 or three sections along the cavity [20]. The tuning function is provided by varying the injection current ratio into two sections. A typical geometry for this kind of laser is shown in Figure 5.18. Tuning in these devices results from the combined effects of shifting the effective Bragg wavelength of the grating in one or several sections and the accompanying change of the optical path length for the change of the refractive index itself. Combining these effects with thermal discontinuous tuning, a bandwidth on the order of 15 nm can be achieved, which is much more than pure thermal tuning, but it is not enough to cover the entire C band. Much wider tuning is achieved using multisection DBR lasers. A principal scheme of a tunable DBR is represented in Figure 5.19 [21]. The central region of the laser is an active region where a heterojunction is realized to allow population Technology for Telecommunications: Integrated Optics and Microelectronics p-InP FIGURE 5.18 n-InP Lateral section of a multisection tunable DFB laser. Bragg Phase Active reflector matching FIGURE 5.19 Lateral section of a multisection tunable DBR laser. inversion and optical gain. The laser cavity is realized by cleaving one of the active region facets and providing, on the other hand, wavelength-selective feedback through a Bragg grating realized in a portion of the laser waveguide external to the active region, called the Bragg grating region. Between the active region and the Bragg grating region there is an intermediate part of the waveguide that is used to adapt the field phase to the cavity length through the change of the refraction index caused by carrier injection. This zone is called phase adaptation zone. In order to satisfy the resonance requirements, the laser mode has to satisfy the relation 01-O2=2k where O1 and O2 are phase change of the Bragg reflector in the active and the phase control regions, respectively k is an integer Phase change O2 can be written as (5.14) where subscripts a and p denote the active and phase control 