ependent of the propagating intensities. This section examines a number of ways in which optical fiber transmission systems conform to rather more complicated behavior, designated "nonlinear optics." Nonlinear Refractive Index Transmission in single-mode optical fibers can differ significantly from the simple operating principles of linear optics, because of a number of factors (Agrawal, 2001): The single-mode fiber maintains a high optical intensity over possibly many kilometers. Because of the need to provide good power spectral density at the detector, the launched powers in a high-bitrate system might often have to be several milliwatts per channel. DWDM systems can carry many channels, often with close wavelength separations. Longer-span systems require the use of optical amplifiers, which periodically increase the optical powers. The required pulses are shorter as the bitrate is increased and SO the instantaneous powers at their peaks can be very high. Collectively, these factors cause the propagation in the fiber to differ markedly from the linear regime via a number of effects. In this section, we describe the factors that are based on electron oscillations, commonly called "electronic nonlinearities." The phenomenon of (linear) refractive index results from the excitation of oscillations of bound electrons within the optical fiber's glass by the traveling wave. However, when the optical intensity is high, the oscillations are anharmonic and SO must be modeled by nonlinear equations (Butcher & Cotter, 1990). (The term "nonlinear optics" comes from these wave equations.) The anharmonic electronic oscillations produce several effects depending Nonlinear Optical Transmission on the optical intensities, the wavelengths present, the states of polarization, the nature of the fiber's glass, and the temporal characteristics of the pulses. An underlying phenomenon is the intensity-dependent refractive index, which commonly called the "nonlinear Kerr effect." An additional component is added to the refractive index