## 3 Fabrication of Graded Index Glass Waveguides (Fraunhofer IZM)
### End Facet Quality Analysis on Glass Waveguides  
The quality of the processed end facets depends on several aspects including layout and process parameters. As mentioned above, the method used by Fraunhofer IZM to cleanly dissect the glass was laser dicing by CO\({}_{2}\)-laser and a cooling nozzle, whereby the glass panel was scored with the laser and subsequently cooled so it could be cleanly snapped, providing optical quality surface finish without subsequent polishing. This process was developed for display glasses with a homogeneous glass matrix, however since the glass matrix is selectively changed by the ion exchange process, a non-homogeneous material profile is created at the cut-line and this must be considered. In order to mitigate any deleterious effects of direct laser exposure to the waveguides themselves, the laser scoring process takes place on the opposite surface of the glass panel to the surface where the glass waveguides were formed. The laser dicing process was followed by manual glass separation, which comprised snapping the glass at the interface using a scoring and snapping principle. This method, however, is not only dependent on the cut- and waveguide-process, but also on the operator. Therefore, repeatability of the process would be variable.  
To investigate this issue, IZM followed the well-known cut-back procedure to comparatively characterize the end-facet coupling loss performance on both glass waveguide sample sets under test. The cut-back measurement results are shown in Figure 9 and the derivation of propagation loss is shown in Table 1. Measurements were made in accordance with IEC measurement standard [11].  
These results indicate a strong dependence of the coupling and propagation losses of ion-diffused glass waveguides on the NA and core size.  
Unneglectable are also the comparatively large error bars for most of the measurements. Even for set 7 with lower errors, measurements at 1310 nm are only meaningful because of the measurement at the shortest length. Nevertheless, a conclusion can be drawn that the cut-back procedure gave good insight into the value of the measurement data on IZM's straight waveguides. Besides this  
Figure 8: Refractive index profiles of selected glass waveguides with fabrication parameter sets 1 and 7: (**a**) Refractive index near field (RNF) scan of waveguide cross-section from parameter set 1, (**b**) RNF scan of waveguide from parameter set 7, (**c**) refractive index profiles along the horizontal axis of sets 1 and 7, (**d**) refractive index profiles along the vertical axis of sets 1 and 7.  
investigation, another issue should be investigated in the future: Due to the change of the ion-radius during the thermal diffusion step, small sodium ions (Na+) are replaced by much larger silver ions (Ag+). This results in tension due to higher mechanical stresses within the waveguide vicinities. These tensions are not a reliability issue, but pose a risk for varying pitch densities. The crack during manual breaking, after the laser dicing process, propagates from one side of the glass interface to the other as required. This crack, however, will be disturbed by high changes of density. Thus, high amounts of silver, e.g., due to a small pitch, a large array or several small arrays can give rise to a larger variation in end facet quality during the breaking process. This influence was discovered during processing of the glass used in this paper and is currently undergoing further investigation.