, the fourth is the background noise, which is the current generated by the detection of environ- mentally generated photons. Telecommunication Networks Summarizing, the photocurrent is composed of four terms, represented in the following equation: c(t)=RpP(t)+nsh(t)+ndark(t)+nth(t)+ne(t) (5.27) The noise term can be considered in general as Gaussian white noise. Equation 5.27 holds as far as either the incoming signal bandwidth does not exceed the photodiode bandwidth or the incoming power does not go above the photodiode satura- tion power. In the first case, due to the finite time response of the photodiode essentially determined by spurious and junction capacities, the photodiode works as a lowpass filter, cutting too high frequencies. In the second case, the linear relation between average current and optical power does not hold anymore and the photocurrent tends to saturate, increasing the incoming optical power. The second type of photodetector is the so-called APD [30]. APDs are high-sensitivity, high-speed semiconductor light sensors. Compared to regu- lar PIN photodiodes, APDs have a depletion junction region where electron multiplication occurs, by application of an external reverse voltage. The resultant gain in the output signal means that low light levels can be measured and that the photodiode bandwidth is very large. Incident photons create electron-hole pairs in the depletion layer of a photodiode structure and these move toward the respec- tive p-n junctions at a speed of up to 105 m/s, depending on the electric field strength. If the external bias increases this localized electric field to about 105 V/cm, the carriers in the semiconductor collide with atoms in the crystal lattice with sufficient energy to have a nonnegligible ionization probability. Ionization creates more electron-hole pairs, some of which cause further ionization, giving a resultant gain in the number of electron-holes generated for a single incident photon. Naturally the avalanche process is not a deterministic one and the g