# Photonic Packaging: Transforming Silicon Photonic Integrated Circuits into Photonic Devices
## 5 Emerging Technologies for Photonics Packaging  
In addition to the established technologies described in the previous sections, it is interesting to look at emerging technologies and how they may impact photonic packaging in the future. For example, the same polymer waveguide technology that has been used to demonstrate evanescent coupling can also be combined with electrical FR4 (fire-resistant, version 4) PCBs to create a platform capable  
Figure 16: _COMSOL_ thermal simulations of the EIC and PIC in the FP7-FABULOUS ONU; (**a–c**) For accurate convergent models, the simulation mesh must span from the 10 cm-scale of the full module (including TEC and baseplate) to the 10 \(\upmu\)m-scale of the CPB interconnects. The results of these simulations can be validated against thermal-imaging, and used to test new designs for better cooling of the EIC and PIC in the photonic module. Figure 15: Temperature changes of the EIC and PIC in the FP7-FABULOUS optical network unit (ONU) from the thermal microscope analysis; (**a**) The increase in EIC and PIC temperature, when the ONU is powered-up, and the TEC (thermo-electric cooler) is not activated. After 30 s, the PIC temperature reaches a steady-state at 20 \({}^{\circ}\)C above the ambient; (**b**) Once the TEC is activated (in Cooling#1), the hot PIC rapidly returns to the ambient set-point temperature, reaching \(\pm\)0.1 \({}^{\circ}\)C of the set-point in 30 s. In Cooling#2, the ONU and TEC are powered-up at the same moment, and the PIC never exceeds the set-point by more than 5 \({}^{\circ}\)C.  
of routing electrical and optical signals from external connectors to single or multiple Si-PICs [36]. Here, the main advantages over glass interposers is improved cost-effectiveness and processing flexibility of the polymer waveguides (i.e., direct laser-writing or photolithography), especially for large areas. The anticipated propagation distances in these 'optical PCBs' is in the range of 10s of cm, so the principle challenge to be overcome is reducing waveguide propagation losses to below 0.1 dB/cm. Another highly innovative solution for PIC-to-PIC [58] and multi-core Fiber-to-PIC [59] optical connectivity is that offered by the photonic wire bonds (PWBs) developed at the Karlsruhe Institute of Technology. These PWBs are formed by tightly focused femtosecond laser pulses that expose photoresist along a freeform 3D path connecting two optical interfaces (e.g., inverted-tapers on a Si-PIC to the inner-core of a single-mode fiber). Since the shape of the PWBs can be easily adapted, there is no need for high-precision mechanical alignment between connected PICs and fibers, which is an advantage for future scale-up. Writing PWBs is currently a serial process, and it remains to be seen if it can be economically implemented at the wafer-level, where thousands or tens-of-thousands of PWBs are needed per wafer. There are also many innovations around the basic grating-coupler designs, including replacing single-line trenches with sub-wavelength lithographic features that allow for a high degree of index engineering. In some cases, these sub-wavelength features are designed to create an apodized structure that almost perfectly matches the profile of the incident fiber-mode [60], and so allows for very low (sub-decibel) insertion losses. An alternative strategy is to use the sub-wavelength lithographic features to create a region with highly non-linear patterning that creates a patch of meta-material in the SOI (silicon-on-insulator)-layer [61]. When the pattern in this meta-material is properly optimized it can couple light from free-space into the SOI-waveguide, and can even couple light of different polarizations, in analogy to 2D grating-couplers. The efficiency of these meta-material couplers each device is at least comparable to more standard grating-couplers, and can offer a broader bandwidth, because multiple guided modes are responsible for the coupling, which makes the meta-material coupler less sensitive to wavelength shifts.