# Edge Couplers in Silicon Photonic Integrated Circuits: A Review
## 1 Introduction  
As the size of electronic integrated circuits scales down to the physical limit and the demand for high data rates rises tremendously, photonic integrated circuits (PIC) has gradually taken the stage due to its inherent high speed and low power consumption. Silicon has the potential to construct compact devices benefiting from its high refractive index, and silicon-based devices are well compatible with current mature and standardized complementary metal oxide semiconductor (CMOS) platform. Thus, silicon is a promising material for novel PIC, and silicon photonics has been a hot research topic in recent years [1; 2; 3; 4; 5]. In cases of complex optical transmission systems, optical interconnects between fiber and the photonic integrated circuits happen frequently in the whole system [6], so efficient fiber-to-chip coupling is an important factor to pay attention to for system performance [7; 8; 9; 10]. To date, the feature size of silicon waveguide can be as tiny as tens of nanometers while the typical diameter of a single mode fiber (SMF) is around 125 \(\upmu\)m with a core diameter near 10 \(\upmu\)m [11]. Figure 1 shows the huge size mismatch between a fiber core and the Si waveguide, which causes considerable optical transmission loss when light emitting from the fiber core enters the Si waveguide directly. Fiber-to-chip couplers are a type of key photonic component to deal with this issue in optical interconnects. Fiber-to-chip couplers can be applied in many application scenarios where optical interconnects are required. They are passive devices that can transmit and guide light. The operation mechanism is mainly the transmission and conversion of mode in the photonic waveguides. Generally, there are two main approaches of fiber-to-chip coupling, namely vertical coupling (or off-plane coupling) and butt coupling (or edge coupling/in-plane coupling) [12; 13], according to the relative position of fiber and the photonic chip. For vertical coupling, grating couplers are mostly used, and fiber is placed above the device vertically or slightly slanted at a certain degree to ensure high coupling efficiency [14; 15; 16; 17; 18; 19; 20; 21; 22; 23]. Grating couplers have some major advantages including compact size, wafer-level testing capability, and flexible coupling position, while there are also some drawbacks such as a relatively low coupling efficiency typically below 3 dB, narrow bandwidth, and high wavelength sensitivity. As for the butt coupling regime, under which the fiber is typically placed at the wafer facet and aligned with the Si waveguides horizontally, edge couplers are commonly applied [24; 25; 26; 27; 28; 29; 30; 31; 32]. Edge couplers can achieve rather high coupling efficiency, broad bandwidth, and polarization independence, but they also have some limitations including relatively a larger footprint than grating couplers, fixed coupling position, and more strict requirements of the coupling facet. The main functions and applications of fiber-to-chip couplers in silicon photonic circuits are briefly introduced above. In this review paper, we focus on silicon photonic edge couplers, and Section 2 will deliver an overview of the operation mechanisms, performance metric parameters, and design principles of edge couplers. Sections 3 and 4 comprehensively describe the structural transformations of a basic edge coupler in horizontal and vertical directions, respectively. In Section 3, several types of edge couplers with sophisticated horizontal structures are included such as edge couplers based on nonlinear profile tapers, couplers composed of multi-tip tapers, couplers consisting of multiple tapers, and couplers with subwavelength gratings structures.