ns, from telecommunications to consumer electronics to sensors and SO on. Technology for Telecommunications: Integrated Optics and Microelectronics Wavelength (nm) FIGURE 5.10 Spectrum of an FP InGaAsP laser emitting in a quasi-single-mode regime. In telecommunications such lasers are adopted at transmitters either to generate the carrier or to generate the information-carrying signal through direct modulation when single-mode laser operation is not required but the cost of the source is a key issue. As a matter of fact, due to the simpler structure and the great volume production, FPs are cheaper with respect to single-mode lasers like DFB. Thus, FPs are the standard optical sources in several types of access systems, from point- to-point to A and B class G-PON. Moreover, such lasers are used in the low-performance client cards of transmission systems, where it is needed to connect systems in the same central office and the connection speed is not SO high. The typical emission spectrum of an FP InGaAsP laser emitting in a quasi single-mode regime is plotted in Figure 5.10. The side modes suppression ratio (SMSR) is about 19 dB, which is much lower than the typical 35-40 of a DFB laser, but is enough for several applications, especially if the laser has not to be modulated. The presence of a few lasing modes in an FP laser generates the growth of a particular noise phenomenon when it is directly modulated: the mode partition noise [13]. Mode partition noise is due to the random fluctuations of the lateral modes that causes a change in the spectrum and a fluctuation in the power of the main mode due to the fact that the steady-state power emitted from the laser in a first approximation is constant and depends only on the pump and on the laser quantum efficiency. The effect is an enhancement of the RIN well above the limit given by the spectrum in a strict single mode operation. This effect is shown in Figure 5.11 where the fact that the sensible amount of RIN is mainly due to the mode partition noise is clear from