## 2 Current CORNERSTONE Platforms
### CONERSTONE Partner Highlights
#### 2.4.1 CONERSTONE Partner: Tyndall National Institute, Ireland--High Efficiency Optical Packaging  
Photonics packaging is one of the major steps to bringing integrated photonics to huge volume markets mostly due to the high assembly cost and the constraints related with the optical and mechanical connections [70]. Grating couplers (GCs) are an appealing solution to overcome these issues due to the reasonably relaxed in-plane alignment tolerances (\(\pm\)2.5 \(\upmu\)m at 1 dB) and the possibility of placing these structures anywhere on the PICs without the need for a specific customized PIC design [70; 71]. The main drawback regarding the packaging of GCs has so far been the mechanical limitations due to the bulkiness and fragility of glass fibers vertically interfaced with the PIC. This light coupling geometry is known as the vertical-fiber scheme (V-Fb), shown in the left panel of Figure 9. A more robust solution is represented by a horizontal light coupling scheme (H-Fb) shown in the right panel of Figure 9. However, up to now, no GCs have been optimized to minimize the insertion loss (IL) for the H-Fb scheme. Figure 8: Eye diagram for MZI modulator at 1956.5 nm wavelength. (**a**) Data rate is 10 Gb/s and extinction ratio is 12.7 dB. (**b**) Data rate is 20 Gb/s and extinction ratio is 10.3 dB. (**c**) Data rate is 25 Gb/s and extinction ratio is 6.25 dB. Figure 9: Light coupling geometries for (**a**) left panel, V-Fb, the vertical fiber scheme; (**b**) right panel, H-Fb, the horizontal fiber scheme. To fill this technological gap, we designed GCs optimized in the C-band (\(\lambda_{\rm c}\) = 1550 nm) for the H-Fb scheme to be competitive in terms of IL with the most used V-Fb scheme. Apodized GCs [72; 73] represent the easiest and most efficient solution to achieve this goal and, consequently, e-beam lithography was targeted as the required technique to resolve the small features (around 55 nm). A customized particle swarm optimization (PSO) algorithm [74] was implemented inside LumericalFDTD, a commercial software based on a finite difference time domain (FDTD) method. The SOI-platforms under investigation were either the 220 nm or the 340 nm Si thicknesses, which are considered the most interesting at the moment for the majority of applications in integrated photonics. For the sake of completeness, a 340 nm-thick SOI GC was also optimized for the V-Fb, and all the GCs were designed as focusing gratings [71] to reduce their footprints, which is a technological advantage for the scalability of PICs. An epoxy layer was used to secure the fiber position on the PIC surface, matching the refractive index between the core and the top oxide cladding, maximizing the amount of light shined on top of the GC. The measured IL for both platforms are reported in Figure 10, where panel (a): 220 H-Fb refers to the GC optimized for the 220 nm SOI platform with the H-Fb scheme measuring an IL(\(\lambda_{\rm c}\)) = 2.5 dB (56%); panel (b): 340 H-Fb denotes the structure optimized for the 340 nm SOI platform with the H-Fb scheme measuring an IL(\(\lambda_{\rm c}\)) = 0.8 dB (83%), and panel (c): 340 V-Fb indicates the optimized structure for the 340 nm SOI platform with the V-Fb scheme measuring an IL(\(\lambda_{\rm c}\)) = 0.7 dB (85%). The 220 nm SOI platform and H-Fb scheme shows an IL slightly higher (0.3 dB) with respect to the theoretical values reported in literature due to a wider mode field diameter proper of the H-Fb scheme. It is well-known that a thicker Si layer shows better performances; thus, for the 340 nm SOI platform the difference is reduced to 0.1 dB. Due to these differences, the H-Fb scheme can be considered a competitive solution for both platforms.