# Edge Couplers in Silicon Photonic Integrated Circuits: A Review
## 6 Conclusions  
Research works in recent years have witnessed the significance and practicality of silicon photonics in the field of photonic integrated chips. Optical interconnects in silicon photonic integrated circuits  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Description** & **Coupling** & **Bandwidth** & **Polarization** & **Comments** \\ \hline Shallow etched SOI & & Approximately & & Feasible one-step shallow etch with lithography, \\ grating coupler [109] & 3.1 & & 40 nm @1 dB & TE & standard fabrication, and good performance. \\ \hline Tri-level SiN-on-SOI & 5.6 & Approately & TE & Leading research with fair loss and low polarization \\ grating coupler [134] & 5.6 & & 45 nm @1 dB & TM & platform fabrication process. \\ \hline SiN grating coupler & & & & \\ without SOI & 1.3 & 80 nm @1 dB & TE & Novel design, low loss, wide bandwidth, and \\ reflector [136] & & & & simplified fabrication. \\ \hline Edge coupler based on & 1.37 & Approximately & TE & Low loss, broadband width, and easy fabrication \\ SOI inverse taper [54] & 2.13 & 100 nm @1 dB & TM & practical for mass production. \\ \hline Edge coupler assisted & & & & \\ with SiN multiple upper & 0.9 & \textgreater{}50 nm @1 dB & TE and TM & Ultra-low loss, limited bandwidth, and too large \\ waveguides [74] & & & & footprint for integration. \\ \hline \hline \end{tabular}
\end{table}
Table 3: Comparison of coupler performance in multilayer SiN platforms and silicon-on-insulator (SOI) platforms.  
is a critical issue to focus on to achieve efficient data transmission. The two mainstream paradigms of fiber-to-chip optical interconnects, namely vertical coupling and edge coupling, have different characteristics, while edge coupling is superior in the aspects of higher coupling efficiency, broader operation bandwidth, and lower dependence on polarization status. This review firstly goes through the research background and applications of silicon photonic fiber-to-chip couplers. The operation mechanism and design principles are illustrated, and then the overall performance metrics of edge couplers are summarized. Furthermore, the structural transformations of edge couplers in the horizontal and vertical dimensions are classified and described in detail. In addition, we make a brief comparison between grating couplers and edge couplers and overview the packaging issues in optical coupling. Lastly, we discuss some of the experimental issues and potential perspectives regarding edge couplers, including polarization manipulation, the multilayer integrated photonic platform, and the inverse design method. It is convincing that silicon photonic edge couplers will be increasingly widely used and contribute to more improvements in the field of silicon photonic integrated circuits.  
Conceptualization, All Authors; Investigation, X.M., S.W. and L.C.; Original Draft Preparation, X.M.; Review and Editing, All Authors; Supervision, H.Y.F.; Project Administration, H.Y.F.; Funding Acquisition, H.Y.F. All authors have read and agreed to the published version of the manuscript. This research is funded by the Shenzhen Science and Technology Innovation Commission (Project: JCYJ20180507183815699, JCYJ20170818094001391,KQJSCX20170727163424873), Tsinghua-Berkeley Shenzhen Institute (TBSI) Faculty Start-up Fund, and Shenzhen Data Science and Information Technology Engineering Laboratory. The authors declare no conflict of interest.