M. Settembre

Sideband instability induced by periodic power variation in long-distance fiber links

Periodically spaced amplifiers along a transoceanic cable provide the phase-matching condition for a four-wave mixing process owing to Kerr nonlinearity. A new kind of sideband instability, shows up in both positive and negative dispersion regimes that is similar to modulation instability. A comparison with the sideband instability that was recently discovered for solitons is carried out.

Comparison of the performance of optically amplified transmission systems

In this paper, we show a comparison among the performance of single- and multichannel optical systems considering NRZ and soliton signals. The results have been obtained by means of numerical simulations, taking into consideration the chromatic dispersion, the Kerr nonlinearity, the fiber loss, and the ASE noise of the optical amplifiers. The performance of IM-DD and coherent asynchronous ASK systems have been evaluated in terms of the Q factor. We have considered the propagation in links encompassing conventional step-index fibers, DS fibers and in links with two different dispersion management techniques in which the chromatic dispersion is varied along the propagation distance both in randomly and in deterministically way. The effects of the in-line filtering process are mainly investigated in soliton propagation.

Cascaded optical communication systems with in-line semiconductor optical amplifiers

The optimization of the performance of installed standard-monomode fibers based optical transmission systems with in-line semiconductor optical amplifiers (SOAs) is reported in this paper. Both solitons and nonreturn-to-zero (NRZ) signals are studied and their capacities are compared. This investigation is based on numerical simulations by solving the nonlinear Schrodinger equations with the split-step method while the SOA is simulated with two different models that we show to be in a good agreement, transmissions over distances of the order of several hundreds of kilometers are shown to be possible, and very long distances can be reached especially if in-line sliding filters are used.

Single-channel transmission in dispersion management links in conditions of very strong pulse broadening: application to 40 Gb/s signals on step-index fibers

The dynamic behavior of single-channel transmission on standard fibers with strong dispersion management has been theoretically and numerically analyzed. A single pulse and a pseudorandom pulse sequence have been compared in order to highlight the relevant role played by pulse interaction. A semianalytical theory of the bandwidth evolution demonstrates that the introduction of prechirp is very important for controlling the single pulse propagation and numerical results show that such a chirp also permits to limit the nonlinear pulse interaction when other pulses are present. Simulations of a 40 Gb/s return-to-zero (RZ) system operating in links encompassing step-index fibers, by adopting a periodical compensation of the chromatic dispersion have been performed and results show that a record distance of 1300 km can be achieved with an amplifier spacing of 100 km. A compensation of the fiber third order dispersion would extend the transmission to 1800 km, which corresponds to the limits imposed by the amplified spontaneous emission (ASE) noise of the optical amplifiers.

Compensation of polarization mode dispersion by means of the Kerr effect for nonreturn-to-zero signals

We demonstrate numerically that a compensation of the polarization mode dispersion can be observed for nonreturn-to-zero signals as a result of a trapping effect, in analogy to the well-known soliton behavior. Conditions for such compensation are shown, and a comparison with the soliton case is reported.

Field demonstration of 40 Gb/s soliton transmission with alternate polarizations

A 40-Gb/s single-channel soliton transmission was performed on a dispersion-shifted fiber (DSF) embedded in a link connecting Rome to Pomezia. The soliton system was based on the transmission of pulses with orthogonal states of polarization (alternate polarizations) to limit the impairments due to the soliton interaction. The performance of the system operating in the field has been compared with results obtained in the laboratory. While in the laboratory the system showed a nearly-stable error-free transmission over 700 km, strong temporal fluctuations of the performance have been observed in the field trial. Numerical simulations have shown that such a degradation, and in particular the fluctuation of the Q factor, is mainly due to a polarization mode dispersion of the installed cables larger than that of the fibers used in the laboratory. However, 500 km error-free transmission was observed in the field for time interval longer than one hour, demonstrating the potentiality of a 40 Gb/s soliton system with alternate polarizations operating with long amplifier spacing (100 km) without in-line control.

Strategy for protection and restoration of optical paths in WDM backbone networks for next-generation Internet infrastructures

This paper proposes and analyzes a strategy for protection and restoration of optical paths in wavelength division multiplexing (WDM) networks for next-generation Internet infrastructure. Assuming a network model in which a multiprotocol label switching (MPLS) layer is overlaid on top of a WDM layer, and a segregation of the traffic on a wavelength basis, the strategy is based on a network dimensioning aiming at: 1) the support of different types of traffic, relating to different service categories; 2) the guarantee that all the wavelength paths carrying mission-critical services (with stringent quality of service requirements) are protected against failure; and 3) the possibility of restoring a large amount of wavelength paths carrying Internet best-effort services, i.e., low-priority (LP) preemptible traffic with no QoS requirements. The reference network scenario is an overlay model in which the optical network interacts with an MPLS network with a separated control plane; the optical network manages its resources to better serve the traffic coming from the MPLS network. Two path protection schemes, namely, disjoint path (DP) and single-link basis (SLB), and a link protection mechanism, namely, local repair (LP), are investigated. Furthermore, the paper considers both the case of using wavelength conversion in the optical nodes and the case in which wavelength conversion is not used at all. The analysis compares the performance of the different strategies in terms of the percentage of optical paths carrying LP traffic not served by the optical network after a failure and of the dimensions of the optical nodes for all the schemes. The analysis reveals that the proposed approach allows the restoration of a large amount of best-effort traffic with a reasonable increase of network redundancy.

Performance of optically amplified dispersion-compensated links: reduction of the time jitter for return to zero signals

Abstract We report on a numerical study of the performance of optically amplified systems operating in links with dispersion management, evaluating the Q factor and the time jitter. By a comparison of the return to zero signals and nonreturn to zero signals, and considering different sawtooth distributions of chromatic dispersion we have found the best propagation conditions for links operating at transoceanic distances. The results show that soliton signals permit to achieve the highest performance when they propagate in links with a sawtooth distribution with a low anomalous average GVD and when the parameters of the dispersion management satisfy some conditions. The main advantages of dispersion management for soliton signals are the increase of the tolerance of the signal power and the reduction of the time jitter.

High-speed DPSK coherent systems in the presence of chromatic dispersion and Kerr effect

The error probability for a single-channel coherent optical differential phase-shift keying (DPSK) transmission system based on repeaterless links in the presence of fiber chromatic dispersion and Kerr effect is evaluated. An accurate model for both the optical signal propagation and the probability distribution of the receiver decision variable is obtained by using a numerical solution of the nonlinear Shrodinger equation and the characteristic function method. The results show that the selection of an optimized IF filter bandwidth is crucial to obtaining the best system performance. When chromatic dispersion dominates, the best performance is achieved in the normal dispersion region whereas when the Kerr effect has the most limiting effect on system performances, the lowest error probability is attained in the anomalous regime. The maximum link length is limited by the presence of Kerr effect, independently of the amount of transmitted optical power, to be shorter than a threshold length if an error probability of 10/sup -9/ is to be achieved. >

Nonlinear compensation of chromatic dispersion for phase- and intensity-modulated signals in the presence of amplified spontaneous emission noise

We numerically study the evolution of nonsoliton signals in fiber links in the presence of the Kerr effect, chromatic dispersion, and the amplified spontaneous emission of optical amplifiers. Conditions in which the amplified spontaneous emission noise does not deeply affect the signal propagation are found, and the nonlinear Kerr compensation of the distortion induced by the chromatic dispersion is shown.