# Integrated Photonics on Glass: A Review of the Ion-Exchange Technology Achivements
## 2 Ion-Exchanged Waveguides
### Waveguide's Performances
#### 2.3.1 Passive Glasses  
Since the first waveguides demonstrated by [PERSON] and [PERSON] [1], huge efforts have been made to reduce the losses of the waveguides. Historically, scattering represented the main source of losses. Indeed, the quality of the photolithography used for the realization of the masking layer before the ion-exchange was an issue as well as scratches or dirt deposited on the glass surface or refractive index inhomogeneities, such as bubbles. These problems are typical optical glass issues that are encountered when a custom-made glass is realized for the first time in small volumes, but they are easily handled by glass manufacturers when a higher volume of glass is produced. Therefore, state-of-the-art ion-exchanged waveguides are nowadays based on glass wafers specifically developed for this application or at least for microtechnologies. Among them, the more used are BF33 by Schott because of its compatibility with MEMS process, GO14 by TeemPhotonics SA and BGG31 by Schott [32], which have both been developed specifically for silver-sodium ion-exchanges. The interest of silver-sodium ion-exchange is that it allows the realization of buried waveguides solving, hence the problem of scattering due to surface defects or contaminations while dramatically improving the coupling efficiency with optical fibers. Nonetheless, silver-based technologies present also challenges since Ag\({}^{+}\) has a strong tendency to reduce into metallic Ag creating metallic clusters that are absorbing the optical signals. The glass composition should therefore be adapted not only to remove reducing elements like Fe, As, or Sb, but also to create a glass matrix where Na\({}^{+}\) ions are not linked to non-bridging oxygens [33]. The choice of the material for the masking layer should also be made with caution because the use of metallic mask can also induce the formation of Ag nanoparticles at the vicinity of the diffusion apertures [34]. Therefore, the use of Al or Ti mask is now often replaced by Al\({}_{2}\)O\({}_{3}\)[35,36], SiO\({}_{2}\), or SiN [37] ones.  
Table 1 presents the main characteristics of single mode waveguides realized on GO14, BGG31, and BF33, respectively. GO14 and BGG31 that have been optimized for telecom applications and ion-exchange present very low propagation losses and birefringence that are key characteristics for data transmission. BF33 is not a glass that has been designed for ion-exchange but it is a relatively low-cost glass that presents a quite good refractive index  
Figure 6: (**a**) Refractive index distribution of a thermally diffused waveguide, diffusion aperture width is 2 \(\upmu\)m, exchange time is 2 min, \(D_{B}\) = 0.8 \(\upmu\)m\({}^{2}\)/min; (**b**) refractive index profile of the waveguide (**a**) after an electrically assisted burying in a pure NaNO\({}_{3}\) molten salt, process duration is 1 h30 for an applied electric field of 180 kV/m.  
change and that is specifically indicated by its manufacturer for MEMS and microtechnology applications. Therefore, it is an excellent candidate for sensor realization and is mainly used for that. The relatively high propagation losses observed in BF33 is mainly due to the fact that this parameter is not very important in sensors and has, hence, neither been optimized nor measured accurately.  
We deliberately did not mention Tl\({}^{+}\)/K\({}^{+}\) ion-exchanged waveguides although the process is indeed the first one that has been used and the first one to be tentatively implemented in a production line. However, the advantages of a Tl\({}^{+}\)/K\({}^{+}\) ion-exchange, namely a high refractive index change and the absence of clustering and absorption, are strongly counterbalanced by its toxicity, which implies dedicated safety procedures and waste treatments. It is therefore very scarcely used.