# InP-Based Foundry PICs for Optical Interconnects
## 3 Building Blocks targeting Hybrid-Integration Applications
### Grating Couplers on InP Technology  
Vertical coupling schemes are attractive because they enable deployment in free-space-optic applications, but also they enable on-wafer processing, characterization and testing, and vertical optical in/out coupling virtually at any site on the PIC chip without the need for prior chip cleaving and coating procedures. In silicon photonics, vertical grating couplers represent an established IO interface, however this is not yet the case for InP PICs, mainly due to the fact that compact grating couplers for coupling to an SMF are not possible because of the low index contrast waveguides. On the other hand, for hybrid integration employing surface-coupling approaches, there is no limitation on coupling to SMFs, and it is actually an advantage to have larger mode sizes with larger alignment tolerances. In addition, grating couplers can be designed to generate collimated beams with very low beam divergences, which means that the InP PIC and the interconnect PIC can be much farther apart without introducing a significant penalty to the coupling loss. The drawbacks of using grating couplers to couple light from the InP PIC to an interconnect PIC (as opposed to the other two coupling approaches described in this section) is that they are polarization- and wavelength dependent, which is an issue that PIC designers should take into account. An example of the utilization of InP grating couplers to realize a 2D beamsteering PIC is described in a previous study [31].  
Figure 9 shows our implementation of grating couplers into the generic-foundry fabrication process for maximum coupling of the light through the bottom side of the PIC. This implementation does not require any change to the generic-foundry process, because such gratings can be implemented by using 2 nd-order Bragg gratings (instead of 1 st-order) in the Bragg Grating BB already available on the generic-foundry process. In the implementation shown in Figure 9, the light is coupled to the grating coupler, and the grating coupler starts emitting light to both the InP substrate and the InP top cladding in equal powers. On top of the InP cladding, a highly-reflective coating is added to redirect this light towards the bottom. If the top cladding has the correct thickness, then the light coming from the top cladding will interfere constructively with the light that was directly guided downwards. The coupling coefficient of the Bragg gratings currently offered on the foundry platform (for instance, on the first-order gratings on the DBR lasers) is around 80 cm\({}^{-1}\), which results in a grating length of around 300 \(\upmu\)m for 100% reflection. Considering the fact that the gratings on gratings couplers are second-order gratings (i.e., twice the first-order grating period) with the same coupling coefficient, this would result in a grating coupler length of 600 \(\upmu\)m, and therefore, a mode size of 600 \(\upmu\)m coming out of the grating coupler along the propagation direction. We believe that this length is too large for most practical free-space applications, and therefore in order to shorten the grating coupler length, we have increased the grating depth from 30 nm to 150 nm to increase the coupling coefficient of the gratings on the grating couplers (to reduce the grating coupler length from 600 \(\upmu\)m to 200-300 \(\upmu\)m).