# Photonic Packaging: Transforming Silicon Photonic Integrated Circuits into Photonic Devices
## 2 Optical Packaging
### Fiber-to-PIC Coupling
#### 2.1.2 Grating-Coupling  
The most common alternative to edge-coupling is grating-coupling, where a sub-\(\upmu\)m periodic structure is lithographically etched into the waveguide-layer of the PIC, to create a coherent interference condition that diffractively couples the fiber-mode into the PIC waveguide [24; 25]--see Figure 4. The 10 \(\times\) 10 \(\upmu\)m footprint of the grating-coupler is chosen to match the 8-10 \(\upmu\)m MFD of a standard telecom fiber, and consists of a (periodic) array of \(\approx\)20 trenches partially-etched into the 220 nm Si-layer [12]. A simple relation exists between the peak wavelength of the coupler (\(\uplambda\)), the pitch of the trenches (\(P\)), the effective index of the grating-coupler region (\(n_{e}\)), the index of the oxide-layer (\(n_{o}\)), and the angle-of-incidence (\(\uptheta\)) of the fiber-mode: \(\uplambda=P\left(n_{e}-n_{o}\ \sin\theta\right)\), where the value of \(n_{e}\) is determined by the etch-depth and duty-cycle of the trenches, as well as the polarization of the fiber-mode. Typically, a near-normal angle-of-incidence (\(\uptheta\approx 10^{\circ}\)) is used, to provide directionality to the coupled-mode, and to reduce back-reflections into the fiber. However, from a packaging perspective, Fiber-to-PIC coupling at near-normal incidence--i.e., 'piptailing'--can lead to a bulky device with poor mechanical performance. To address this issue, a 'quasi-planar' approach has been developed for Fiber-to-PIC coupling, where the fiber facet is polished to \(\approx\)40\({}^{\circ}\) to create a total internal reflection condition the directs the fiber-mode onto the grating-coupler at the correct angle of 10\({}^{\circ}\)[26; 27]--see Figure 5. This geometry creates an almost '2D' package in which it is easier to respect the minimum-turn-radius of the telecom fibers (\(\approx\)5 cm), while maintaining a reasonable device footprint. Grating couplers offer significantly more relaxed alignment tolerance than an edge-coupler, typically \(\pm\)2.5 \(\upmu\)m of in-plane misalignment for a 1 dB penalty [13]. Grating-couplers can also be placed at any point on the PIC surface, not just at the edge of the chip, do not require any post-processing steps, such as precision dicing and polishing of the PIC, thereby facilitating wafer-scale testing and characterization of PIC before dicing and packaging. However, until recently, these practical advantages have been somewhat offset by minimum insertion-losses that are higher than those that can be achieved demonstrated by edge-couplers. Fortunately, recent advances in grating-coupler design have started to close this performance gap, with reports of 1.6 dB and 1.2 dB (measured) insertion-losses from uniform and apodized grating-couplers in the silicon-on-insulator platform [28; 29]. More advanced, though less commercially-scalable grating-coupler designs, where a metallic back-reflector layer is added in a post-processing step, allows for insertion-losses of 0.6 dB [30], showing that best-in-class grating-couplers offer the same level of performance as the best edge-couplers, while still offering relatively relaxed alignment tolerances. Standard, or '1D' grating-couplers usually exhibit a strong polarization sensitivity, which can make them unsuitable telecom and datacom connections, due to the unknown and unstable polarization state from telecom fibers. One solution is 2D grating-coupler, which is formed by the superposition of two orthogonally orientated 1D grating-couplers, and can accept a fiber-mode of any polarization-state, diffracting it into a pair of on-PIC waveguides that are both TE-polarized [31]. Recent work on 2D  
Figure 4: Fiber-to-PIC grating-coupling; **(a)** Side-view schematic of a silicon-on-insulator (SOI) grating-coupler, with a 2 \(\upmu\)m bottom-oxide (BOX) layer. The \(\approx\)10 \(\upmu\)m MFD (mode-field diameter) fiber-mode is incident on the grating-coupler with \(a\approx\)10\({}^{\circ}\) angle-of-incidence; **(b)** Top-view schematic of a 1D focusing grating-coupler.