 wider than 5 nm and never exceeds 10 nm, since it is essentially due to the change of the index in the Bragg section (the other indexes are changed to satisfy the phase matching and to select the correct mode, and do not directly contribute to tuning); thus the relationship AN/2 ~ Ar/r still holds and the limited changing range of the refraction index defines the changing range of the emitted wavelength. It is possible to combine index-driven tuning with thermal tuning and in this way a tuning range as wide as 22 nm has been reached with simple DBR structures. In order to achieve much wider tuning ranges, the tuning cannot be based only on index changes. Now it is useful to remember that only three causes can generate the tuning of a semiconductor laser structure [23]: 1. Index changes 2. Cavity length changes 3. Mirror wavelength-selective reflectivity tuning Technology for Telecommunications: Integrated Optics and Microelectronics Somehow all these three techniques have to be used if a tuning range twice the DBR maximum of 22 nm has to be reached. This is possible by using a structure with multiple cavities and a short grating as reflec- tor for each cavity. Sampled-grating DBR (SG-DBR) and super-structure grating DBR (SSG- DBR) lasers implement this idea in two different manners [24]. The sampled-grating design uses two different multielement mirrors to create two reflection combs with different free spectral. The laser operates at a wavelength where a reflection peak from each mirror coincides. Since the peak spacing is different for various mirrors, only one pair of the peaks can line up at a time, SO that, when the lasing wavelength passes from a couple of peaks to the oth- ers a big change of emitted wavelength is obtained with a small change of the mirror peak positions (Vernier effect [25]). The scheme of a discrete tuning SG-DBR and a plot showing the idea of the Vernier effect are shown in Figure 5.20. Even if this simple explanation of the SG-DBR principle seems to limit this kind of laser to step tu