## 3 Fabrication of Graded Index Glass Waveguides (Fraunhofer IZM)  
### Two-Step Ion Diffusion Fabrication Process  
The glass waveguide fabrication consists of a two-step thermal ion-exchange process between salt-melt and display glass suitable for large panel and batch processing. The glass waveguide panel processing line at Fraunhofer IZM (Berlin, Germany) is shown in Figure 7. Process steps like sputtering (PVD), lithography (Dip-Coater, LDI, etc.) and glass panel separation (Laser-Cutter) are suitable for board formats of up to 610 \(\times\) 457 mm\({}^{2}\). The glass panel areas that can be processed this way are limited to 305 \(\times\) 457 mm\({}^{2}\). Fraunhofer IZM have already processed waveguide panels with a maximum size of 305 \(\times\) 440 mm\({}^{2}\). Figure 5: Photo-addressed waveguide termination of photo-addressed waveguides with PMT connector (**left side**) and out-of-plane coupling array (**right end**) Source: Sumitomo Bakelite. Figure 6: Photo-addressed waveguides with waveguide layout showing multiple cross-overs at varied crossing angles (Source: Sumitomo Bakelite). The test waveguides produced for this research were patterned on chemically untreated Corning Gorilla Glass 1 (distributed by Schroder Spezialglas, Ellerau, Germany), which was only available in a thickness of 550 \(\upmu\)m. The glass waveguides were fabricated as follows. An aluminum layer of 400 nm thickness was deposited on both the top and bottom surfaces of the glass foil through DC-sputtering by Creavac Creamet 600 physical vapor deposition (PVD) equipment (Creavac, Dresden, Germany). The glass panel was then dip coated to deposit photoresist on both sides of the panel and the top surface patterned by an Orbotec Paragon Ultra 200 laser direct imaging (LDI) system (Orbotec, Tokyo, Japan), which transferred the waveguide layout and alignment mark patterns to the photoresist layer, the uncured areas subsequently being developed away. Then, the exposed aluminum layer on the top-surface was etched through an acid treatment and the photoresist removed completely. The glass panel was vertically lowered into a furnace containing a hot salt melt. The salt melt for the first diffusion step comprised a diluted AgNO\({}_{3}\) mixture. During this step, sodium ions in the glass matrix were exchanged with silver ions in the salt mixture, giving rise to a localized graduated increase in the refractive index of the glass. The concentration gradient of the silver ions into the bulk glass was proportional to the resulting refractive index gradient with the highest refractive index change occurring at the exposed glass surface interface to the mixture and the refractive index decreasing in a graduated manner to that of the bulk glass. This gave rise to an isotropic refractive index profile emanating from the exposed glass panel section. The aluminum mask layers were subsequently removed from the top and bottom glass surfaces by wet chemical etching. A single step ion diffusion process of glass waveguide fabrication was reported by [PERSON] [10] for sensing applications, whereby the refractive index maxima was on the top surface of the glass, however for optical communication applications a second ion diffusion step is required to generate a graded index profile with the index maxima buried in the glass. In order, therefore, to \"round off\" the waveguide profile and shift the refractive index maximum to a certain depth below the glass surface, a second ion diffusion step was implemented whereby salt ions were leached back out of the glass matrix into a second solution. Following this two-step ion diffusion process, an MDI LD600-H system CO\({}_{2}\)-laser scribing system (Mainz, Germany) was used to score the glass panel into smaller sections with identical waveguide layouts. The individual sections were then manually snapped off along the score lines, producing very high-quality facets. This process is described in more detail in the next section.