# Photonic Packaging: Transforming Silicon Photonic Integrated Circuits into Photonic Devices
## 2 Optical Packaging
### Laser-to-PIC Integration
#### 2.2.1 Micro-Optical Bench  
Both Luxtera and the Tyndall National Institute have demonstrated micro-optical bench (MOB) [40; 41]. These MOBs consist of an AlN (or Si) sub-mount on which an edge-emitting laser-chip, a ball \(\upmu\)Lens for collimating and focusing, and a mirror (or a total internal reflection element) for beam-steering, are mounted--see Figure 9. The hermetically-sealed Luxtera MOB also includes a micro-optical isolator to reduce feedback to the laser-chip. The MOB functions by re-imaging light from the edge-emitting laser onto the grating-coupler on the surface of the PIC with the required near-normal angle-of-incidence. Standard 1D grating-couplers on the PIC can be used, but custom grating-couplers designed to better match the elliptical spot from the MOB offer even lower insertion-losses.  
In the Tyndall MOB, the 300 \(\upmu\)m ball \(\upmu\)Lens self-aligns in a precision laser-drilled hole, and which then provides a fixed reference-point for the alignment and mounting of the \(\upmu\)Prism reflection element. The overall footprint of the MOB is approximately \(1\times 1\) mm, with the individual components of laser-chip, \(\upmu\)Lens, and \(\upmu\)Prism having a maximum dimension of 300 \(\upmu\)m. Once assembled, the  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Coupler Type** & \begin{tabular}{c} **Insertion** \\ **Loss** \\ \end{tabular} & \
\begin{tabular}{c} **1 dB** \\ **Bandwidth** \\ \end{tabular} &
\begin{tabular}{c} **Polarization** \\ **Sensitivity** \\ \end{tabular} & **1 dB Alignment Tolerance** & **MPW Availability** \\ \hline
1D-GC 1 & 1.6 dB [28] & 40 nm [13] & TE 1/TM 1 & \(\pm\)2.5 \(\upmu\)m (in-plane) [13] \& \\  & & & 10 \(\upmu\)m (out-of-plane) [26] & CEA-Leti \&lmce \\ \hline
2D-GC 1 & 3.2 dB [32] & 35 nm [32] & TE \& TM & \(\pm\)2.5 \(\upmu\)m (in-plane) [32] & CEA-Leti \& Imce \\ \hline Edge Coupler & 1.2 dB [17] & \
\begin{tabular}{c} 200 nm TE \& \\ 150 nm TM [17] \\ \end{tabular} & TE \& TM & \(\pm\)0.5 \(\upmu\)m (in-plane) [17] & Imce (taper only) \\ \hline
\begin{tabular}{c} Evanescent \\ Coupler \\ \end{tabular} & 1.0 dB [36] & \(>>\)40 nm [36] & TE \& TM & \(\pm\)2.5 \(\upmu\)m (in-plane) [36] & Imce (taper only) \\ \hline \hline \end{tabular}  
* 1D-GC = one-dimensional grating-coupler, 2D-GC = two-dimensional grating-coupler, TE = transverse-electric, and TM = transverse-magnetic.  
\end{table}
Table 1: Typical insertion loss, bandwidth, polarization sensitivity, alignment tolerances, and MPW (multi-project wafer) availability for different Fiber-to-PIC (photonic integrated circuits) couplers.  
Figure 8: III–V semiconductor optical amplifiers (SOAs) integrated on a Si-PIC; (**a**) Image of a packaged SOA module, with Fiber-to-PIC grating-coupling of the input and output channels in the QPC configuration, to evaluate the optical gain of the SOA; (**b**) Microscope image of the packaged SOA showing the QPC Fiber-to-PIC coupling, and the thermistor used to provide feedback for the integrated thermo-electric cooler (TEC) mounted under the PIC.  
MOB is actively aligned with respect to the grating-coupler on the Si-PIC, and has a 1 dB alignment tolerance comparable to that of Fiber-to-PIC coupling (\(\pm\)2.5 \(\upmu\)m). Without adequate thermal-sinking, the laser-chip on the MOB is likely to overheat, leading to reduced performance or burning-out. Good thermal contact between the MOB and the PIC, as well as thermo-electric cooler (TEC), are needed for stable operation of the MOB [42].