# Photonic Packaging: Transforming Silicon Photonic Integrated Circuits into Photonic Devices
## 2 Optical Packaging
### Fiber-to-PIC Coupling
#### 2.1.2 Grating-Coupling
The footprint of the coupler matches the MFD of the fiber-mode; **(c,d)** Microscope images of one-dimensional (1D) and two-dimensional (2D) grating-couplers, respectively. Figure 5: Quasi-planar Coupling (QPC) geometry to allow for the ‘horizontal’ Fiber-to-PIC grating-coupling; **(a)** Single-channel in/out QPC fiber-coupling to a Si-PIC, with an inset showing a schematic of the polished fiber facet; **(b)** QPC fiber-coupling using a multi-channel fiber-array. grating-coupler optimization for Si-photonics has predicted insertion losses of 1.0 dB and 2.0 dB for designs with and without back-reflectors and polarization dependent losses as low as 0.3 dB [32; 33]. For many telecom and datacom applications, it is useful to have multiple channels packaged to the same PIC, with current projects targeting even 128 and 256 channels. Instead of aligning individual channels, it is possible to use an 'optical shunt' to simultaneously align each fiber with a matching grating-coupler [34]. In this approach, a precision fiber-array is constructed, consisting of a glass-plate in which a series of parallel v-groves are precision-etched, typically with a pitch of 127 \(\upmu\)m or 250 \(\upmu\)m, to match the diameter of standard telecom fibers, and a flat glass 'lid'--see Figure 6. Once the fibers are inserted into the v-groove channels, and the lid is applied, the three-point-contact condition ensures the precise position and spacing of the different channels. The nominal centricity of the inner-cores of the fibers in these fiber-arrays is \(\pm\)0.5 \(\upmu\)m, which is well within the \(\pm\)2.5 \(\upmu\)m 1 dB alignment tolerance of the grating-couplers. Therefore, when the first and last grating-couplers are connected by a shunt waveguide, a single active alignment of the fiber-array that maximizes the shunt transmission also aligns all intermediate fiber-channels with respect to their grating-couplers (with a \(\pm\)0.5 \(\upmu\)m tolerance). Once the single- or multiple-channel Fiber-to-PIC alignment has been made, it is locked into place using a low-shrinkage index-matched ultraviolet (UV)-cured epoxy. Although shunt alignment helps reduce the per-channel cost of Fiber-to-PIC packaging, it still involves a single active-alignment step, while a credible route to scalable packaging calls for a passive Fiber-to-PIC alignment scheme. The recent demonstration of 'flip-chip' Fiber-to-PIC alignment is one promising approach [35]. Here, a beam-splitter system is used to visually align the inner-cores in a fiber-array to the matching grating-couplers on a PIC, with a tolerance of \(\pm\)1 \(\upmu\)m, without active alignment. Since this is within the 1 dB alignment tolerance of the grating-couplers, this approach still offers low insertion-loss, but is at least one order of magnitude faster than active alignment. Alternative schemes for passive alignment involving large-footprint (30 \(\upmu\)m \(\times\) 30 \(\upmu\)m) grating-couplers that offer relaxed 1 dB alignment tolerances of \(\pm\)10 \(\upmu\)m are also under investigation. Here, the principle drawback is a reduction in the spectral bandwidth, which can exclude this approach for wavelength division multiplexing applications. Figure 6: Optical shunt to support the active-alignment of a multi-channel fiber-array to a Si-PIC; (**a**) Microscope image of an optical-shunt and three intermediate couplers—2 \(\times\) 1D grating-couplers, and 1 \(\times\) 2D grating-coupler); (**b**) Schematic of a multi-channel fiber-array showing how three-point contact allows for precise alignment of the fibers in the v-grooves; (**c**) The transmission spectrum of the optical-shunt, showing the bandwidth of the grating-couplers. As the shunt is symmetric and has negligible waveguide-losses, the insertion-loss (IL) in units of dB is simply half of the optical-shunt transmission in units of dB.