ter different refractive indices, they Chapter 6 Optical Fiber Transmission Transmission Fiber Fig. 6.17 Schematic of pulse spreading due to chromatic dispersion, as represented by packets of wavelengths. travel at fractionally different speeds and therefore the pulse spreads in time, as illustrated in Figure 6.17. For this reason we often speak of "Group Velocity Dispersion" (GVD), which means the same as "chromatic dispersion." When a pulse propagates a distance, L, in a fiber, two of its spectral components with a wavelength separation, AA, experience transit times that differ by AT, known as the "group delay," given by Wilner and Hoanca (2002): AT = DLAN (6.26) in which D is called the "dispersion coefficient" of the fiber. D can be specified in wavelength space by (6.27) Neff is the fiber's effective index and a is the average wavelength of the spectral compo- nents. If we were to substitute expressions for Neff from fiber waveguide theory (Ramaswami & Sivarajan, 2002), we could identify the two main constituents of D: D=Dmat+DWG, (6.28) They are the material and waveguide components, the physical significance of which is as previously discussed. (In fact, there is a third term, the "profile dispersion," but it is small and can often be neglected [Mynbaef & Scheiner, 2001].) Equation 6.26 describes a linear increase of the group delay with the fiber length. The parameter D is measured in ps/(km.nm), which can be interpreted as picoseconds of group delay per kilometer traveled in the fiber per nanometer of bandwidth. Dmat and DWG are both wavelength dependent and can be positive or negative. This means that D can be posi- tive, negative, or zero. When D is negative, the fiber is said to be in the "normal dispersion regime"; when D is positive, the fiber is in the "anomalous dispersion regime." It is very important to stress that propagating pulses broaden for all nonzero values of chromatic dispersion. When D is negative, the long-wavelength components travel fastest but when it is positive, the short-wavelen