Sergio Pinna

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.

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.

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.

Multifunctional Current-Controlled InP Photonic Integrated Delay Interferometer

We demonstrate a novel and flexible InP monolithically integrated optical circuit with multiple applications. The circuit is an interferometric delay loop and its particular configuration allows to perform a number of different functions, namely, optical buffering, differential phase-shift keying (DPSK) demodulation, intensity modulation, and differential XOR logic operation. By properly controlling the current supplied to the active elements in the circuit loop (i.e., a semiconductor optical amplifier and a variable optical attenuator), the relative phase of the propagating signals can be adjusted, changing the interference condition between the input signal and its delayed copies. In this way the four different functions are enabled. Experimental results are reported for all the different functions of the device. When the circuit is operated in buffer configuration, up to 13 circulations (corresponding to a 1.62 ns delay) of an input bit at 12.5-Gb/s data rate are shown before the output signal degrades due to excessive ring losses. Its behavior as a current-controlled DPSK demodulator is demonstrated for 8-Gb/s signals, and the resulting bit error rate measurements are compared with those of a thermally tuned commercial demodulator, showing no significant power penalty. A proof-of-concept demonstration as an intensity modulator is reported for relatively low-frequency signals at 10 and 20 MHz, being limited by electrical components of the setup. Finally, error-free operation for a 1-, 2-, and 4-b differential XOR logic gate has been demonstrated at 8, 16, and 32 Gb/s, respectively.

Generation of a flexible optical comb in a periodically poled lithium niobate waveguide

We propose and demonstrate a technique for the generation of an optical comb with tunable line spacing in a periodically poled lithium niobate (PPLN) waveguide. The technique is implemented with four input continuous waves (CWs), which generate a 19-line comb tuned to the spacing of 25 and 20 GHz. We show that each additional CW switched on out of the quasi phase-matching band at the PPLN waveguide input generates the growth of six new lines. The performance of the comb is tested modulating the lines with a 40 Gb/s differential quadrature phase shift keying data, demonstrating error-free operation. Nonuniform spacing of the input seed CWs improves the efficiency of the line generation process.

Flexible frequency comb generation in a periodically poled lithium niobate waveguide enabling optical multicasting

We propose and demonstrate a technique for the generation of a coherent optical comb, with tunable line spacing in a periodically poled lithium niobate (PPLN) waveguide. A single continuous wave laser is modulated to generate three phase-locked seed lines, which are injected into a PPLN waveguide, to obtain line multiplication. The line spacing is set acting on the frequency of the electrical signal driving the modulator. The quality of the comb is verified measuring the autocorrelation, the phase noise, and the linewidth of the generated lines. With the same scheme, we demonstrate optical multicasting. From a single quadrature phase shift keying signal, modulated at 12.5 and 25 GBaud, five replicas are generated, with spacing 25 and 37.5 GHz. The performance of each signal replica is measured after transmission through 80 km of a single-mode fiber, demonstrating operation with a bit error rate value lower than the forward error correction threshold.

Defragmentation and grooming on 85.4 Gb/s by simultaneous format and wavelength conversion in an integrated quad SOA-MZI

In this paper, we propose and experimentally demonstrate simultaneous format and wavelength conversion of multiple fragmented signals suitable for gridless and multi-granular networks. Conversion of NRZ and large-RZ to short-RZ-OOK by means of a single commercial Quad SOA-MZI is demonstrated for grooming 85.4 Gb/s frame. The output RZ-OOK signal is suitable for further tributary multiplexing onto a 170.8 Gb/s OTDM stream. The resultant OTDM channel is transmitted over a 110-km dark fiber link.

Field trial of the first photonic-based radar for maritime border security and harbor protection

The field trial results of the first photonics-based radar system in a real maritime environment are presented in this paper. The exploited prototype is the final outcome of the ERC-funded project PHODIR (PHOtonics-based fully DIgital Radar). The system exploits photonic technologies for both the generation and the detection of radar RF signals, allowing increased performance and an unprecedented potential flexibility. This paper details the system capabilities and presents the field trial campaign conducted at the port of Livorno (Italy), detecting naval targets on both long range and harbor maneuvering. The results prove the effectiveness of the proposed scheme and the benefits of photonics highlighting the excellent performance and promising improvements.

A beam-forming network for 5G systems based on precise optical clock and phase shifting

A novel, power-efficient optical beam-forming network is proposed exploiting wavelength-division-multiplexing and phase-shifting. A mode-locked laser acts as both laser comb and local oscillator. Integrated frequency-selective phase shifters perform precise beam steering of a modulated signal.