after detection by a photodiode. After amplification, the signal has the expression (4.72) After detection, several random components are added to the term proportional to the optical power and that constitute the overall noise: The square of the quantum noise due to the input coherent state fluctuations (generally called shot noise) The square of the amplifier quantum noise (also called ASE) A beat term between the signal and the shot noise A beat term between the signal and the ASE noise A beat term between shot and ASE noise indicating these current contributions with c;(t), (i=1,..., 5), we have that the overall current after the photodiode has the expression (4.73) where Rp is the photodiode quantum efficiency. As the noise terms are uncorrelated, their variances can be added. Evaluating all the variances and calculating the SN, the expres- sion of NF can be easily found. In telecommunication however, the used amplifiers are high-gain, low-noise compo- nents; thus, the signal arriving to the photodiode after amplification is orders of magni- tude greater than the power of the shot noise. On this ground, for practical cases, all the terms containing the shot noise can be neglected, besides the term containing the square of the ASE noise, which has quite a smaller power with respect to the term containing the beat between the ASE and the signal. Retaining only the fourth term in the list given earlier, NF is immediately evaluated as (4.74) It is apparent that in the ideal case, perfect population inversion and almost infinite gain, the minimum value of NF is 2. Of course, practical amplifiers are characterized by values of NF greater than 2 and generally, greater the gain, greater the value of NF since in the presence of great power inside the amplifiers the spurious phenomena causing a signal power decrease or a noise increase are more evident. Telecommunication Networks 4.3.1.3 Dynamic Behavior of a Two-Level Amplifier In practical systems, the optical signal to be amplified is modulated to transmit informa-