can be done in several ways, among which distributed feedback (DFB) is by far the most used. 5.2.1.2 Semiconductor Laser Modeling and Dynamic Behavior Once confinement is achieved both for the optical field and for the injected charges, the laser can be considered as a waveguide inserted into a resonant cavity with the core sub- ject to population inversion to generate optical gain. A physical modeling of the semiconductor laser would require a quantum representa- tion of both the optical field and the pseudo-particle population inside the crystal. In this way the laser equations will appear as a couple of interacting equations, one for the density matrix elements representing the electron population and the other for the quadrature operators of the field [3]. Even though this analysis is possible, it is quite complicated and, from the point of view of the physical comprehension of laser dynamics, is not more effective with respect to a semiclassical model. In a semiclassical model, the laser behavior is represented with a set of rate equations for the propagating field and for the carrier density. Telecommunication Networks The only element that cannot rise from a semiclassical analysis is the spontaneous emis- sion noise, which is a pure quantum element (compare Section 4.3.1.1). Thus the noise terms have to be added phenomenologically to the semiclassical rate equations [4]. With this in mind, the rate equations can be derived by the coupled field wave equation and electrons balance equation and by applying the slowly varying and rotating wave approximation to the wave equation [5]. At the end of these procedures, calling N(x,y,z,t) the carrier population in the conduction band, S(x,y,z,t) the optical energy inside the cavity, which is proportional to the photon density, and (x,y,z,t) the field phase, the coupled equations of a semiconductor laser can be written as (5.1) The symbols that appear in the rate equations have the following meanings: DN is the carrier diffusion coefficient. In many problems the carr