red end of the spectrum. This causes an overall spreading or smearing of the transmitted signal- a phenomenon known as chirp. It occurs in fiber systems that transmit a single pulse down the fiber and is proportional to the amount of chro- matic dispersion in the fiber: the more chromatic dispersion, the more SPM. It is counteracted with the use of large effective area fibers. Cross-Phase Modulation (XPM) When multiple optical signals travel down the same fiber core, they both change the refractive index in direct proportion to their individual power levels. If the signals happen to cross, they will distort each other. While XPM is similar to SPM, there is one significant difference: While self- phase modulation is directly affected by chromatic dispersion, cross- phase modulation is only minimally affected by it. Large effective area fibers can reduce the impact of XPM. Four-Wave Mixing (FWM) FWM mixing is the most serious of the power/refractive index-induced nonlinearities today because it has a catastrophic effect on DWDM- enhanced systems. Because the refractive index of fiber is nonlinear and because multiple optical signals travel down the fiber in DWDM sys- items, a phenomenon known as third-order distortion can occur that seriously affects multichannel transmission systems. Third-order dis- tortion causes harmonics to be created in large numbers that have the annoying habit of occurring where the actual signals are, resulting in their obliteration. Four-wave mixing is directly related to DWDM. In DWDM fiber sys- items, multiple simultaneous optical signals are transmitted across an optical span. They are separated on an ITU-blessed standard transmis- sion grid by as much as 100 GHz (although most manufacturers today have reduced that to 50 GHz or less). This separation ensures that they do not interfere with each other. Consider now the effect of dispersion-shifted fiber on DWDM sys- tems. In DSF, signal transmission is moved to the 1,550 nm band to ensure that dispersion and loss are both minimized with