Measurement of Nonlinear Crosstalk in Multi-Span WDM Systems 575 modulation efficiency f XPM =P k ð0Þ. The wavelength spacing between the pump and the probe in this measurement was 3.7 nm. Figure 5.6.15(a) was obtained in a system with two fiber spans, each having 25 km standard SMF. For Figure 5.6.15(b) there were three fiber spans in the sys- tem, again with 25 km standard SMF in each span. In both cases, optical power at the input of each span was set to 7 dBm. It is obvious that if there is no optical amplifier between fiber spans, the XPM index versus modulation frequency decreases monotonically showing a lowpass characteristic. This is shown in Fig- ure 5.6.15 as the dotted lines that were calculated with a single 50 km (a) and 75 km (b) fiber span. When an optical amplifier was added at the beginning of each 25 km fiber span, the XPM indices varied significantly over the modulation fre- quency. Clearly, this is due to coherent interferences between XPM-induced phase modulations produced in different fiber spans. Compared to the XPM-induced intensity modulation, XPM-induced phase modulation tends to diminish at high modulation frequency, especially when the chromatic dispersion of the fiber is high. From this point of view, XPM-induced intensity modulation discussed in the last section is probably the most damaging effect on high-speed optical systems. It is important to point out that XPM is the crosstalk originated from the inten- sity modulation of the pump, which results in the intensity and phase modulations of the probe. In optical systems based on phase modulation on all wavelength chan- nels, XPM will not exist, in principle, because the pump has a constant optical