to the analysis of error correction and compensators for transmission systems and the analysis of the base elements of electronics switching. Telecommunication Networks Only the analysis of the switch fabric itself, starting from the elementary cross-point, is delayed to Chapter 7, in the more suitable framework of the switching and routing machine architectures. 5.2 Semiconductor Lasers 5.2.1 Fixed-Wavelength Edge-Emitting Semiconductor Lasers Semiconductor lasers are the most diffused laser category, whose application ranges from consumer products (like compact disc players) to highly specialized industrial applica- tions like transmitters in optical fiber systems and sensor equipment [1]. Here we will limit our review to two categories of semiconductor lasers that are instru- mental in developing optical transmission systems: transmission lasers and pump lasers. Transmission lasers are used to provide the optical carrier over which the information is coded. Different applications have different requirements for the optical carrier parameters like power, linewidth, wavelength stability, and SO on, as well as different cost targets; thus there are a wide variety of transmission lasers. Depending on the specific application, the modulation of the carrier can be performed by an external modulator (a component completely different from the source laser), by a modulator integrated into the same chip of the laser or by the laser itself, with the so-called direct laser modulation. Something similar happens for pump lasers, whose application ranges from the cheap erbium-doped fiber amplifiers (EDFAs) that could be used in the optical access network to the high-performance distributed Raman amplifiers used in ultra-long haul dense wave- length division multiplexing (DWDM) systems. 5.2.1.1 Semiconductor Laser Principle The principle at the base of the working of a semiconductor laser is to create population inversion between valence and conduction electrons (or holes, depending on the material), exciting electrons from t