wer loss in the DBR structure. In Figure 5.14 three different VCSEL architectures are shown that essentially differ for the way in which the Bragg reflector is realized and, as a consequence, in the mirror's structure and position. Technology for Telecommunications: Integrated Optics and Microelectronics implantation Silicon region oxide Substrate Substrate Back emission Substrate Silicon oxide Electrode FIGURE 5.14 Scheme of three different VCSEL architecture: A VCSEL relying on ion implantation zones for current confine- ment is shown in (a), A VCSEL achieving current confinement via a passivation zone is shown in (b) while a structure using two confinement zones on both sides of the wafer is shown in (c). The architecture shown in Figure 5.14a is the simplest to realize, but also the one with lower performance. One of the fundamental problems of this architecture and, in some way, of all VCSELs, is the fact that the optical mode and the current are parallel, traversing the laser-active zone from top to bottom. To realize current confinement, generally in this type of structure the active region surrounded by a region where the current cannot penetrate, for example, a zone where a proton implantation has been done. The architecture shown in Figure 5.14b presents a passivation level immediately on top of the active zone that has the role of confining the injected current, as the proton-bombed zone in architecture Figure 5.14a. Finally, architecture Figure 5.14c is more complex, requiring to process both sides of the wafer, but provides also better current confinement and in general better performances. In their telecommunication application, where long wavelength lasers are needed to match the fiber transmission windows, VCSELs have also some challenge to overcome to be suitable substitutes of edge-emitting lasers in low-performance applications. The first and perhaps more important point is the emitted power. While long-wavelength VCSELs emit several milliwatts, moving the wavelength toward the telecommunicatio