# Edge Couplers in Silicon Photonic Integrated Circuits: A Review
## 5 Discussions and Perspectives
### Packaging Issue of Optical Couplers  
Optical interconnects are necessary for photonic integrated circuits working as a component in an optical fiber network. For a photonic chip design from proof of concept to commercial prototype, packaging and assembly is a crucial point and may be one of the most significant bottlenecks on the way to commercialization [119]. It is essential to provide an interface for packaged photonic integrated circuits to connect with fibers in the fiber-to-chip coupling regime. Corresponding to the aforementioned coupling strategies, there are generally two types of fiber-to-chip coupling in packaging, namely edge coupling and grating coupling. Lateral coupling is appropriate for planar photonic integrated chips, but it is challenging for lateral edge coupler to provide a desirable alignment [120]. Vertical coupling with grating couplers are mainly adopted since fibers can be placed at multiple positions on the chip to connect with grating couplers. The packaging of photonic chips has high demand in the aspects of alignment precision, thermal management, and the feasibility of integrating active and passive devices together. Coupling efficiency is the most important figure of merit in packaging and bandwidth, and polarization dependence should also be taken into consideration. Edge coupling can reach high coupling efficiency, broad bandwidth, and low sensitivity to polarization. However, the key limit of edge couplers is that it can only support optical coupling at the chip facet with no flexibility. There are two major drawbacks of packaging with edge coupling--that is, low misalignment tolerance and complicated fabrication processes. Firstly, at the 1550-nm wavelength, which is widely used in telecommunications, the typical 1-dB alignment tolerance of a single mode fiber is about 1.75 \(\upmu\)m [39]. Since the modal field diameter of edge couplers is comparable to this value, a small deviation in the aligning fiber with the edge coupler facet can bring about a sharp decrease in coupling efficiency. Secondly, the packaging of edge coupling usually applies optical fiber pigtailing combined with laser welding [121]. The fiber requires cleaving, dicing, and tapering to lensed fiber in some cases. In addition, the chip facet needs delicate polishing to reduce the facet surface roughness and improve the coupling efficiency. An anti-reflection coating should be introduced further after the polishing process sometimes. These disadvantages make edge-coupling insufficient to be utilized in mass production at low cost. Grating couplers dominate in fiber-to-chip packaging in most cases. From the industrial point of view, non-contact probing and the testing of packaged circuits is critical for flexible measurement and tests at low cost. Grating couplers have much larger mode spot sizes than edge couplers, which is suitable for coupling with SMF [121]. It is worthwhile to mention that edge coupling can suit for some particular photonic applications, including scenarios requiring high power transmission such as nonlinear silicon photonics and situations where broad bandwidth is needed to enable integrated devices with high optical input/output (I/O) counts and high-frequency characteristics [39]. As for the developing tendency of packaging with edge coupling, it is feasible to use HNA fibers to improve the coupling performance and loosen the alignment constraints. V-grooves are also used to passively place and align the fiber. However, a V-groove may occupy too large an area of the chip, impacting the integration density and cost. In addition, since the misalignment tolerance of edge couplers is comparable with the fabrication tolerance of fiber arrays, edge coupling is only applicable for