Page 264 - Optical Communications Essentials

Performance Impairments

254 Chapter Fifteen

to send an NRZ-encoded signal over this distance and the power-penalty
requirement is that the pulse spread can be no more than 10 percent of a pulse
width T. In this case the maximum possible data rate is
0.1
Maximum data rate 20Gbps
∆t PMD
Whereas several methods exist for mitigating the effects of chromatic disper-
sion, it is more difficult to compensate for polarization mode dispersion. This is
so because PMD varies with wavelength and slowly drifts with time in a ran-
dom fashion. Since this factor requires any compensation technique to adapt
dynamically to polarization state changes while the system is running, no prac-
tical PMD compensation method has been implemented as yet (for further
details see the paper by Sunnerud et al.).

15.5. Nonlinear Effects

This section addresses the origins of the two nonlinear categories and shows the
limitations they place on system performance. The first category encompasses
the nonlinear inelastic scattering processes. These are stimulated Raman scat-
tering (SRS) and stimulated Brillouin scattering (SBS). The second category of
nonlinear effects arises from intensity-dependent variations in the refractive
index in a silica fiber. This produces effects such as self-phase modulation
(SPM), cross-phase modulation (XPM), and four-wave mixing (FWM). In the lit-
erature, FWM is also referred to as four-photon mixing (FPM), and XPM is
sometimes designated by CPM. Table 15.1 gives a summary of these effects.
The SBS, SRS, and FWM processes result in gains or losses in a wavelength
channel that are dependent on the optical signal intensity. These nonlinear
processes provide gains to some channels while depleting power from others,
thereby producing crosstalk between the wavelength channels. In analog video
systems, SBS significantly degrades the carrier-to-noise ratio when the scattered
power is equivalent to the signal power in the fiber. Both SPM and XPM affect only
the phase of signals, which causes chirping in digital pulses. This can worsen pulse
broadening due to dispersion, particularly in very high-rate systems ( 10Gbps).
Viewing these nonlinear processes in a little greater detail, Sec. 15.5.1 first
shows how to define the distances over which the processes are important.
Sections 15.5.2 and 15.5.3 then qualitatively describe the different ways in
which the stimulated scattering mechanisms physically affect a lightwave

TABLE 15.1. Summary of Nonlinear Effects in Optical Fibers

Origin Single-channel Multiple-channel
Index-related Self-phase modulation Cross-phase modulation
Four-wave mixing
Scattering-related Stimulated Brillouin scattering Stimulated Raman scattering

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