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.

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.

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.

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.

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.