imit of LNL 8. This expression is another example of the more general rule to analyze the widening of a pulse traveling through a cascade of filters that is derived in Appendix D. As a matter of fact, the factor that is added to the unit under the square root is exactly the square of the width of an ideal filter that has on the optical pulse the same effect of the dispersion. Thus, AT = T(z) - T(0) is minimum at a distance L from the transmitter, if T(0) = /B2L For an SSMF this means, at a distance of 40 km, T(0) = 0.028 ns that is a short pulse with respect to the duration of 0.1 ns of the bit interval at 10 Gbit/s, but it is not sufficiently short for 100 Gbit/s transmission, where the bit interval is 0.001 ns. Moreover, increasing the distance the optimum pulse becomes longer. This can be understood by observing that, for very large distances, the term containing dispersion is SO high that the other term under square root can be neglected and the final pulse width is inversely proportional to the square of the initial width SO that shorter launched pulses produce longer output pulses. For example, at 10 Gbit/s and with B2=20ps2/km, the optimum pulse at 300 km is 77 ps. Since this is the half width at 1/e, this means that we are quite near NRZ, with a pulse whose real width is 50% wider than the bit interval (if a real transmission would be set up, naturally only the part of the pulse within the bit interval would be transmitted). In the following analysis of 10 Gbit/s systems, we will consider practical pulses never shorter than 10ps and never longer than half the bit interval whatever the distance and the bit rate. In order to define the pulse width we will use the so-called duty cycle of the RZ modu- lation that is defined as (2TR) SO to be in practical cases always smaller than one SO to be measured in percentage unit. In a 10 Gbit/s RZ modulation with a duty cycle of 30% the full width at 1/e of the used pulses is 30 ps out of the 100 ps of the bit interval. The sensitivity penalty of an unrepeated and unc