To describe the laser behavior on the grounds of Equation 5.1 what we have assumed up to now is not sufficient. As a matter of fact, these equations are stochastic equations that cannot be solved if the distribution of the noise term is not known. The most correct distribution of the terms Fj (j = S, N, 0) is not Gaussian: it is sufficient to consider that the term representing the shot noise (i.e., the fluctuation of the optical power) should be distributed following a Poisson distribution. However, in the case of a stable lasing operation, it is possible to demonstrate that, due to the fact that the number of excited carriers and of the photons is very high, the Gaussian approxi- mation implies a small error, in line with the overall accuracy of the semiclassical model. Starting from Equations 5.1 and from the laser structure the main characteristics of a specific type of semiconductor laser can be derived. Despite the intrinsic complexity of their behavior, gain saturation happens also in semiconductor lasers. In particular, due to the presence of the saturation constant, increasing the power in the cavity, the laser inner gain decreases SO that the dependence of the emitted power from the pump is not exponential as foreseen in a laser without gain saturation. Moreover, the dependence of the carrier density on a temperature typical of a p-i-n junction induces a sensible dependence of the curve relating the pumping current and the emitted power on temperature. TABLE 5.1 Values of the Microscopic Parameters of a DFB Used at the Transmitter in DWDM Systems Active layer thickness (da) 100-200 nm Optical confinement factor (y) Carrier lifetime (TN) Photons lifetime (Tp) Gain coefficient (GN) 3x10-12 m³/s Carrier density at transparency (NT) 1024 mm3 Coherence time (s) Saturation carrier density (no) (m-3) 10-23 Local saturation power (1/0) (mW) Linewidth enhancement factor (x) 6 (1.5 MQW) Telecommunication Networks A typical emitted power versus current characteristic of a DFB laser is shown in Figure 5.6. From the