JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 22, NO. 5,MAY 2004 1263 Comparison of Nonlinear Pulse Interactions in 160-Gb/s Quasi-Linear and Dispersion Managed Soliton Systems Andrea Del Duce, Robert I. Killey, Member, IEEE, and Polina Bayvel, Senior Member, IEEE Abstract—The understanding and development of 160-Gb/s transmission systems requires the study of the impact of dif- ferent dispersion compensation schemes on pulse propagation in nonlinear fiber. In this paper, we present an investigation of 160-Gb/s optical transmission systems, focusing on optimal propagation regimes, and in particular, we analyze different transmission limitations and dominant nonlinear effects by comparing quasi-linear and dispersion managed soliton systems. Two quasi-linear systems, one using nonzero dispersion-shifted fiber (NZDSF) and the other single-mode fiber (SMF), and one short-period (1 km) dispersion managed soliton (DMS) system are studied, both for single-channel and wavelength-division-mul- tiplexed (WDM) transmission. First, the performance of the two quasi-linear systems in single-channel transmission are compared and it is shown that the NZDSF and SMF systems allow similar error-free transmission distances with only small differences in the intrachannel four-wave mixing (IFWM) induced amplitude jitter. The effect of pulsewidth on transmission performance in this regime was investigated and the use of shorter pulses was found to result in lower amplitude jitter. We analyzed the behavior of the DMS system and showed that the reduced pulse broadening during transmission allowed a significantly longer single-channel transmission distance with a smaller impact of nonlinearities compared to quasi-linear propagation. The sensitivity of the DMS system performance to statistical fluctuations in the fiber dispersion was studied and the results show the level of accuracy in the dispersion management map which must be ensured in these systems. Finally, the performance of the DMS in WDM transmission was investigated and it was found that it was subject to very large penalties increasing the minimum channel spacing possible because of the strong impact of interchannel cross-phase modulation (XPM). Index Terms—Dispersion-managed solitons, dominant non- linearities, nonlinear fiber optics, nonlinear optics, optical communications, optical fiber communications, optical transmis- sion, quasi-linear transmission, wavelength-division multiplexing (WDM). I. INTRODUCTION W HILE the dominant single-channel and multiwavelength nonlinear limitations in optical transmission at bit rates up to 40 Gb/s have been investigated and are well under- stood, the transmission of pulses at 160 Gb/s remains largely Manuscript received May 6, 2003; revised December 11, 2003. This work was supported by EPSRC and the Royal Society. The authors are with the Optical Networks Group, Department of Electronic and Electrical Engineering, University College London (UCL), Torrington Place, London, WC1E 7JE, U.K. Digital Object Identifier 10.1109/JLT.2004.826384 unexplored. A key area of research for the understanding of 160-Gb/s transmission systems is the investigation of optimal transmission schemes, identifying the impact on performance of parameters such as pulse shape, pulsewidth, and dispersion compensation map. To date, two competing approaches have been proposed for the transmission of ultrahigh bit rates of 40-160-Gb/s – soliton-like transmission where the pulse shape is maintained and quasi- or pseudolinear transmission where the pulses are allowed to disperse. Which of the two regimes is optimal, in terms of penalties introduced by nonlinearities, for 160-Gb/s single-channel and wavelength-division multiplexing (WDM) optical transmission systems remains the subject of active research. As shown in [1]–[3], systems operating at very high bit rates which use dispersive fiber and lumped dispersion compensation are affected by nonlinear interactions between pulses in the same channel, the so-called intrachannel nonlinearities, leading to timing and amplitude jitter of the transmitted pulses. These intrachannel nonlinearities are, respectively, intrachannel cross-phase modulation (IXPM) and intrachannel four-wave mixing (IFWM) and occur in high bit rate systems because of the strong impact of dispersion which spreads short pulses over adjacent bit slots, combined with the nonlinear phase shift introduced by the nonlinear refractive index. Systems that operate in this regime are referred to as quasi- or pseudolinear systems [4]. To date, distances of 300 and 480 km, respectively, have been demonstrated experimentally in single-channel transmission in [5] and [6], while transmission over 400 km was reported with WDM in [7], all using quasi-linear transmission at 160 Gb/s (without forward-error correction). Numerical studies have been carried out to understand how to reduce the impact of intrachannel nonlinearities in these systems through an optimal choice of fiber dispersion and the use of precompensation [8]. However, it would be useful to quantify the impact of intrachannel non- linearities on the maximum transmission distance for specific quasi-linear systems, considering practical issues such as slope compensation and maximum span loss. An alternative scheme which reduces intrachannel nonlinear- ities is the dispersion managed soliton regime [9]. For very high bit rate transmission this requires dispersion maps with a disper- sion management period shorter than the amplifier spacing. In such systems pulse spreading, and therefore intrachannel non- linear interaction, is limited through the use of distributed dis- persion compensation. The feasibility of using DMS systems operating at a bit rate of 160 Gb/s has, to date, only been investi- gated numerically [10]–[12]. Good performance is achieved by 0733-8724/04$20.00 © 2004 IEEE