 the form of the RIN spectrum. A selection of characteristics of practical FP diode lasers for different applications is reported in Table 5.2. From the table the wide span of applications covered by FP lasers corresponding to the possibility of matching different set of requirements is evident. 5.2.1.5 Source DFB Lasers In a DFB laser the feedback is obtained by building a grating immediately on top of the active zone or on the side of it, influencing the propagation of the tail of the guided mode. Telecommunication Networks RIN (dB) FIGURE 5.11 RIN spectrum induced by relevant mode partition noise in a multimode FP laser. (After Wentworth, R. H. et al., J. Lightwave Technol., 10(1), 84, 1992.) Frequency (GHz) TABLE 5.2 Characteristics of Practical FP Diode Lasers for Different Applications Source for Free Space Wireless Systems Transmitter for Datacom Remote Sensing Client Card Short High-Power External Client Interface Application Reach or CWDM Cavity Lasers Telecom Fiber window C band (L optional) C band Second window Direct modulation 2.5 Gbit/s 2.5 Gbit/s CW optical power 160 mW 0.3 mW Cooling Package TO 18 Threshold current 50 mA Operating current 450mA 16mA (CW) 32mA (pulse, duty rate 50%) Peak wavelength 1530, 1550, 1570nm Custom in C band 1310nm Spectral width 2.5nm Operating temperature -40°C + 85°C -20°C + 70°C 20°C+70°C Notes MQW active region MQW active region All data at 25°C All data at 25°C All data at 25°C Typical values Typical values Typical values Via interaction with the mode tail, the grating blocks the amplification of all the wave- lengths, save the one resonating with the grating itself, thus guaranteeing single-mode operation. The scheme of a DFB semiconductor laser is shown in Figure 5.12 while its single-mode spectrum is shown in Figure 5.13. Altering the temperature of the device causes the pitch of the grating to change due to the dependence of the refractive index on temperature. This dependence is caused by a change in the semiconductor laser's bandgap with temperature and thermal