Effects of FWM and XPM on Long-Haul Design
FWM is a third-order nonlinearity in optical links that can be compared to the intermodulation distortion in standard electrical systems. FWM is worse for equally spaced WDM systems and at high powers. When three optical channels at frequencies ωi, ωj, and ωk travel such that they are close to the zero dispersion wavelength, they intermingle to produce a fourth signal whose frequency is shown in Equation 4-24.
This ωijk can mix with another WDM channel, causing severe cross-talk. For W wavelengths in a fiber, the number of FWM channels (N) produced is shown in Equation 4-25.
Figure 4-12 Equal Channel Spacing (Three Equally Spaced Channels Generated Nine FWM Signals, Out of Which Three Fall on Top of the Signals)
Figure 4-13 Three Unequal Spaced Channels Generating Nine FWM Signals; None of the Generated Signals Falls on Top of the Original Signals
The solution for minimizing FWM is to use unequal channel spacing in such a way that the generated wavelength does not interfere with the signal channel(s). Use of NZDSF minimizes the effect of FWM.
In multichannel WDM systems, XPM causes intensity-based modulation to adjacent frequency channels. XPM causes fluctuations in pulse propagation due to the effect of other channels. Furthermore, if adjacent channels are traveling at the same bit rate, XPM effects are more pronounced. One way to avoid XPM is by carefully selecting bit rates for adjacent channels that are not equal to the present channel bit rate. When designing WDM links, we typically keep a 0.5 dB power penalty margin for both FWM and XPM. XPM has more impact on certain types of modulation formats. Typically, FSK and PSK have a more pronounced impact than pure NRZ and RZ coding.