The artwork was made in 1395 by [PERSON] and is recorded in the Italian Stained Glass Windows Database (BIVI). Reproduced with permission of Opera di Santa Maria del Fiore, Firenze. and also in conventional optical systems [11] but mostly for laser-to-fiber or fiber-to-fiber coupling [12]. Recently, it was shown that the use of GRIN lens cascades is opening up many surprising applications, spanning from quantum optics to clinical diagnostics [13]. Further theoretical and experimental advances completed the foundations of what is now a fully mature technology for integrated optics and photonics; with time, several very interesting review papers appeared, showing the gradual progresses of this R&D field, up to the use of femtosecond lasers to drive the ion diffusion in glasses [14; 15; 16; 17; 18; 19; 20; 21]. Moreover, as ion exchange in glass allows producing either surface or buried waveguides, this technology is promising for 3D integration, too [22; 23]. A sketch of the K\({}^{+}\)/Na\({}^{+}\) exchange process, which is exploited quite often because it guarantees waveguides with low propagation loss, is shown in Figure 3[24]. The K\({}^{+}\) ions from the molten salt enter the glass, whereas Na\({}^{+}\) ions from the glass move to the melt, in order to keep the electrical neutrality. This type of exchange has two drawbacks: the induced refractive index variation \(\Delta\)n is lower than \(10^{-2}\) (so, only a weak guide confinement is possible), and it introduces some birifringence, due to the compressive stress induced by the difference in ionic radius between Na\({}^{+}\) and K\({}^{+}\) ions. On the other hand, as already mentioned, this stress may also be exploited to increase the mechanical resistance of the glass itself (chemical strengthening). The Ag\({}^{+}\)/Na\({}^{+}\) ion exchange, instead, thanks to the similar size and the difference of polarizability of the two ions, allows a higher \(\Delta\)n (\(10^{-1}\)) without birefringence and also the burial of waveguides in a two-step process, as sketched in Figure 4. Unfortunately, silver, too, has some limits: in the presence of humidity, surface waveguides (which are used in sensing devices, interacting with external medium) suffer higher losses, due to chemical reduction to metal silver. An approach useful to overcome this limitation is to use the Tl/Na process [25]: in this case, a drawback is due to the toxicity of thallium. Thus, we must conclude that the choice of the ion pair to be used in the waveguide fabrication by ion exchange is dictated by the characteristic (optical, mechanical, safety) one wants to privilege. In this paper, the focus is on glass ion-exchanged integrated optical devices: their applications cover broad areas, where optical sensing and optical communications are at the forefront. Thus, in the field of optical sensing, many integrated optical sensors are based on the properties of the evanescent modal optical field, and ion-exchanged waveguides offer the advantage of the proximity between the glass index (\(n=1.5\)) and the index of frequently used organic compounds (around 1.4-1.6) and of aqueous solutions (index close to 1.33). These guided-wave structures lead to large evanescent fields, which maximize the optical sensitivity function. A large amount of optical integrated sensors and biosensors based on ion-exchanged glass waveguides have been proposed [18; 19; 20; 26]  
Figure 2: Schematic view of the ion-exchange process to fabricate a gradient-index channel waveguide in a glass substrate [6]. and keep being implemented in the last years [27; 28; 29; 30]. An interesting example may be represented by a 32-analyte integrated optical fluorescence-based multi-channel sensor and its integration to an automated biosensing system [31]. Figure 5 shows the layout of the fiber-pigtailed sensing chip consisting of a single-mode channel waveguide circuit that distributes the light to 32 separate sensing patches on the chip surface.