7.1 Introduction

Considering that optical sources with wavelength specifications enabling smaller and smaller channel spacings are becoming available, and that components with almost arbitrarily high-channel counts are feasible, one could think that the number of channels would not be limited. Of course, this is not true: Optical nonlinearities set limits on the optical power that can be transmitted on the wavelength channels of the optical fiber.

The nonlinearities are related to:

• Index variation caused by intense electrical fields such as:

  • Self-phase modulation (SPM);

  • XPM;

  • FWM;

• Scattering effects such as:

  • Stimulated Brillouin scattering (SBS);

  • Stimulated Raman scattering (SRS).

All these nonlinear effects are locally rather small, but they become important by accumulation over long fiber lengths. They generally degrade the transmission quality through signal losses, intra- or interchannel crosstalks, pulse broadening or induced jitters. Of course the nonlinear effects can be advantageous in other cases such as: soliton transmission, amplification, and wavelength conversion. In this chapter, we consider the limiting effects only. The other effects are discussed in different chapters of this book (soliton: Section 2.9, amplification: Sections 5.2 to 5.9, conversion: Section 4.5).

7.2 SPM

In long-haul systems high powers are required to reach reasonable optical amplifier spacing at high bit rate. Index variations, due to the intense electrical field of the light signals, cause the Kerr effect to occur, and thus induces changes in the signal phase, leading to SPM. This index variation modulates the signal phase, which, in turn, modifies the signal spectrum. The pulse is broadened by...