# Photonic Packaging: Transforming Silicon Photonic Integrated Circuits into Photonic Devices
## 2 Optical Packaging
### Fiber-to-PIC Coupling
#### 2.1.1 Edge-Coupling  
Edge-coupling is a well-established approach for the commercial packaging of laser-chips [16], but has not been widely adopted in the field of Si-photonics, despite being able to deliver broadband, polarization-agnostic insertion-losses of better than \(-1\) dB after careful alignment [17]. A typical edge-coupler for a Si-PIC consists of an inverted taper (with a length of 100-300 \(\upmu\)m) embedded in an integrated nitride-based spot-size-converter (SSC), or a post-process deposited polymer-based SSC, to increase the effective MFD of the on-PIC waveguide mode to approximately 3 \(\times\) 3 \(\upmu\)m [18; 19]. This creates good modal-overlap with either lensed SMF-28 fibers or ultra-high numerical aperture (UHNA) fibers for Fiber-to-PIC coupling--see Figure 3.  
Since the back-end deposition and etching of a 3-5 \(\upmu\)m SiON-layer needed to create an integrated mode-adapter has proven difficult to add as a standard building block (mainly because of the high strain introduced to the wafers by the SiON layer), edge-couplers are only slowly being introduced to the MPW runs being offered by the Si-Photonic foundries. Adding an edge-coupler onto a Si-PIC also increases post-fabrication processing costs, because it calls for either accurate dicing and polishing of the PIC edges, or an additional deep-etch (>60 \(\upmu\)m) lithography to create a high-quality facet for low insertion-losses [6]. Deep etching techniques can even be taken further, to simultaneously create a facet and v-grove in which the fiber can be passively aligned with respect to the on-PIC waveguides [20]. The principle draw-back of this approach is that the v-grooves can occupy a significant fraction the Si-PIC footprint that would otherwise be available for the active part of the photonic device. Most often, edge-coupling is carried-out between a PIC and a lensed-fiber. The lensing of the fiber facet creates a 3 \(\upmu\)m diameter 'hot-spot' of light that better matches the fiber-mode to the MFD of the on-PIC SSC. The 1 dB alignment tolerance for such an edge-coupler is typically sub-micron (approximately \(\pm\)500 nm), and so requires careful active alignment to minimize insertion-losses [18; 19]. Given that this alignment tolerance is comparable to the fabrication tolerance of multi-channel fiber-arrays, edge-coupling is almost always applied only to single-channel Fiber-to-PIC coupling. In addition, laser-welding of the lensed-fiber (mounted in a metallic ferrule) and the PIC to a kovar package is usually necessary to ensure that the small displacements caused by thermal expansion/contraction do not impact the Fiber-to-PIC coupling (kovar is a Fe-Ni-Co alloy designed to have a thermal expansion coefficient that matches the glass in the fiber)--see Figure 3. In addition, this metal-to-metal bond is less prone to the small alignment-drift that sometimes occurs in epoxy-bonds  
Figure 3: Fiber-to-PIC edge-coupling; **(a)** Kovar ‘butterfly’ package for edge-coupling. Kovar has the same thermal expansion coefficient as glass, so the Fiber-to-PIC alignment can be maintained, when the ambient temperature changes; **(b)** Microscope image of an edge-coupling to a III-V laser-chip using a lensed fiber. The fiber is mounted into a metal ferrule, so it can be laser-welded to the kovar butterfly package; **(c)** Schematic of edge-coupling to a Si-PIC using a lensed-fiber and spot-size converter (SSC), or mode-converter (MC), to help bridge the gap between the mode-size in the fiber and on-PIC waveguide. due to age or environmental effects [16]. Clearly, the alignment tolerance and material requirements of current state-of-the-art edge-coupling are too high to credibly satisfy medium-to-high volumes (\(10^{6}\)-\(10^{7}\) devices per year). Edge-coupling is often used for non-linear applications in Si-photonics, where high optical powers (on the order of 1 W) are needed to drive four-wave mixing and other non-linear interactions.