ompensated RZ system using pulses of 25 ps is shown in Figure 6.14 for propagation on an SSMF and different values of the PMD parameter. The effect of PMD is evident also at short distances due to the short pulses. For uncompensated systems on high dispersion fibers, some working zones can be indi- viduates, where there is a partial compensation between the chromatic dispersion and the Sensitivity penalty (dB) 0.025 (ps/nm/km) PMD (ps/vkm) No PMD PMD 1 PMD = 0.4 Distance (km) FIGURE 6.14 Sensitivity penalty of an unrepeated and uncompensated RZ system using pulses of 25 ps for propagation on an SSMF and for different values of the PMD parameter. System parameters are reported in Table 6.1 but for fibers parameters that are reported in the suitable table of Chapter 4. Telecommunication Networks SPM induced chirps. Although this phenomenon is not particularly useful in unrepeated system, it is important as the base on which the design of nonlinear aided WDM system is founded [8]. 6.2.6 Unrepeated Wavelength Division Multiplexing Systems The huge bandwidth available in the third transmission window of optical fibers naturally move the interest toward frequency multiplexing, that in the field of optical transmissions is called WDM. In general, WDM systems are called DWDM if the channels are a few GHz far, one from the other. Typical channels' spacing for DWDM systems at 10 Gbit/s are 50 or 100 GHz, while for 40 Gbit/s a typical spacing is 200 GHz. DWDM systems require frequency stabilized sources and suffers sizeable interference among adjacent channels due to different phenomena. Differently, CWDM (coarse WDM) systems have 20 nm spaced channels, SO that up to 18 channels can be used in the low attenuation bandwidth of an optical fiber, if the water absorption peak is not present (compare Chapter 4). The 20nm spacing is chosen on the ground of the standard characteristics of semicon- ductor lasers, SO to be able to use uncooled sources to reduce the system cost. CWDM systems are mainly used in the metro and metro acc