# Edge Couplers in Silicon Photonic Integrated Circuits: A Review
## 5 Discussions and Perspectives  
### An Overall Overview of Edge Coupler Performance  
The different types of structural transformations of fiber-to-chip edge couplers mentioned in previous sections have been reorganized in Table 1 for convenience of reference and comparison. Table 1 reveals that edge couplers can realize very high coupling efficiency up to 90%, and the device footprint is usually of several hundred microns. Edge couplers generally perform stably in a broad operation wavelength bandwidth, and relatively low polarization dependence for TE and TM modes is shown.  
Due to the low coupling losses of single-edge couplers, a cut-back method is usually applied in the testing of edge couplers to measure the coupling efficiency [26; 68; 108] assisted with multiple auxiliary sets of ultra-long straight waveguides and bends to acquire accurate experimental results. In some situations, fiber array can be customized to place at one facet or both facets to ease the alignment issue. Since edge couplers work for butt coupling, the quality of chip facet is critical in practical experiments and applications. There are two main factors to take care of in the preparation of chip facet. Firstly, there should be a proper spacing between the chip facet and the coupler facet to protect the device structure as well as reduce power evanescence when light propagates within the  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Edge Coupler Configuration** & \begin{tabular}{c} **Coupling** \\ **Loss (dB)** \\ \end{tabular} & **Polarization** & \
\begin{tabular}{c} **Size** \\ **(im)** \\ \end{tabular} & **Comments** \\ \hline Edge coupler based on inverse taper & 1.37 & TE & 250 \\ with various nonlinear profiles [54] & 2.13 & TM & 200 \\ \cline{2-5}  & 2 1.37 & TE & 170 \\ \cline{2-5}  & 2.12 & TM & 140 \\ \cline{2-5}  & 3 1.39 & TE & 170 \\ \cline{2-5}  & 1.87 & TM & 150 \\ \hline Edge coupler consisting of double-tip & 1.10 & TE & 40 \\ inverse taper [60] & 1.52 & TM & \\ \hline Trident edge coupler [64] & \textless{}1.5 & TE & 300 \\ \cline{2-5}  & \textless{}1.7 & TM & \\ \hline SWG edge coupler assisted with SiN & 0.75 & TE & 1500 \\ layer [24] & 0.5 & TE & -500 \\ \hline Si taper under multiple SiN rods and & 0.9 & TM & \\ \cline{2-5}  & 1.5 & TE & 240 \\ \cline{2-5}  & 2.1 & TM & \\ \hline Nano-taper coupler dadded with & 0.66 & TE & 300 \\ polymer waveguide [89] & 0.36 & TM & \\ \hline Knife-edge shape coupler fabricated & 0.35 & TE & -180 \\ using a double-pattern & 0.21 & TM & \\ \hline A cantilevered mode-size & 1.0 & TE & 250 \\ converter [108] & 1.5 & TM & \\ \hline \hline \end{tabular}
\begin{tabular}{c} 1. Taper with linear profile, 2. Taper with exponential profile, 3. Taper with quadratic profile.  
\end{table}
Table 1: Overview of edge couplers with different structures. SMF: single mode fiber, SWG: subwavelength gratings structure, PDL: polarization-dependent loss under-confined spacing. Typically, a spacing of several microns to several tens of microns is acceptable, and refractive index-matching oil can be used for more precise refractive index management. Secondly, the surface roughness of the chip facet should be as low as possible to prohibit excess light reflection and scattering. The facet can be prepared via firstly deep reaction ion etching and then laser dicing or mechanical dicing to obtain a smooth sidewall with low surface roughness even to the order of several hundred nanometers.