onductor Amplifiers Before closing the section on semiconductor lasers a brief presentation of an SOA is needed. If the reflectivity of the chip facets is eliminated from the structure of an FP laser the chip cannot generate self-oscillation due to lack of feedback, but it can maintain an optical gain due to population inversion in the heterojunction depletion zone. The result is a traveling-wave semiconductor amplifier. The dynamics of an SOA is from several points of view similar to that of an ideal amplifier described in Section 4.3.1 with the advantage of having a broad gain curve and a small form fac- tor. Two phenomena limit the use of SOAs in telecommunication systems as signal amplifiers: Telecommunication Networks 1. As reported in Table 5.1 the typical carrier's lifetime is on the order of a few nanoseconds, that is, the order of magnitude of the bit time in 10 and 40 Gbit/s transmissions. Thus we have to expect that gain saturation happens during the amplification of signal pulses creating nonlinear pulse distortion as analyzed in Section 4.3.1.3. 2. The carrier dynamics in a semiconductor creates a link between the phase and the amplitude of the amplifier field SO that pure amplitude gain cannot be achieved, but an induced chirp is always present. Due to these elements the only practical use of SOAs as linear amplifiers is in InP- integrated components where an SOA section is often used to compensate the losses of attenuating components like multiplexers. The nonlinear dynamics of SOAs is on the contrary an opportunity if they are used not as linear amplifiers but as elements in optical processing circuits. This is the reason why a more detailed analysis of SOAs, with a particular attention to their nonlinear dynamics, will be delayed up to Chapter 10, where optical signal process- ing in new-generation networks will be discussed. 5.4 PIN and APD Photodiodes A photodiode is in general terms a diode whose structure has been designed SO as to allow the incoming light beam to reach a zone very near to th