transfer function and a reverse-biased pn-junction waveguide optical phase shifter. The photonic integrated circuit is proved to precisely control the phase of microwave signals in a range of more than 400° with a fast reconfiguration time of 1 ns, with a broad bandwidth of more than 6 GHz around an RF carrier flexibly selectable between 10 and 16 GHz, and limited in-band RF power variations. Moreover, thanks to the periodic behavior of the deinterleaver, the device is demonstrated to correctly control the phase of signals in the mm-wave band (> 30 GHz) as well. The performance guaranteed by the proposed microwave phase shifter suggests its use in demanding applications as rapidly reconfiguring phased array antennas in future wireless communications systems.

The evolution of microwave and mm-wave systems for communication and remote sensing is heading towards a massive deployment of phased-array antennas with beamforming capabilities. 5G wireless access networks are expected to employ beamforming to optimize resources allocation, whereas radars will benefit from precise and fast-switching antennas. Moreover, in the next future, those systems are expected to widely employ photonic technologies for signal generation, acquisition, and distribution. In this scenario, photonics can allow for the development of high-performance beamforming networks, which can easily be integrated in photonics-assisted RF systems, combining broad operational bandwidth, low signal distortion, and high pointing precision with unprecedented flexibility. In this paper, a phase shift-based beamforming network for microwave applications is presented and its performance analyzed. The Si-photonics integrated phase shifters exhibit large phase shift (> 360°) and limited amplitude fluctuations (~ 2.5 dB). The packaged beamforming network elements show extremely low signal distortion, low steering error (<3°), and fast switching time (< 5 ns), demonstrated by the transmission with beam-steering of a 2-Gb/s communication signal and of a 1-GHz chirped radar waveform. A comparison with commercial RF devices shows that the photonics-assisted beamforming network is well-suited for high bit-rate 5G communications and for wideband radar applications.

In the last years, beamforming has become a hot research topic, due to the perspective of its massive deployment in the RF access segment of future 5G mobile networks. This paper is a review of some possible implementations reported in literature of photonic integrated circuits for the realization of phase shift-based beamforming networks for radiofrequency signals, suitable for future 5G communications. The results demonstrate the possibility of ultrafast beamsteering reconfiguration (~1 ns), large applied phase shift (> 400°), stable operation at high carrier frequencies (tens of GHz) over a wide signal bandwidth (~6 GHz), and low power consumption (tens of mW).