regions, respectively indicates a propagation constant a is the wavelength of the lasing mode r is the refraction index Telecommunication Networks The phase 1 accumulated by the lasing mode passing through the Bragg reflector can be derived starting from the properties of Bragg reflectors. The characteristic parameter y of the Bragg reflector can be defined, following [22], as (5.15) where A, K, a, and rb denote the corrugation period, the corrugation coupling coefficient, the loss of the corrugated region, and the equivalent refractive index for the Bragg reflec- tor, respectively. The phase change experimented by the field when reflected by the Bragg reflector can be expressed as a function of the characteristic parameter y and the overall cavity length L as (5.16) Substituting Equations 5.14 and 5.15 in Equation 5.16 the relation determining the possible lasing wavelength of the DBR laser can be obtained [1,21]. In particular, the laser will pass from one wavelength to the other when the index of the Bragg reflector section is changed by changing the carrier density with the injection of a current in the area; in general, however, the mode will start free oscillations only if the index of the phase matching section is suitably adjusted to satisfy the phase-matching condition (Equation 5.15). It is clear that the phase-matching conditions can be attained contemporarily by a set of modes, due to the presence of the addendum 2k in the phase-matching condition. Among the phase-matched modes only one starts laser oscillation: the mode having the highest cavity gain. The cavity gain of a mode in the DBR structure is given by the gain of the gain section minus the losses in the phase-matching and Bragg sections. But these losses are again controlled by the carrier densities; thus the specific mode to start laser oscillations among those permitted by the phase-matching condition can be selected by correctly polarizing the three laser regions. The DBR laser is a good tunable laser, but the tuning range is just slightly