It is clear from the pseudo-linear lightwave systems tutorial that intrachannel XPM and FWM can limit the performance of a pseudo-linear system. Both of these effects may occur even for systems making use of dispersion-managed (DM) solitons because pulses overlap partially during each map period. It is thus important to find ways to reduce their impact through a proper system design. In this tutorial, we focus on several such schemes.
Pseudo-linear lightwave systems operate in the regime in which the local dispersion length is much shorter than the nonlinear length in all fiber sections of a dispersion-managed link. This approach is most suitable for systems operating at bit rates of 40 Gb/s or more and employing relatively short optical pulses that spread over multiple bits quickly as they propagate along the link. This spreading reduces the peak power and lowers the impact of SPM on each pulse. There are several ways one can design such systems. In one case, pulses spread throughout the link and are compressed back at the receiver end using a dispersion-compensating device. In another, pulses are spread even before the optical signal is launched into the fiber link using a DCF (precompensation) and they compress slowly within the fiber link, without requiring any post-compensation.
Dispersion management is employed commonly for modern WDM systems. Since such systems use fiber sections with both normal and anomalous GVDs, one may ask what happens to solitons in this case. It turns out that solitons can form even when the GVD parameter β2 varies along the link length but their properties are quite different. This tutorial is devoted to such dispersion-managed solitons. We first consider dispersion-decreasing fibers and then focus on fiber links with periodic dispersion maps.
Solitons form in optical fibers because a balance between the chirps induced by GVD and SPM, both of which limit the system performance when acting independently. To understand how such a balance is possible, we follow the analysis of the dispersion-induced pulse broadening tutorial and the nonlinear optical effects tutorial. As shown there, the GVD broadens an optical pulse during its propagation inside an optical fiber, except when the pulse is initially chirped in the right way. More specifically, a chirped pulse is compressed during earlier stages of propagation whenever β2 and the chirp parameter C happen to have opposite signs so that β2C is negative. As discussed in the nonlinear effects tutorial, SPM imposes a chirp on the optical pulse such that C > 0. If β2 < 0, the condition β2C < 0 is readily satisfied. Moreover, as the SPM-induced chirp is power-dependent, it is not difficult to imagine that under certain conditions, SPM and GVD may cooperate in such a way that the SPM-induced chirp is just right to cancel the GVD-induced broadening of the pulse. In this situation, an optical pulse propagates undistorted in the form of a soliton.
With the use of dispersion compensation, dispersion ceases to be a limiting factor for lightwave systems. Indeed, the performance of modern long-haul systems is typically limited by the nonlinear effects. In this tutorial, we focus on the techniques used to manage the nonlinear effects.
The use of dispersion management in combination with optical amplifiers can extend the length of a WDM system to several thousand kilometers. If the optical signal is regenerated electronically every 300 km or so, such a system works well as the nonlinear effects do not accumulate over long lengths. In contrast, if the signal is maintained in the optical domain by cascading many amplifiers, the nonlinear effects such as self-phase modulation (SPM) and cross-phase modulation (XPM) ultimately limit the system performance. For this reason, the impact of nonlinear effects on the performance of dispersion-managed systems has been investigated extensively. In this section we study how the nonlinear effects influence a dispersion-managed system and how their impact can be minimized with a suitable choice of system parameters.
