# Integrated Photonics on Glass: A Review of the Ion-Exchange Technology Achivements
## 2 Ion-Exchanged Waveguides
### Principle and Technology
Nonetheless, this complicates the set-up and might also induce the reduction of the doping ions into metallic clusters that dramatically increase the propagation losses (specifically when silver is involved). The use of a silver thin film has also been employed successfully for the creation of the waveguide's core [15]. The thin film can be either deposited on anexisting mask, as depicted on Figure 1c, or patterned directly on the glass substrate [16]. An applied electric field ensures an efficient electrolysis of Ag\({}^{+}\) ions into the glass by the consumption of the silver film anode. These three different processes allow realizing waveguides whose core is placed at the surface of the glass wafer and whose shape is, depending on the process parameters, semi-elliptical with a step refractive index change at their surface and diffused interfaces inside the glass. Intrinsically, such waveguides are supporting modes that are prone to interact with the elements present on the wafer surface. Interesting and even maximized for the realization of sensors, this interaction is often a drawback when dealing with telecom devices where the preservation of the quality of the optical signal is a key factor. For this reason, ion-exchanged waveguide cores are usually buried inside the glass. Figure 2 depicts the two main processes that can be used: the first one consists of plunging the wafer containing surface cores in a molten salt containing only the ions that were originally present in the glass. A reverse ion-exchange is then occurring, removing doping ions from the surface of the glass [17]. This process entails a quite important decrease of the refractive index change and an increase of the waveguide's dimension because of thermal diffusion, which practically limits the depth of the burying to one to two micrometers. In order to reach a deeper depth and ensure a good optical insulation of the guided mode, the reverse ion-exchange is quite often assisted by an electric field that forces the migration of the core inside the glass preventing hence a loss of refractive index variation. Moreover, by a proper tuning of the process parameters, circular waveguide cores can be obtained in order to maximize the coupling efficiency with optical fibers. Nonetheless, it must be noticed that the applied voltage can be close to 1 kV, which requires on one hand, a proper and well secured dedicated set-up, and on the other hand, an excellent quality of the glass wafer in order to prevent percolation path formation and short circuits. Figure 3 depicts an optical image of a buried optical waveguide realized on a Teem Photonics GO14 glass by a silver-sodium ion-exchange. Burying depth as high as 47 \(\upmu\)m have been realized, as shown in Figure 4, but such extreme values are rarely required in practical devices where the burying depth is of the order of 10 \(\upmu\)m.  
Figure 1: Three main processes used to realize surface waveguides by an ion-exchange on glass. A\({}^{+}\) and B\({}^{+}\) represent the ions contained in the glass and the ones replacing them, respectively. (**a**) the glass wafer is dipped into a molten salt containing B\({}^{+}\) ions entailing a thermal diffusion on the exchange ions through a diffusion aperture; (**b**) the diffusion process is assisted by an electric field; (**c**) an electrolysis of a silver thin film is used to generate Ag\({}^{+}\) ions that are migrating by diffusion and conduction inside the glass.