fier). Unfortunately, as will be explained when we discuss nonlinear optical transmission, high propagating powers increase the likelihood of unacceptable nonlinear cross-talk in the optical fiber. The nonlin- ear optical effects can sometimes cause even greater bit corruption than the various noise terms. In real systems what we observe is a "BER floor." Increasing the incident optical power reduces BER to an extent, after which nonlinear optical effects become dominant and improvement is no longer possible. A key challenge for optical designers is to optimize their systems to provide acceptable BER within a design window in which the transmitted powers are sufficiently high to ensure good SNRs but sufficiently low to avoid nonlinear crosstalk. Some of the issues that designers must consider are explained in the final section of the chapter. Chromatic Dispersion: System Implications Chromatic dispersion causes pulses to spread in time, and this can limit the capacity of a system. This section provides an overview of the origin and implications of chromatic dis- persion and discusses the tolerance of systems toward it. This section refers exclusively to single-mode fibers. Multimode operation is explained in Chapter 5. Origin of Chromatic Dispersion The most important (and most easily understood) design of optical fiber is the step index structure. This type has only two refractive indices: a relatively high value for the core and a lower value for the cladding. Light propagates in optical fibers as guided waves. High- capacity optical systems nearly always operate in the fundamental waveguide mode, which has an intensity distribution that is a rotationally symmetrical bell-shaped function (Mynbaef & Scheiner, 2001). The main constituent of optical transmission fibers is vitreous silica (SiO2) because when purified it is highly transparent. The core region is created by adding a few percent germania (GeO2) or other dopants to raise the refractive index. Alternatively, fluorine (F) used as a dopant can lower the r