nt but much smaller) in the adjacent bit times, the modulation is called non-return to zero (NRZ). As a matter of fact, the optical power does not return to zero between the transmission of two consecutive ones and a series of con- secutive ones corresponds to a constant signal. If the pulse m(t) occupies a part of the bit time, leaving a guard time among consecutive pulses SO that the signal returns to zero (or to a small value) between two consecutive ones, the modulation is called return to zero (RZ). Logically, the modulation operation is divided into two steps: the binary message is coded on the electrical carrier (operation that in real systems is done by the remote signal originator) and the obtained current is coded by the modulator onto the CW field emitted by the transmitting laser. The field is then coupled with the transmission line injecting it in an optical fiber. After coupling loss, the signal at the fiber input contains the information in the slow varying part of the optical field. In telecommunication systems, almost only single-mode fibers are used; thus, the propagating field is also single mode. From Equation 4.2, we can write this signal as follows, where the mode transversal shape and the modulation pulses have been considered normalized to one and Pin is the average optical power injected into the fiber in the absence of modulation (6.8) After ideal propagation, the field that arrives in front of the receiver has the following expression: (6.9) where L is the fiber length a is the attenuation T is the propagation time M is the Jones matrix that in the ideal case does not depend on frequency After detection by the receiving photodiode, the electrical current is proportional to the optical power; thus, in the ideal case in which all the incoming power is detected by the photodiode, the electrical current writes (6.10) where Cbias is the detection circuit bias current Rp is the photodiode sensitivity Pout=Pine-aL ne(t) is the detector noise process Transmission Systems Architectures and Perfo