Control of XPM Interaction

Since XPM affects the performance of all WDM systems, a lightwave system must be designed to control its impact so that it can operate reliably. In practice, the dominant contribution to XPM affecting the performance of a specific channel comes from the two channels that are its nearest neighbors in the spectral domain. The XPM interaction between neighboring channels can always be reduced by increasing the channel spacing. A larger channel spacing increases the mismatch between the group velocities at which pulses in each channel propagate through the fiber link. As a result, pulses cross each other so fast that they overlap for a relatively short duration, resulting in a much reduced XPM interaction. This scheme is effective but it reduces the spectral efficiency as channels must be spaced farther apart. XPM effects can also be reduced by lowering channel powers. However, a reduction in the channel power also lowers the SNR at the receiver. In practice, channel powers cannot be reduced below a critical value set by the SNR requirements.

A simple scheme, often employed in practice, controls the state of polarization (SOP) with which each channel is launched into the fiber link [52]. More specifically, individual channels are launched such that any two neighboring channels are orthogonally polarized. In practice, even- and odd-numbered channels are grouped together and their SOPs are made orthogonal before launching them into the fiber link. This scheme is sometimes referred to as the polarization channel interleaving technique. The XPM interaction between two orthogonally polarized does not vanish but its strength is reduced significantly. Mathematically, the analysis is complicated because one must take into account the vector nature of the electromagnetic field within the fiber [4], It turns out that the coupled NLS equations, Eqs. (4.2.2) and (4.2.3), can still be used, provided the factor of 2 appearing in the XPM term is replaced with 2/3. It is this reduction in the XPM strength that reduces the magnitude of the XPM-induced phase shift and improves the system performance when neighboring channels of a WDM system are orthogonally polarized.

It may appear surprising that such a scheme works in spite of birefringence fluctuations that change the SOP of each channel and produce polarization-mode dispersion (PMD). Indeed, the SOP of all channels changes in a random fashion along the fiber in any realistic lightwave system. The vector theory of XPM capable of including the PMD effects (see Section 4.7) shows that the XPM-induced crosstalk is reduced considerably even for two copolarized channels simply because the channels do not remain copolarized as they propagate within the fiber link [53]. For the same reason, the effectiveness of polarization-interleaving technique is reduced. However, this technique is still useful in practice for dense WDM systems in which any two neighboring channels differ in their wavelengths by a relative small amount (typically <1 nm). Because of a small difference in the carrier frequencies, the SOPs of the two neighboring channels can remain nearly orthogonal over relatively long distances, and XPM effects can be reduced by launching alternate channels with orthogonal SOPs [52],

0 0

Post a comment