mitter dynamic range: In this case the difference between the "one" level and the "zero" level is not sufficient to assure correct system working. This effect can be due to a wrong drive of the modulator or by the use of a modulator with unsuitable characteristics. Since a limited dynamic range reflects in a proportional increase of the EOP this has to be specified with great attention before accepting a certain transmitter in the system design. Transmitting laser linewidth: Even if semiconductor lasers are high performing and very stable sources, nevertheless, they have a finite linewidth due to homogeneous broadening of the emitted mode. Generally, lasers used for DWDM transmission have a linewidth on the order of 1 MHz or less and the impact on system perfor- mances is negligible. However, if the linewidth should increase it can disturb the receiver PLL causing timing jitter [2]. Polarization dependent losses: Imperfect connectors positioning or other problems along the line (like transit through patch panels in the metro area) can cause polarization depending losses (PDL). PDL also affects the system performances increasing effective losses and creating unbalance between the output polariza- tion principal states. Excess losses due to passive optics: In a realistic DWDM system there is a lot of pas- sive optics (like connectors, patchcords, etc.) to bring the signal from one card to the other. All these elements introduce losses that can be important in case of bad mounting or of a damaged connector. Amplifiers gain curve ripple: Although gain flattening filters are usually adopted in optical amplifiers to smooth the gain curve, cascading several amplifiers could evidence also small imperfections. If a channel gain is globally greater it can depress the gain of nearby channels creating a relevant penalty. 6.3 Intensity Modulation and Direct Detection Systems Using Optical Amplifiers In the previous section, we have analyzed optical transmission systems without optical amplifiers. Even if all the transmission 