Valeria Vercesi

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.

Sliceable transponder architecture including multiwavelength source

A multiflow transponder in flex-grid optical networks has recently been proposed as a transponder solution to generate multiple optical flows (or subcarriers). Multiflow transponders support high-rate super-channels (i.e., connection composed of multiple corouted subcarriers contiguous in the spectrum) and sliceability; i.e., flows can be flexibly associated to the incoming traffic requests, and, besides composing a super-channel, they can be directed toward different destinations. Transponders supporting sliceability are also called sliceable transponders or sliceable bandwidth variable transponders (SBVTs). Typically, in the literature, SBVTs have been considered composed of multiple laser sources (i.e., one for each subcarrier). In this paper, we propose and evaluate a novel multirate, multimodulation, and code-rate adaptive SBVT architecture. Subcarriers are obtained either through multiple laser sources (i.e., a laser for each subcarrier) or by exploiting a more innovative and cost-effective solution based on a multiwavelength source and micro-ring resonators (MRRs). A multiwavelength source is able to create several optical subcarriers from a single laser source. Then, cascaded MRRs are used to select subcarriers and direct them to the proper modulator. MRRs are designed and analyzed through simulations in this paper. An advanced transmission technique such as time frequency packing is also included. A specific implementation of a SBVT enabling an information rate of 400 Gb¿s is presented considering standard 100 GbE interfaces. A node architecture supporting SBVT is also considered. A simulation analysis is carried out in a flex-grid network. The proposed SBVT architecture with a multiwavelength source permits us to reduce the number of required lasers in the network.

Up to 40 Gb/s Directly Modulated Laser Operating at Low Driving Current: Buried-Heterostructure Passive Feedback Laser (BH-PFL)

A directly modulated 1.55-μm buried-heterostructure passive feedback laser exhibits a high modulation bandwidth of up to 34 GHz at moderate distributed-feedback (DFB) section currents between 20 and 60 mA. A very flat frequency response and a low alpha parameter have been demonstrated in the small signal modulation analysis. The device has open eyes at data rates of 25 and 40 Gb/s with reduced frequency chirp.

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.

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.

Spectral-Efficient Flexible Optical Multicasting in a Periodically Poled Lithium Niobate Waveguide

We experimentally demonstrate enhanced spectral efficiency (SE) within flexible multicasting operation in a periodically poled Lithium Niobate (PPLN) waveguide. Replicas of quadrature phase-shift keying (QPSK) and 16 quadrature amplitude modulation combs are generated in the C-band exploiting second-order nonlinear effects in a PPLN waveguide as second harmonic generation and sum frequency generation cascaded with difference frequency generation. This technique allows creating equally spaced replicas of the signal to be multicast, being independent of its modulation format. The bit error rate is measured for the signal replicas after transmission through 80 km of a standard single-mode fiber span. The SE obtained with the multicasting of a 25 Gb/s QPSK on 37.5 GHz spaced channels is about 1.3 b/s/Hz. By introducing the time-frequency packing technique, we demonstrate optical multicasting of a 60 Gb/s QPSK signal with 20 GHz spacing among the replicas by achieving a SE of 2.7 b/s/Hz.

Fully photonics-based radar demonstrator: Concept and field trials

This work shows the concept, performance, and field-trials of the first photonics-based radar. The comparative in-field experiments in aerial and naval scenarios against a state-of-the-art commercial system show the photonics potentials in enabling software-defined radars.

Tunable dual-frequency lidar exploiting a mode-locked laser for integrated coherent radar-lidar architectures

A mode-locked laser-based dual-frequency lidar with tunable tones separation is demonstrated, allowing a dynamic tradeoff among robustness and sensitivity of measurements and enabling integration with photonic-based radar. Velocity measurements for different tones separation are demonstrated.

Integrated multi-frequency lidar / radar system for precise and robust Doppler measurements

A novel architecture of an integrated photonic assisted radar-lidar system based on a single mode-locked laser is proposed and demonstrated. The lidar exploits a multi-frequency optical signal with tunable tones separation allowing a dynamic tradeoff among robustness and sensitivity of measurements. The radar, which employs the mode-locked laser for generating and receiving the radio-frequency signal operates in the X-band and simultaneously with the lidar. Velocity measurements for different tones separation are successfully demonstrated with good agreement between lidar and radar.

Electronically synthesized Nyquist pulses for photonic sampling of microwave signals

We report electronic generation of optical Nyquist pulses using an arbitrary waveform generator (AWG) followed by a Mach Zehnder modulator (MZM), providing a simple, highly stable and flexible technique to perform photonic sampling. Here, we demonstrate the generation of 10 GHz periodic optical Nyquist pulses by synthesizing both all-positive and alternate positive-negative electrical pulse trains using a 25 GHz bandwidth AWG. Biasing the MZM at null ensures the meeting of the Nyquist ISI-free criterion in the optical domain and allows for pulse compression. Moreover, we report the first photonic sampling and demodulation of 1 Gbaud 16- and 32-QAM signals up to 22.5 GHz using 10 GHz optical Nyquist sampling pulse trains.