valence band, thus causing population inversion. However, if a simple junction is used, creating the conditions for lasing operation is quite difficult. As a matter of fact, in a structure like that of Figure 5.3, there is no lateral confinement either for the injected current or for the optical field. Thus on the one hand, there is a constant electron loss due to the leak current flowing away from the inversion zone on the side of the device and, on the other hand, the optical mode is very large and the superposition of the mode with the inverted area of the junction where optical gain is concentrated is quite scarce. Telecommunication Networks Light Cleaved facets p-InP Light n-InP 300-500 um FIGURE 5.3 Simpler possible structure of a semiconductor laser. In order to achieve effective lasing operation, both the current and the optical mode have to be effectively confined in the junction area. Several architectures have been proposed to achieve vertical and lateral confinement, at present the most used solution is constituted by heterostructure index-guided architecture [1]. The heterostructure solution consists in realizing a p-i-n junction where the neutral zone is realized in a different alloy with a smaller band gap with respect to the doped zones. The band profile across the heterostructure is shaped approximately as shown in Figure 5.4 when an inverse polarization is applied. The fact that the intrinsic material has a smaller energy gap with respect to the surround- ing doped layers creates a sort of trap for the injected carriers: when the carriers arrive in the intrinsic zone, they fall in the potential energy hole and are confined to the intrinsic zone. If the heterostructure provides confinement in the direction of the current injection, con- finement in the lateral direction has to be achieved via some other system. The most used structure for lateral confinement is the so-called index-confined architec- ture, where the optical mode is confined in the laser-active region by realizing an optical wavegu