Francesco Laghezza

A fully photonics-based coherent radar system

The next generation of radar (radio detection and ranging) systems needs to be based on software-defined radio to adapt to variable environments, with higher carrier frequencies for smaller antennas and broadened bandwidth for increased resolution. Today’s digital microwave components (synthesizers and analogue-to-digital converters) suffer from limited bandwidth with high noise at increasing frequencies, so that fully digital radar systems can work up to only a few gigahertz, and noisy analogue up- and downconversions are necessary for higher frequencies. In contrast, photonics provide high precision and ultrawide bandwidth, allowing both the flexible generation of extremely stable radio-frequency signals with arbitrary waveforms up to millimetre waves, and the detection of such signals and their precise direct digitization without downconversion. Until now, the photonics-based generation and detection of radio-frequency signals have been studied separately and have not been tested in a radar system. Here we present the development and the field trial results of a fully photonics-based coherent radar demonstrator carried out within the project PHODIR. The proposed architecture exploits a single pulsed laser for generating tunable radar signals and receiving their echoes, avoiding radio-frequency up- and downconversion and guaranteeing both the software-defined approach and high resolution. Its performance exceeds state-of-the-art electronics at carrier frequencies above two gigahertz, and the detection of non-cooperating aeroplanes confirms the effectiveness and expected precision of the system.

Photonic Generation of Phase-Modulated RF Signals for Pulse Compression Techniques in Coherent Radars

A novel and flexible photonics-based scheme is proposed for generating phase-coded RF pulses suitable for coherent radar systems with pulse compression techniques. After selecting two modes from a mode-locked laser (MLL), the technique exploits an optical in-phase/quadrature modulator driven by a low-sample rate and low-noise direct digital synthesizer to modulate the phase of only one mode. The two laser modes are then heterodyned in a photodiode, and the RF pulse is properly filtered out. The scheme is experimentally validated implementing a 4-bit Barker code and a linear chirp on radar pulses with a carrier frequency of about 25 GHz, starting from an MLL at about 10 GHz. The measures of phase noise, amplitude- and phase-transients, and autocorrelation functions confirm the effectiveness of the scheme in producing compressed radar pulses without affecting the phase stability of the optically generated high-frequency carriers. An increase in the radar resolution from 150 to 37.5 m is calculated. The proposed scheme is capable of flexibly generating software-defined phase-modulated RF pulses with high stability, even at very high carrier frequency, using only a single commercial device with potentials for wideband modulation. It can therefore allow a new generation of high-resolution coherent radars with reduced complexity and cost.

Phase Coding of RF Pulses in Photonics-Aided Frequency-Agile Coherent Radar Systems

An innovative optical scheme to generate software-defined phase-modulated radio frequency (RF) pulses with carrier frequency agility from a mode-locked laser (MLL) is proposed. The technique exploits a direct digital synthesizer and a Mach-Zehnder modulator to apply an intermediate frequency modulation to the MLLs modes. The heterodyne detection of the optical signal allows the generation of amplitude- and phase-modulated RF carriers with very high phase stability, suitable for coherent radar applications. Further, a single MLL can be used to generate carriers simultaneously at different frequencies, enabling frequency hopping or multifunctional radars, with no need to increase the complexity of the transmitter. Results show chirped and Barker-coded pulses at around 10 or 40 GHz in a single setup, without any performance degradation while increasing the carrier frequency. The proposed technique allows the practical realization of compressed pulses for coherent radars over a wide carrier frequency range, allowing the development of software-defined radar systems with improved functionalities.

Jitter-limited photonic analog-to-digital converter with 7 effective bits for wideband radar applications

A photonic analog-to-digital converter exploiting a sample time-interleaving approach is presented, showing more than 7 effective bits for input signals up to 40GHz and bandwidth up to 200MHz. The effective 1-to-4 optical sample parallelization scheme does not contaminate the low sampling jitter with limited crosstalk. A real-time digital post-processing is also added to increase the ADC linearity and to minimize the spurious tones induced by the digital data interleaving. The results show that the scheme is fundamentally limited by the sampling jitter and it approaches the theoretical limits for the considered signal frequencies. Discussions are also reported, demonstrating that the proposed photonic ADC can be upgraded to accept signals with larger bandwidth without performance reductions.

Photonics-Assisted Multiband RF Transceiver for Wireless Communications

In this paper, the concept of a fully photonics-based multiband radio-frequency transceiver is presented. In the proposed architecture, the precision, wide bandwidth, and flexibility of the photonic technologies allow generating and detecting simultaneous multiple wireless signals in an extremely wide frequency range, up to the millimeter waveband. This approach is therefore promising for future wireless base stations that will need to handle huge data traffic distributed on several different transmission protocols, while minimizing the hardware components. In the proposed system, the capability of easily treating multiple signals simultaneously, and the architecture based on a single pulsed laser for both the transmitter and the receiver sections, permit to reduce the complexity of the transceiver while increasing its functionalities, with a potential benefit in terms of system costs. The paper details the principle of operation of the proposed solution, and describes the implementation of a dual-band wireless transceiver simultaneously working in the X and S bands. The results from the characterization of the transceiver and from its verification in a wireless transmission experiment confirm the potentials of the solution and set the basis for a new paradigm of RF transceivers.

Multi-Band Software-Defined Coherent Radar Based on a Single Photonic Transceiver

In this paper, a photonics-based architecture of a multi-band coherent radar system is proposed and validated. The precision and flexibility of photonic technologies are exploited for generating and detecting simultaneously multiple radar signals in an extremely wide frequency range. Moreover, the fully digital approach enables the software-defined radio paradigm, allowing the flexible use of several advanced radar techniques such as waveform diversity or frequency hopping. The proposed architecture is therefore promising for future radar systems that need to adapt to different scenarios for improved situation awareness. The proposed system exploits a single laser unit for the multiband transmitter and receiver sections, reducing the architectural complexity with potential benefits on system dimensions, cost, and reliability. This paper details the principle of operation of the proposed multi-band coherent radar system, and describes the implementation of a proof-of-concept dual-band transceiver operating in the X- and S-bands simultaneously and independently. The results from the characterization of the transceiver are presented. The system validation through the coherent detection of moving targets confirms the suitability of the proposed solution, laying the basis for a new paradigm of radar systems.

In-Field Experiments of the First Photonics-Based Software-Defined Coherent Radar

The complete scheme of the first photonics-based fully digital coherent radar system demonstrator is presented. The proposed architecture relies on a novel flexible photonic transceiver, based on the software-defined radio paradigm, capable of generating and receiving signals with arbitrary waveform and carrier frequency. The system core is a single mode-locked laser, whose inherent phase and amplitude stability allows generating high-quality carriers over an extremely broad frequency range, as well as directly digitizing high-frequency signals with unprecedented precision. The implementation of the field trial demonstrator is presented in detail, focusing on both the photonic transceiver and the antennas front-end, as well as on the employed digital signal processing. The excellent performance is proved by the results of the in-field experiments carried out with noncooperative targets in real scenarios. The outcomes from the aerial and naval target detections are here presented and discussed.

Photonic generation and independent steering of multiple RF signals for software defined radars

As the improvement of radar systems claims for digital approaches, photonics is becoming a solution for software defined high frequency and high stability signal generation. We report on our recent activities on the photonic generation of flexible wideband RF signals, extending the proposed architecture to the independent optical beamforming of multiple signals. The scheme has been tested generating two wideband signals at 10 GHz and 40 GHz, and controlling their independent delays at two antenna elements. Thanks to the multiple functionalities, the proposed scheme allows to improve the effectiveness of the photonic approach, reducing its cost and allowing flexibility, extremely wide bandwidth, and high stability.

Photonics for Radars Operating on Multiple Coherent Bands

The introduction of photonics in microwave systems is setting new paradigms in radar architectures, providing new features potentially improving the surveillance effectiveness. In particular, photonics is enabling a new generation of the multiband radars able to manage multiple coherent radar signals at different frequencies simultaneously, with high and frequency-independent quality, enabling multispectral imaging for advanced surveillance systems. In fact, thanks to its high stability and huge bandwidth, photonics matches the urgent requirements of the performance and flexibility of the next-generation software-defined radar architectures, and it guarantees system compactness, thanks to the use of a single shared transceiver for multiband operations and to the potentials for photonic integration, which also promises reduced power consumption. In this paper, we present the first field trial, in a maritime scenario, of a fully coherent multiband radar enabled by the use of photonics. The paper reviews the basic concepts exploited for the photonic generation and the detection of the radar signals, and describes the extension to the multiband operation. We present details on the implementation and testing of a dual-band coherent radar system, discussing the potentials for a software-defined radio approach. Moreover, the results obtained after a simple digital data fusion are discussed, highlighting the capability of the coherent photonics-based multiband radars in exploiting the extended observation bandwidth for improving the system detection resolution with minimum computational costs.

Flexible receiver for multiband orthogonal frequency division multiplexing signals at the millimeter waveband based on optical downconversion

A novel (to our knowledge) and flexible photonics-based downconversion scheme is proposed for wireless receivers in base stations. It allows simultaneous detection of multiple signals at carriers up to tens of gigahertz, enabling communications at millimeter waves. Experiments demonstrate the effective downconversion of Wi-Fi signals at 2.4 and 39.8 GHz with the error vector magniture <-43 dB.