Mirco Scaffardi

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.

All-optical regeneration and demultiplexing for 160-gb/s transmission systems using a NOLM-based three-stage scheme

An all-optical pulse regenerator suitable for 160-Gb/s transmission systems, including three stages based on nonlinear optical loop mirrors, is proposed. The idea of splitting the regeneration process in three different steps allows the use of easy and well-known ultrafast subsystems. This approach offers advantages in terms of maximum bit rate of the signals that can be regenerated. The significant signal improvement in terms of noise reduction, pulse reshaping, and jitter suppression introduced by the regenerator is experimentally evaluated. A Q-factor increase from 3.2 to 6.3 was measured for the eye diagram. Since the presented scheme can also be exploited as a regenerating demultiplexer, simultaneous regeneration and demultiplexing functions are implemented for data signals up to 160 Gb/s.

Nonlinear optical loop mirrors: investigation solution and experimental validation for undesirable counterpropagating effects in all-optical signal processing

The ultrashort response time of the Kerr effect in the optical fiber suggests the possibility of using nonlinear optical loop mirrors (NOLMs) in applications of ultrafast all-optical signal processing. Roughly speaking, only one of the signal halves in the fiber loop undergoes a phase shift, due to its own power [self-phase modulation (SPM)] or to a copropagating pump light [cross-phase modulation (XPM)]. But even the other signal half is affected by SPM or XPM induced by the mean power of the strong counterpropagating signal. When considering ultrashort pulse trains at low bit rate, the phase shift due to the mean power can be neglected. But as the repetition rate of the pulse train gets higher, as in an optical time-domain multiplexing) frame, the effect of counterpropagating power must be taken into account, as it tends to reduce the efficiency of the NOLM. Up to now, some schemes have been proposed in the literature to eliminate these limitations, but they usually add further complexity. In this paper, we will investigate the impact of the undesirable effects due to the counterpropagating power on the NOLM performance for different duty-cycle values. We will present a simple and low-cost solution to overcome these impairments for both SPM- and XPM-based NOLMs. The solution consists of a proper NOLM design, using non-polarization maintaining) fiber and including a polarization controller into the loop, in order to compensate for the counterpropagating effects. Finally, the proposed solution will be experimentally validated.

Photonic Combinatorial Network for Contention Management in 160 Gb/s-Interconnection Networks Based on All-Optical 2

A modular photonic interconnection network based on a combination of basic 2times2 all-optical nodes including a photonic combinatorial network for the packet contention management is presented. The proposed architecture is synchronous, can handle optical time division multiplexed (OTDM) packets up to 160 Gb/s, exhibits self-routing capability, and very low switching latency. In such a scenario, OTDM has to be preferred to wavelength division multiplexing (WDM) because in the former case, the instantaneous packet power carries the information related to only one bit, making the signal processing based on instantaneous nonlinear interactions between packets and control signals more efficient. Moreover, OTDM can be used in interconnection networks without caring about the propagation impairments because of the very short length (<100 m) of the links in these networks. For such short-range networks, the packet synchronization can be solved at the network boundary in the electronic domain without the need of complex optical synchronizers. In this paper, we focus on a photonic combinatorial network able to detect the contentions, and to optically drive the contention resolution block and the switching control block. The implementation of the photonic combinatorial network is based on semiconductor devices, which makes the solution very promising in terms of compactness, stability, and power consumption. This implementation represents the first example of complex photonic combinatorial network for ultrafast digital processing. The network performance has been investigated for bit streams at 10 Gb/s in terms of bit error rate (BER) and contrast ratio. Moreover, the suitability of the 2times2 photonic node architecture exploiting the earlier mentioned combinatorial network has been verified at a bit rate up to 160 Gb/s. In this way, the potential of photonic digital processing for the next generation broad band and flexible interconnection networks has been demonstrated.

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This paper describes recent research activities and results in the area of photonic switching carried out within the framework of the EU-funded e-Photon/ONe+ network of excellence, Virtual Department on Optical Switching. Technology aspects of photonics in switching and, in particular, recent advances in wavelength conversion, ring resonators, and packet switching and processing subsystems are presented as the building blocks for the implementation of a high-performance router for the next-generation Internet.

2 Switching Elements

Linear feedback shift registers (LFSRs) are essential components of communications systems such as pseudo random bit sequence (PRBS) generators, convolutional encoders, scramblers and randomizers [1, 2]. Fig.1 (left) shows a LFSR scheme in which the XOR gate is operated among two tapped bits i and i+k within the shift register. The result bit is fed to the shift register at the last position (n) and the sequence in the register is 1-bit shifted as well. The experimental setup for demonstrating an all-optical LFSR is shown in Fig.1 (middle). The main components of the scheme are an optical buffer [3], a bit selecting circuit, and a XOR gate. The data vector (A) is generated by modulating the continuous wave (CW) λs with a 10Gbps non-return-to-zero (NRZ) electrical pattern whose period is TP=33.3168µs. Only 5 bits are used as the initial sequence for the shift register. The travelling time of the packet in the buffer is TL=1.85 µs, meaning that up to 18 round trips can be accommodated by the patterns period TP but one of those should be preserved for erasing the buffer before it is fetched a new sequence. The initial sequence enters the buffer as well as propagates directly to the bit selecting circuit. Every round trip, a new sequence as a replica of the previous one plus the feedback bit comes out. At the last round trip, a pump signal inserted into the buffer through the circulator will strongly reduce the gain of the semiconductor optical amplifier (SOA). Both the circulating packet and the accumulating in-band amplified spontaneous emission (ASE) noise will be suppressed. The buffer is now completely cleaned for a new cycle. The first and the last bits of the 1-bit shifted sequence are selected for the XOR operation. Referring to Fig.1 (left), i=1 and k=4, n=5. With this setup, the bit selecting circuit requires 2 clock signals (C) for tap the proper bits from the incoming sequence. The sampling process is done by exploiting four-wave-mixing (FWM) in a 250m-long highly nonlinear fiber (HNLF). Since at every round trip the sequence must be 1-bit shifted, the sampling clocks must be accordingly delayed for 1 bit, resulting the clock period of TC=TL+TB where TB=100ps is the bit time. The outputs of the bit selecting circuit are fed as pump signals to port a and d of a SOA Mach Zehnder interferometer (MZI). They are synchronized with a clock at λs used as the probe signal (F) at port b. The output of the XOR gate is then fed back into the buffer. The feedback bit should be precisely allocated at the end of the packet circulating inside the buffer, meaning that the feedback time should be TF=TL+TB+Tseq in which Tseq is the original sequence duration and equal to 500ps in our setup. Fig.1 (right) depicts the results at the 10th round trip for two different input initial sequences (10001) and (10011) taken at point (B). The equalization between bits in the sequence and the quality of the pulses is strongly limited by the accumulating ASE noise inside the buffer, the long fiber-based structure and the bits polarization. The fiber-based setup also causes misalignment between the clocks, spoils the essential synchronism and degrades the performance of the FWM and the XOR operation. One possible solution for solving all of these problems and hence improving the performance is the integration approach. The bit selection could be implemented exploiting nonlinear effects into an SOA or a periodic-poled lithium niobate (PPLN) instead of using a long HLNF. Moreover the extinction ratio of the pulses at the SOA-MZI XOR output (&#60;13.5dB) could be improved, e.g. with a saturable absorber at its output, thus increasing the number of allowed round trips into the shift register and reducing the noise on the final bit sequence.

Photonics in switching: enabling technologies and subsystem design

A complete and effective tool for the analysis of semiconductor optical amplifiers (SOAs) in ultrafast telecommunication applications is presented. This model includes gain dynamics, the current induced gain saturation effect, the current-dependent gain dispersion, and the amplified spontaneous emission. Experimental results concerning both stationary and dynamic conditions confirm the model accuracy and its effectiveness in the design of all-optical signal processing schemes based on SOAs. Finally, we study the characteristic times and the efficiencies of SOA fast dynamics in order to investigate the bandwidth limits of these devices. A numerical evaluation, validated by high-resolution measures makes the SOAs suitable to process optical signals up to 160 Gb/s.

Optical linear feedback shift register

We experimentally demonstrate several high-speed wavelength division multiplexing (WDM) to time division multiplexing (TDM) conversion schemes by using different nonlinearities in highly nonlinear fiber (HNLF). 40-160-Gb/s WDM-to-TDM conversion using cross-phase modulation (XPM) and 160-320-Gb/s conversion based on supercontinuum generation are shown. Furthermore, we investigate a different kind of HNLF, and demonstrate 40-80-Gb/s WDM-to-TDM conversion using XPM in a 0.8-m bismuth oxide HNLF (Bi-HNLF). Less than 3-dB power penalty is obtained for each case.

Modeling and measurement of noisy SOA dynamics for ultrafast applications

An all-optical ADD- DROP multiplexer able to carry out ultrafast channel extraction, clearing, and insertion operations for time-interleaved optical signals is presented. The proposed ADD-DROP multiplexer exploits polarization rotation induced by cross-phase modulation in 1-m-long highly nonlinear bismuth oxide-based optical fiber with a nonlinear coefficient of 1250 W/sup -1/ /spl middot/ km/sup -1/. Penalties lower than 1.5 dB in a 4 /spl times/ 10 Gb/s optical-time-division-multiplexed system have been measured for all operations.