h a dielectric material, the glass, and under suitable conditions the behavior of the glass departs from a linear characteristic. This is illustrated in Figure 5.20, where the input to output relationship is no longer linear. The study of nonlinear optical effects belongs to the field of nonlinear optics, but because the effects on optical fiber communications are serious, a brief outline will be pre- sented here (see also Chraplyvy et al., 1984). The origin of nonlinear optical effects lies in the response of the atoms and molecules of the glass dielectric in high electric fields. One might be surprised that in spite of the low optical powers used in fiber communications, the nonlinear behavior of the fiber impairs system performance. The electric field in a medium with effective area Aeff and power P is given by (5.48) With a semiconductor laser of 10 mW and a fiber with an effective area of 53 um ², the electric field in the fiber is 22010-3010 The electric field experienced by the electron in the hydrogen atom is of the order of X 1011 V/m, SO one could think that the field generated by a laser beam of 10 mW would not give rise to nonlinear optical effects. Although the optical power is low, the interaction length between the optical radiation and the glass dielectric can be very long when the fiber operates with optical amplifiers. The effect is cumulative, and system performance is impaired. The channel power used in this simple example refers to one laser, but in a WDM system the number of channels can exceed 100, SO the total power can be very high. In nonlinear effects associated with the propagation of light in fibers there are two fiber parameters that dimension the impact of the effects. The first is the effective area of the radiation mode, Aeff. It is defined as shown in equation 5.51: Nonlinear Optical Effects in Fibers Radius (Aeff/It) 1/2 Radius Fig. 5.21 Concept of the effective area of Aeff. Nonlinear Nonlinear Linear Propagation Propagation Propagation Threshold Linear Propagation Fiber Length