# Edge Couplers in Silicon Photonic Integrated Circuits: A Review
## 5 Discussions and Perspectives
### A Brief Comparison between Edge Couplers and Vertical Couplers  
As mentioned previously in the Introduction, under the vertical coupling regime, fiber is placed above certain coupling structures (mostly diffractive gratings) on the top of planar photonic chip. A vertical grating coupler changes the off-plane wave-vector direction of light to the in-plane waveguide direction via grating structures with either periodic or chirped pitches, and it couples the light into on-chip waveguide via a spot-size converter. The wave-vector conversion can be described by the Bragg condition, which relates the pitch of grating, structure effective index, and beam incident angle with a certain wavelength that satisfies the condition [34]. Grating couplers are advantageous considering their compatibility to wafer-level testing, flexibility for aligning and measurement, and relatively compact size [109; 110]. However, it is also important to figure out some drawbacks of grating couplers. Firstly, the coupling efficiency is rather low due to the inherent diffraction and scattering that occurs in the grating structure. The component of light that does not satisfy the phase match condition is eliminated during propagation inside the grating. Secondly, also owing to the phase match condition, only light within a particular wavelength range can travel through the grating. This property leads to the limited narrow bandwidth of grating couplers. Thirdly, the teeth of the grating structure have high directionality, which is very sensitive to the oscillation direction of the electromagnetic vector. This presents as the TE or TM polarization status from the macro perspective. Thus, the wave-vector conversion has the underlying drawback of polarization sensitivity for grating couplers.  
High-efficiency grating couplers have been also reported, but they usually include complex fabrication processes such as a bottom mirror below the grating or silicon overlay layer above the grating [111; 112; 113]. To deal with wavelength sensitivity and polarization sensitivity issues, polarization splitting grating couplers and dual-wavelength-band grating couplers are proposed [114; 115]. In the meantime, these designs usually bring increased coupling loss. When it comes to edge couplers, despite the apparent advantages such as high coupling efficiency, broad bandwidth, and low dependence on polarization, the main limits lie in that edge coupling only allows the alignment of fiber with the coupler at the chip facet, which is less convenient compared with grating couplers that support wafer-level testing at different positions on the wafer plane [116; 117; 118]. Meanwhile, edge couplers generally have larger footprints in the longitudinal dimension. In addition, the particular designs of edge couplers with the aforementioned cascaded multi-stage tapers, 3D tapers, or cantilever structure need complex and difficult fabrication processes. The performance of typical vertical grating couplers and edge couplers are summarized and shown in Table 2 for quick comparison.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Description**} & **Coupling** & \multirow{2}{*}{**Bandwidth**} & \multirow{2}{*}{**Polarization**} & \multirow{2}{*}{**Comments**} \\  & **Loss (dB)** & & & \\ \hline Shallow etched grating coupler [109] & 3.1 & Approximately & & Standard 248 nm DUV plus shallow \\  & 40 nm @ 1 dB & & etch, good coupling efficiency. \\ \hline Subwavelength grating coupler [110] & 3.7 & 60 nm @ 1 dB & TM & 193 nm DUV, good efficiency, wide \\  & & & & bandwidth, sensitive to fabrication. \\ \hline Grating coupler with silicon overlay [111] & 1.6 & 80 nm @ 3 dB & TE & 193 nm DUV plus amorphous \\  & & & & silicon overlay available at Imec \\  & & & & and IME \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison between edge couplers and grating couplers. DUV: deep ultraviolet, EBL: electron beam lithography.