610 Optical System Performance Measurements dispersion maps by choosing different values of the inline dispersion compen- sators in the system. The accumulated chromatic dispersions of the transmis- sion system (excluding pre- and post-compensations) are 941 ps/nm, 1465 ps/ nm, 1734 ps/nm, and 2189 ps/nm for the four dispersion maps. Then, by fine adjusting the pre- and post-compensations through switch selection, the resid- ual dispersion of the optical link between the transmitter and the receiver can be varied around the value of each dispersion map. In this particular example, the transmission performance is optimized when the residual dispersion is in the vicinity of 1500 ps/nm, which depends on the transmitter and the receiver design as well as the chirp of the optical signal. If 1-dB OSNR penalty is allowed, the dispersion window of this system is approximately 1000 ps/nm as illustrated in Figure 5.8.3. 5.8.2 Measurement of R-SNR Due to Fiber Nonlinearity The R-OSNR shown in Figure 5.8.3 was measured with only a single wave- length channel, and the average optical power at the input of each fiber span is approximately þ3 dBm, whereas the optical power at each dispersion compen- sator was at 0 dBm. It is worthwhile to mention that the optimum residual dis- persion value may also depend on the signal optical power level used in each fiber span because the effect of SPM is equivalent to an additional chirp on the optical signal. When the optical power level is low enough, the system is lin- ear and the effect of chromatic dispersion can be completely compensated for, no matter where the dispersion compensator is placed along the system. In a nonlinear system, on the other hand, the nonlinear phase modulation created by the SPM process is converted into an intensity modulation through chro- matic dispersion in the following fiber spans. Therefore, the location of disper- sion compensation is important. Similar to the case of XPM, which was discussed in Section 5.6, per-span dispersion compensation usually works better than lumped compensation either in the transmission side or in the receiver side. Figure 5.8.5 shows the measured R-OSNR in a 12-span amplified fiber-optic system with 80 km of standard SMF in each span. The chromatic dispersion is also compensated at the end of each fiber span. The experimental setup is simi- lar to that shown in Figure 5.8.2, but the R-OSNR measurements were con- ducted at the end of each fiber span. The output optical power level, P, at the output of each inline optical amplifier was adjusted to observe the effect due to nonlinear SPM. Figure 5.8.5 indicates that when the signal optical power level is less than ­4 dBm, the system can be considered linear. A slight R- OSNR degradation can be observed when the signal optical power increases from ­4 dBm to ­2 dBm near the end of the system. R-OSNR degradation becomes significant when the signal optical power increases from ­2 dBm to 0 dBm.