ide whose core is the active region itself. In this way the different materials pro- vide confinement of the current and the diffraction index difference provides confinement of the field in the same region. A transversal section of a buried heterostructure laser is represented in Figure 5.5. The gray area represents the active waveguide that is buried below the surface structure and : Conduction band edge Ef: Pseudo-Fermi level Optical transition E: Valence band edge FIGURE 5.4 Schematic band profile of a heterojunction in inverse polarization. Technology for Telecommunications: Integrated Optics and Microelectronics n-InP n-InP p-InP p-InP i-InGaAsP p-InP n-InP Figure 5.5 n-InP Transverse section of a buried heterostruc- ture laser. assures contemporary good lateral current confinement and good superposition between the optical mode and the inverted region. Once inversion is effectively realized, in order to start the lasing operation, a positive counterreaction has to be introduced via an optical cavity to sustain spontaneous oscilla- tions of the structure. The easier way to do this is to cleave the front and rear facets of the laser to create a reflectivity. The inner laser waveguide behaves like a Fabry-Perot (FP) cavity assuring the needed feedback to the laser. The structure of an FP laser is schematized in Figure 5.3. With the typical dimensions of semiconductor lasers, the FP cavity has a free spectral range SO small that several modes are enhanced by the optical gain. Thus in general, FP semiconductor lasers oscillate in multimode regime even if, with some care in designing the laser, few modes can be selected. Even when single-mode oper- ation is achieved by using strongly wavelength-dependent mirrors (that can be realized by multiple layer coating of the FP facets) this is not a stable condition and side modes can start to oscillate due to environmental fluctuations or laser modulation. In order to achieve real single-mode operation a strongly wavelength-selective feedback has to be introduced. This 