n region the emitted power decreases and common VCSELs at 1550nm emit a power on the order of 0.1 mW [15], even if particular architectures have demonstrated larger emitted powers. An example of high-performance VCSELs [16] emitting at 1551 nm, whose optical power versus current characteristic is plotted in Figure 5.15, is reported in [16,17], demonstrating that progress in this direction is rapid and products are almost ready to hit the market with suitable emitted power characteristics. A second important challenge is constituted by the operating temperature [18]. Long- wavelength VCSELs are quite temperature-sensitive, while their application target requires uncooled sources. An example is shown in Figure 5.16 [102], where the temperature sensitivity of the bias current of the VCSEL considered in Figure 5.15 is shown. Also from this point of view progress seems to be rapid. Telecommunication Networks Optical power (mW) Temperature (C) FIGURE 5.15 Long-wavelength VCSEL (emitting at 1550nm) cur- rent optical power characteristic. (After RayCan, 1550 nm Vertical-cavity surface-emitting laser, 090115 Rev 4.0.) Bias current (mA) Threshold bias current (mA) FIGURE 5.16 Long-wavelength VCSEL (emitting at 1550nm) current temperature characteristic. (After RayCan, 1550 nm Vertical-cavity surface-emitting laser, 090115 Rev 4.0.) Temperature (C) The third point is direct modulation, which is another key characteristic for low-end source lasers for telecommunication equipment. This point, which was a key issue for a certain time, seems to be solved with more recent VCSEL structures, SO that direct modula- tion at 1.25 and 2.5 Gbit/s seems a consolidate feature. Moreover, the frequency response of long-wavelength direct-modulated VCSELs has an interesting flat characteristic. As a matter of fact, from the analysis of a high-confinement VCSEL it is possible to evalu- ate its response to frequency modulation by obtaining the following approximate equation that is valid for small modulation regimes [19]: H(f)=Ho- (5.13) Techn