Coherent multi-band MIMO radar: robustness analysis to SSMF-based RF signal delivery.

A numerical evaluation is conducted to assess the impact of distributing radio frequency (RF) signals through optical fiber links on the performance of a coherent multi-band multiple-input multiple-output (MIMO) radar system. The analysis focuses on scenarios where the antennas are widely separated in comparison to the employed signal wavelengths. The development of a model to quantify the phase noise (PN) induced on each RF band due to the signal transmission through optical fiber links between the centralized base station and each radar peripheral is described. Monte Carlo simulation results are collected to estimate the key performance indicators (KPIs) for varying standard single-mode fiber (SSMF) length and different PN contributions. The main contributors to the PN are revealed to be chromatic dispersion (CD), double Rayleigh scattering (DRS), and mechanical vibrations. In a shipborne scenario, a significant performance degradation occurs only when the length of the fiber links reaches approximately 20 km. Further, the PN impact has also been studied in a shipborne scenario to analyze the robustness of the system for worse phase noise level assumptions. The results reveal excellent robustness of the proposed centralized acquisition and processing approach in the presence of both very long fiber links and economically employed RF oscillators.

Phase noise mitigation in photonics-based radio frequency multiplication.

Two photonics-based radio frequency multiplication schemes for the generation of high-frequency carriers with low phase noise are proposed and experimentally demonstrated. With respect to conventional frequency multiplication schemes, the first scheme induces a selective cancelation of phase noise at periodic frequency-offset values, whereas the second scheme provides a uniform 3-dB mitigation of phase noise. The two schemes are experimentally demonstrated for the generation of a 110-GHz carrier by sixfold multiplication of an 18.3-GHz carrier. In both cases, the experimental results confirm the phase noise reduction predicted by theory.

Coherent MIMO radar network enabled by photonics with unprecedented resolution.

The conventional concept of radar is based on stand-alone and independent apparatuses. Superior performance is possible, exploiting distributed points of view (i.e., distributed radars) and centralized data fusion, but systems based only on radio-frequency technology are not able to guarantee the requested degree of coherence and high capacity links among radars. In the current distributed systems, radars act almost independently from each other. Thus, data fusion, which must be performed on locally pre-processed information, can only exploit partial information content, harming the imaging capability of the distributed system. Here we present, to the best of our knowledge, the first extended analysis and experiment of a distributed coherent multiple input-multiple output radar system, enabled by photonics, which maximizes the information content extracted by a centralized data fusion, providing unprecedented resolution capabilities. Stepping from previous achievements, where photonics has been demonstrated in single radars, here photonics is used also for providing coherence and high capacity links among radars. The numerical analysis also demonstrated the benefits of coherent multi-band operation for sidelobe reduction, i.e., false alarms reduction.

Penetration capability of near infrared Laguerre-Gaussian beams through highly scattering media.

The higher capability of optical vortex beams of penetrating turbid media (e.g., biological fluids) with respect to the conventional Gaussian beams is, for the first time to our knowledge, demonstrated in the 1.3 µm wavelength range which is conventionally used for optical coherence tomography procedures in endoscopic intravascular scenarios. The effect has been demonstrated by performing transmittance measurements through suspensions of polystyrene microspheres in water with various particulate concentrations and, in reflection, by using samples of human blood with different thicknesses. The reduced backscattering/increased transmittance into such highly scattering media of Laguerre-Gaussian beams with respect to Gaussian ones, in the near infrared wavelength region, could be potentially exploited in clinical applications, leading to novel biomedical diagnoses and/or procedures.

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.

Software-defined reconfigurable VCSEL-based transmission.

We propose and demonstrate a programmable and self-adaptive VCSEL-based transponder for short-reach applications that is easily extensible up to access and metro scenarios. The transponder presents a monitoring system that feeds the transponder controller in order to maintain the proper transmission performance. The emerging NETCONF protocol including the YANG model controls and manages the transponder. NETCONF messages are reported for two reference scenarios - uncooled and cooled systems - together with performance at varying environment conditions. For the uncooled scenario heating processes on the board are emulated with various dynamics, the effect on the performance of a 25 Gbps WDM channel is checked through optical power monitoring and the control plane reacts so as to optimize the performance by suitably controlling the bias current. For slow temperature variations the system is able to avoid service outage whereas for variations faster than 1.3 °C/s outage occurs and corresponding notifications are opportunely triggered. For the cooled scenario, an optical power loss is emulated with consequent service outage, leading to a reconfiguration of the transponder data rate from 25 to 10 Gbps, so as to recover successful transmission.

3 × 3 optical switch by exploiting vortex beam emitters based on silicon microrings with superimposed gratings.

A silicon-on-insulator microring with three superimposed gratings is proposed and characterized as a device enabling 3×3 optical switching based on orbital angular momentum and wavelength as switching domains. Measurements show penalties with respect to the back-to-back of <1 dB at a bit error rate of 10-9 for OOK traffic up to 20 Gbaud. Different switch configuration cases are implemented, with measured power penalty variations of less than 0.5 dB at bit error rates of 10-9. An analysis is also carried out to highlight the dependence of the number of switch ports on the design parameters of the multigrating microring.

Dual use architecture for innovative lidar and free space optical communications.

An innovative and effective architecture for lidar systems is presented and experimentally demonstrated. The proposed scheme can also be easily exploited for optical communications. In particular, the system includes an innovative lidar software-defined architecture based on optically coherent detection, overcoming current drawbacks of time of flight incoherent systems. The experiments demonstrate the ability to perform long range detection resorting to the waveform compression on the continuous wave approach, obtaining a range resolution of 15 cm with a sensitivity of -95 dBm. Beside the bulk implementation, the system has been also implemented in a photonic integrated circuit using complementary metal-oxide-semiconductor-compatible silicon on insulator technology with an extremely reduced footprint of 1.5 mm×3.5 mm. The testing of the integrated device confirms the effectiveness of this proof-of-concept realization.

Frequency-agile dual-frequency lidar for integrated coherent radar-lidar architectures.

We propose a novel architecture for implementing a dual-frequency lidar (DFL) exploiting differential Doppler shift measurement. The two frequency tones, needed for target velocity measurements, are selected from the spectrum of a mode-locked laser operating in the C-band. The tones' separation is easily controlled by using a programmable wavelength selective switch, thus allowing for a dynamic trade-off among robustness to atmospheric turbulence and sensitivity. Speed measurements for different tone separations equal to 10, 40, 80, and 160 GHz are demonstrated, proving the system's capability of working in different configurations. Thanks to the acquisition system based on an analog-to-digital converter and digital-signal processing, real-time velocity measurements are demonstrated. The MLL-based proposed architecture enables the integration of the DFL with a photonic-based radar that exploits the same laser for generating and receiving radio-frequency signal with high performance, thus allowing for simultaneous or complementary target observations by exploiting the advantages of both radar and lidar.

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.

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.

NRZ/RZ 40 Gbit/s optical regenerator based on a photonic two-level nonlinear device.

A compact device with a two-level transfer function (TF) implemented with two semiconductor optical amplifier (SOA)-based stages is proposed and characterized. Each stage exploits nonlinear polarization rotation and self-phase modulation. The obtained improved TF with very flat top and bottom levels makes the scheme suitable for working as a reshaper in all-optical regeneration. The effectiveness of the device is verified in regenerating both nonreturn-to-zero (NRZ) and return-to-zero (RZ) data signals up to 40 Gb/s. Bit error rate measurements demonstrate increased threshold margin and extinction ratio improvement.

PPLN-based OOK and DQPSK optical grooming by amplitude and phase-signal multiplexing through pump depletion.

We propose and characterize a simple, integrable, and wavelength-preserving scheme able to groom a 40 Gbps (D)QPSK signal with a 20 Gbps OOK one into a 20 Gbaud (60 Gbps) 8-APSK signal. The proposed all-optical scheme is based on the second-order nonlinear signal-depletion effect in a single periodically poled lithium niobate (PPLN) waveguide. Performance of the device, characterized by means of BER measurements, attests error-free operation and a power penalty below 4.1 dB.

All-optical simultaneous drop and wavelength conversion of DPSK data.

We demonstrate an all-optical scheme for the simultaneous drop and wavelength conversion of bursts of data from a continuous stream of differential phase-shift keyed (DPSK) signals. This function is obtained in a single semiconductor optical amplifier Mach-Zehnder interferometer thanks to proper nonlinear interaction of the data stream and an optical gate signal at different wavelength. Fast switching-time enabling wavelength shifting operation on continuous DPSK data stream at 10 and 40 Gb/s without any bit loss is reported. Corresponding measured power penalties are negligible at 10 Gb/s and about 1.7 dB at 40 Gb/s.

Colorless all-optical sum and subtraction of phases for phase-shift keying signals based on a periodically poled lithium niobate waveguide.

A colorless all-optical scheme performing the subtraction and addition of phases between phase-shift keying (PSK) signals exploiting cascaded sum and difference frequency generation in a periodically poled lithium niobate waveguide is introduced and experimentally demonstrated. The subtraction of phases of two 40 Gb/s differential quadrature PSK signals has been experimentally tested and performances have been analyzed in terms of bit error rate measurements.

High-speed addition/subtraction/complement/doubling of quaternary numbers using optical nonlinearities and DQPSK signals.

We propose an innovative approach to implementing multiple arithmetic functions of quaternary numbers using optical nonlinearities and differential quadrature phase-shift keying (DQPSK) signals. By adopting 100 Gbit/s DQPSK signals (A, B) and exploiting nondegenerate four-wave mixing (FWM) for addition/subtraction and degenerate FWM for complement and doubling in a single highly nonlinear fiber, we demonstrate 50 Gbaud/s simultaneous quaternary addition (A+B), dual-directional subtraction (A-B, B-A), complement (-A, -B), and doubling (2B). Power penalties less than 4 dB (addition), 3 dB (dual-directional subtraction), 2 dB (complement), and 3.1 dB (doubling) are observed at a bit-error rate of 10(-9).

160 Gbit/s binary-to-quaternary amplitude shift keying encoding in the optical domain.

160 Gbit/s all-optical binary-to-quaternary amplitude shift keying format conversion is carried out in a nonlinear optical fiber. This scheme, which also acts as a 2 bit digital-to-analog convertor, has been confirmed through Q-factor measurements.

640 Gbits/s photonic logic gates.

We demonstrate 640 Gbits/s all-optical A AND B, and A AND B logic functions using pump depletion in a periodically poled lithium niobate waveguide. Bit-error-rate measurements show the effectiveness of the scheme, with a penalty of <2 dB.

Eightfold 40-320 Gbit/s phase-coherent multiplexing and 320-40 Gbit/s demultiplexing using highly nonlinear fibers.

We experimentally demonstrate 40-320Gbit/s phase-coherent multiplexing based on cross-phase modulation and subsequent 320-40Gbit/s demultiplexing using cross-phase-modulation-induced frequency shift in highly nonlinear fibers. An average penalty of approximately 7dB at a 10(-9) bit-error rate is observed after the nonlinear processes. We also show the generation of a phase-coherent 320GHz optical pulse train. A delay-line interferometer based on differential group delay is used to verify the phase coherence characteristic of the multiplexed 320Gbit/s signal and the generated 320GHz pulses.