Giovanni Serafino

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.

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.

Phase and Amplitude Stability of EHF-Band Radar Carriers Generated From an Active Mode-Locked Laser

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).

Optical xor for Error Detection and Coding of QPSK I and Q Components in PPLN Waveguide

An all-optical scheme based on periodically-poled lithium niobate (PPLN) waveguide for signal processing of the in-phase (I) and quadrature (Q) components of an input quadrature phase shift keying (QPSK) signal is presented. The device is able to work on the I and Q components without any additional demodulation stage, and makes use of cascaded second harmonic and difference frequency generation in the PPLN to obtain the logical operation xor (I, Q). A single continuous wave signal is needed in addition to the input signal to generate the output signal, in which the information is coded in a binary phase shift keying modulation. The logical xor (I, Q) potentially enables data coding, error detection, and encryption of sensitive information in all-optical networks. Bit error rate measurements are provided to evaluate the system performance for a 20-Gb/s differential-QPSK input signal, and tunability of the output wavelength has been attested with almost constant optical signal-to-noise-ratio penalty along the C-band.

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.

All-Optical Gated Wavelength Converter-Eraser Using a Single SOA-MZI

We propose and experimentally demonstrate an original scheme for simultaneous all-optical wavelength conversion and data erasing under an optical gate signal, suitable for on-off keying (OOK) signals. This function is obtained in a semiconductor optical amplifier Mach-Zehnder interferometer (SOA-MZI) exploiting nonlinear interaction between a continuous data stream and an optical gate signal at a different wavelength. In correspondence of the gate signal, a burst of data from the optical stream is converted at the optical gate wavelength and, at the same time, cancelled at the input stream wavelength. Fast switching time enabling selective wavelength shifting on a data stream at 10 Gb/s without any bit loss is demonstrated. Error-free operation with 0.2-dB power penalty for the wavelength-preserved output data and 1.8 dB for the shifted data is reported.

Novel Architecture for a Photonics-Assisted Radar Transceiver Based on a Single Mode-Locking Laser

A novel solution for the optical generation of radio-frequency signals for coherent radars is proposed, which exploits the same mode-locking laser of the photonic-assisted radar receiver. The scheme, which is based on the concept of shifting the frequency of a laser mode, exploits a commercial optical in-phase/quadrature modulator to allow using a single mode-locking laser for both the radar transmitter and receiver, making the solution attractive for immediate and practical implementation in photonics-assisted transceivers.

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.

Phase-Preserving Amplitude Noise Compression of 40 Gb/s DPSK Signals in a Single SOA

Wavelength-preserving and wavelength-converting amplitude regeneration of 40 Gb/s DPSK data in a saturated semiconductor optical amplifier (SOA) with limited excess phase noise feature is reported and investigated. By injecting simultaneously in the amplifier the noisy data and a CW beam with proper power levels, both the pass-through (PT) signal and the wavelength-converted data by four-wave-mixing (FWM) can be regenerated. The strongest regenerative effect is observed for the converted FWM data. The effect of input power variations on the regenerative performance of the device is thoroughly investigated in terms of Bit-Error Rate (BER) versus threshold margin improvement. We observe that regenerative behavior is attained for different levels of the data and CW power levels at SOA input. Experimental investigation of small-signal amplitude and phase SOA response reveals indeed the role of data/CW power balance for reducing the impact of excess phase noise from the amplifier. The analysis also confirms the stronger regenerative capability of the FWM process due to the strong amplitude-limiting SOA response at lower input data power in respect to the PT signal. The scheme offers the advantages of simplicity, compactness, and low operating power levels, making it a practical candidate for all-optical regeneration of high-speed DPSK signals.

Regenerative Optical Buffer Based on SOA-Amplified Recirculating Loop

We introduce a novel scheme for all-optical buffering based on a recirculating fiber loop with regenerative amplification by cross-gain compression in a semiconductor optical amplifier. Fifty circulations of 10-Gb/s packets with 1.2-dB power penalty every 10 rounds are reported obtaining more than 150-μs buffering time. A large resilience of the scheme to noisy signals is also demonstrated.