Emma Lazzeri

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.

Photonic Processing for Digital Comparison and Full Addition Based on Semiconductor Optical Amplifiers

An N bit all-optical comparator and an all-optical full adder are presented. These complex circuits, which perform photonic digital processing, are implemented cascading a unique basic gate that exploits cross gain modulation and cross-polarization rotation in a single semiconductor optical amplifier (SOA). Since the interacting signals are counterpropagating in the SOA, they can be set at the same wavelength. Photonic processing improves the speed of the optical networks by reducing the packet latency time to the time-of-flight in the nodes. Digital comparison and full-addition are key functionalities for the processing of the packet labels. Integrated realizations are crucial, thus, SOAs represent a suitable mean both because they allow hybrid integrated solutions and fast operation speed. The performances of the basic gate, the comparator, and the full adder are investigated both in terms of bit error rate and eye opening. To the best of our knowledge this is the first time it is reported on the implementation of an all-optical comparator able to compare patterns longer than 1 bit. Previous works demonstrate the comparison of 1 bit patterns. Only few works report on an all-optical full adder implementation, but with different schemes. In our implementation, sum and carry out do not depend directly on the carry in, thus potentially improving the output signal quality when cascading multiple full adders.

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.

All-Optical

The design and experimental characterization of an all-optical circular shift register are presented. The proposed scheme consists of a fiber loop optical buffer and a bit selecting circuit. A semiconductor optical amplifier (SOA) is used as an active element in the buffer structure, while the bit selecting circuit exploits Kerr effect in a highly nonlinear medium. The scheme is designed to store an N-bit sequence and to return the bits one by one at a lower clock rate, achieving proper circulating shift register functionality. Photonic integration is possible thanks to the presence of SOA in the buffer loop and to the absence of erbium-doped fiber amplifier-based feedbacks. Moreover, the increase in the number of bits to be stored does not increase the scheme complexity. Experimental validation is demonstrated for , and a signal-to-noise ratio per bit penalty equal to 5.6 dB is achieved at a bit error rate equal to with respect to the input sequence.

N

All-optical digital circuits based on loop memories are demonstrated. These circuits are a variable optical buffer, an optical random access memory, an optical shift register, and an optical linear feedback shift register. Buffers are employed in network nodes to solve the contentions without loosing information. Shift registers are used for error detection and correction techniques. Linear feedback shift registers are employed for generation of pseudo-random bit sequences, data scrambling, and encryption/decryption of information in secure communication systems. Since the role of optical processing is gradually increasing within the communication systems, the possibility to store information directly in the optical domain can lead to a systems simplification and to a performance improvement. In order to make the schemes suitable for practical applications, optical integration is necessary. The use of semiconductor optical amplifier as gain element into the memory loop allows all the proposed schemes to be integrated.

-Bits Shift Register Exploiting a Ring Buffer Based on Semiconductor Optical Amplifier

A new approach based on modular blocks to implement all-optical analog-to-digital conversion is presented. The basic blocks exploit cross-gain modulation in semiconductor optical amplifiers. A four-level signal at 20 Gsamples/s is successfully quantized and coded onto 2 bits. The output extinction ratio of the encoded bits is higher than 3.6 dB. The extinction ratio can be improved by means of a thresholder, e.g., a saturable absorber, at each output port.

All-Optical Digital Circuits Exploiting SOA-Based Loop Memories

Elementary blocks, performing logic operations, are the building elements for more complex subsystems implementing all-optical digital processing. They can potentially enable next generation optical networks and optical computing, overcoming the limitations of the electronics bandwidth, also guaranteeing scalability, transparency, easy reconfigurability and modularity. Finally, integrated technologies can reduce power consumption, footprint and cost.

Analog-to-Digital Conversion Based on Modular Blocks Exploiting Cross-Gain Modulation in Semiconductor Optical Amplifiers

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.

Optical logic elementary circuits

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.

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

An all-optical scheme for comparison of N-bit long Boolean numbers is presented. The scheme, wavelength independent, exploits six SO As. Error-free operations and extinction ratio degradation lower than 1.9 dB are obtained for each output signal.