The use of this waveguide immunosensor, based on a K\({}^{+}\)-exchanged BK7 glass, was demonstrated for the detection of Microcystin-LR cyanotoxin in two real lake water samples taken from Fuhai lake and Beihai lake in Beijing [31]. Figure 4: Outline of two different ion-exchange processes involving two ions A and B (e.g., K\({}^{+}\) and Na\({}^{+}\), respectively): the upper drawing refers to a purely thermal diffusion, that produces a surface waveguide. The bottom drawing indicates a field-assisted diffusion, that leads to a buried waveguide. Reproduced with modifications from [25] with permission by Elsevier. Figure 3: Depiction of the K\({}^{+}\)/Na\({}^{+}\) ion-exchange process using KNO\({}_{3}\) molten salt as the source of K\({}^{+}\) ions. The glass considered here is an aluminosilicate glass with an alkali oxide (Na\({}_{2}\)O) as glass modifier. The kinetics of the process is described by diffusion equations. Reproduced from [24] with permission by John Wiley and Sons. In the field of optical communications, the high optical compatibility between glass integrated devices and optical fibers clearly offers an important advantage with respect to other materials. Again, ion-exchanged glass waveguides have constituted a fundamental component of several devices based on modal coupling, such as directional couplers (e.g., for power dividers), gratings (e.g., for contradirectional coupling in integrated lasers), multimode interferometers (MMI), 1 \(\times\) N and 2 \(\times\) N splitters, array waveguide gratings (e.g., for wavelength division multiplexing), and so on [19; 32; 33]. Section 2 summarizes the fundamentals of ion-exchanged channel waveguides and circuits. By using suitable glass substrates, such as rare-earth doped oxide glasses, active integrated optical devices may also be fabricated through ion exchange; this topic is discussed in more detail in Section 3 of this paper. Other interesting aspects concern the manufacturing of nonlinear optical (NLO) integrated devices: [PERSON] et al. [34], in 1990, designed two glasses, containing Ti and Nb to increase the nonlinear index coefficient and an alkali ion (Na\({}^{+}\) or K\({}^{+}\)), so to have a large optical nonlinearity and to be compatible with ion exchange. Spatial soliton propagation in these waveguides was demonstrated, which proved the possibility of implementing an all-optical switch. Almost at the same time, tests were also performed on commercial semiconductor-doped glasses, which had been proven to possess nonlinear optical properties [35; 36]: optical waveguides were fabricated by using a Cs\({}^{+}\) - K\({}^{+}\) exchange [37]. Ion-exchanged waveguides may also be capable of handling high laser powers, e.g., from an integrated Q-switched laser [38], and are therefore suitable, due their intrinsic light confinement properties, for nonlinear applications [39]. In that area, however, hybrid structures may be more efficient and the coupling of a lithium niobate film with a ion-exchanged glass waveguides can lead to the manufacturing of electro-optic modulators or second harmonic generators [40]. Many other applications are possible, also linked to the new topics emerged in the last years, as for example integrated quantum photonics (IQP) for the implementation of tasks in quantum information, particularly, in quantum communications. It has been underlined that in IQP 'the big challenge is how to realize a scalable, convenient platform for practical deployment and commercialization' [41]. Indeed, the growth of this area requires the design and fabrication of passive and active integrated devices to be used, for example, in quantum photonic simulation, quantum cryptography, quantum photonic sensors and so on. It is well known that, since the early experiments on silica-on-silicon structures [42], different platforms have been proposed for the implementation of IQP, including those ones silicon-based, III-V semiconductors and lithium niobate [43; 44; 45; 46].