# Edge Couplers in Silicon Photonic Integrated Circuits: A Review
## 3 Structural Transformations of Edge Couplers in Horizontal Direction
### Edge Couplers Based on Multiple Tapers  
Inspired by the idea of the multi-tip taper, it is natural to propose another kind of edge coupler based on multiple tapers. The trident edge coupler with three separate tapers is the most common form among different types of edge couplers consisting of multiple tapers [61; 62; 63; 64; 65], and its structure is presented in Figure 5b. For trident edge couplers, light exiting from the fiber firstly enters the facet composed by two bilateral taper tips and is split into a superimposed mode due to the high refractive index contrast between Si and SiO\({}_{2}\) when light continues to propagate along the edge coupler. A central Si taper appears and the mode exists in all the three tapers, and the proportion of mode confined by the central taper tends to increase as the width of the central taper becomes larger. Finally, when bilateral tapers end, the mode is totally converted into the central taper and then propagates into successive integrated circuits. Figure 6 compares the modal distribution at the facet of edge couplers based on one taper and trident tapers. As mentioned in Section 3.1, an inverse taper is aligned with the fiber at the narrow end, and the mode distributes in the outer area of the taper, as shown in Figure 6a. In the trident edge coupler, light emitted from the fiber firstly propagates into the two bilateral tapers and exists as a superimposed mode, as presented in Figure 6b. Figure 6 clearly shows that when the other parameters are kept the same, an edge coupler based on multiple tapers has a larger modal area at the facet than that composed of one single taper. An enlarged modal distribution area can contribute to a higher modal overlap and reduce the modal size mismatch between the fiber and the chip facet.  
An edge coupler composed of multiple tapers has a simple structure, high feasibility, and is compatible with traditional lithography and etch processes. Similar to edge couplers with one multi-tip taper, multiple tapers are also more tolerant to misalignment between an optical fiber and the edge coupler than a single inverse taper. Despite the advantages of low modal mismatch and a simple fabrication process, alignment between tapers within the edge coupler needs sophisticated design to prevent excess loss. Take a trident edge coupler for instance [64]; it is prepared by a 248 nm deep ultraviolet (DUV) lithography, and the total length is around 300 \(\upmu\)m. It exhibits a coupling loss of 1.4 dB for the TE mode and 1.5 dB for the TM mode separately, with a loss variation below 0.1 dB over the C-band. Both polarization independence and wavelength insensitivity are revealed. However, the relative position between the bilateral tapers and the central taper has strong influences on the coupling  
Figure 5: Schematic of Si photonic coupler based on (**a**) a multi-tip taper and (**b**) multiple tapers.  
performance. A slight deviation of 1 \(\upmu\)m between the fiber and the trident coupler can introduce 1-dB excess loss. Researchers have applied different numbers and spatial locations of tapers to engineer the mode distribution and adjust the effective spot size to reduce modal mismatch. Moreover, there exists a trade-off between the number of multiple tapers and device performance as more tapers can provide a larger mode size while introducing higher propagation loss and a more complicated fabrication process. Despite general usage in fiber-to-chip optical interconnects, edge couplers based on multiple tapers can also operate as laser-to-chip couplers [63]. This particular application provides an approach to realize a hybrid integrated light source on photonic chips.