ning, a suitable design can achieve also semi-continuous tuning, at the expense of a more complex control algorithm. Most of the features of the SG-DBR are shared by the SSG-DBR [26] design. In this case, the desired multi-peaked reflection spectrum of each mirror is created by using a phase modulation of grating rather than an amplitude modulation as in the SG-DBR. Periodic bursts of a grating with chirped periods are typically used. This multielement mirror structure requires a smaller grating depth and can provide an arbitrary mirror peak amplitude distribution if the grating chirping is controlled. Using SG-DBR or SSG-DBR, discontinuous tuning ranges as wide as 60nm have been reached, arriving in this way to oversatisfy the requirements. On the positive side, they are compact, they maintain the properties of fixed-wavelength lasers, and, perhaps more importantly, they are suitable to be integrated with other devices leveraging the InP platform. FIGURE 5.20 Lateral section of a multisection sampled grating DBR (SG-DBR) tunable laser. In the lower part of the figure the Vernier effect used to tune the laser by superimposing only one passband of the two gratings that create the laser cavity is also illustrated. Telecommunication Networks For example, if the output power has to be increased, it is possible to integrate immedi- ately out of the laser, on the same waveguide, a semiconductor optical amplifier to achieve high output power, or if a compact component that can be directly modulated is needed, an InP modulator (either Mach-Zehnder or electro-absorption [EA]) can be integrated at the laser output. Moreover, the typical reliability of integrated components is higher than that of compo- nents composed by different and possibly moving macroscopic parts. On the negative side, there is the complex electronic control, which is more complex for more performing structures and the difficult processes needed to produce these lasers. 5.2.4.2 External Cavity Lasers Limitation in tuning range experienced by integrated