# Edge Couplers in Silicon Photonic Integrated Circuits: A Review
## 4 Structural Transformations of Edge Couplers in Vertical Direction  
### Edge Couplers Assisted with Multiple Upper Waveguides  
Typical silicon photonic integrated circuits are based on the SOI platform, which is composed of a thick silicon substrate, buried silicon dioxide for isolation, a photonic circuits layer, and top silica cladding for protection from bottom to top. It has the potential to make some changes in the vertical dimension within the buried silicon dioxide and the top cladding to improve the device performance. As for the vertical structural transformation of fiber-to-chip edge couplers, the main aim is to enlarge the effective modal area in the vertical direction to achieve a high modal match between the fiber core and the coupler facet. Figure 10 shows an approach to obtain a large effective modal area where multiple waveguides are placed above the Si inverse taper [74; 75]. Upper assisting waveguides are usually made of materials with a lower refractive index than silicon such as silicon nitride (SiN) and silicon oxynstride (SiON), which are also well compatible with the typical silicon photonic platform and CMOS fabrication process. Figure 10 demonstrates one type of pattern under which assisting multiple waveguides are introduced [76]. In principle, assisting waveguides should be placed above the appearance of an inverse taper so that the propagating mode distributed outside the narrow taper can be transmitted and confined upwards. Due to the existence of upper assisting waveguides with higher refractive indexes than the surrounding silicon dioxide cladding, light tends to be confined into the multiple assisting waveguides. Therefore, the lower Si inverse taper together with the upper assisting waveguides support a superimposed mode, and the modal area becomes enlarged and is comparable to that of the fiber core, which helps efficiently couple the light emitting from the fiber.  
Technically, it is possible to manipulate the ultimate modal distribution by changing the quantity, material refractive index, and orientation of the assisting waveguides, and [PERSON] et al. have discussed the performance of assisting waveguides with different patterns [76]. Taking fabrication issues into consideration, it is feasible to fabricate the multiple SiN or SiON waveguides since they are well compatible with the silicon photonic fabrication process. Nevertheless, alignment should be taken care of to reach the expected performance, and alignment marks need to be carefully designed during the deposition of each layer. Generally, there are several situations under which alignment issues should be considered. Firstly, upper assisting waveguides are required to align with the Si inverse taper laterally to ensure proper modal distribution and conversion. Next, as assisting waveguides are located at various heights (or different layers), interlayer alignment should be achieved to obtain the required waveguide pattern. Lastly, a superimposed mode is formed within the Si inverse tape; thus,  
Figure 9: Schematic of (**a**) fork-shape coupler and (**b**) trident coupler assisted with subwavelength gratings structure.  
the assisting waveguides should also be deposited and aligned longitudinally above the taper part to promote upward modal transition.  
Specifically, a novel spot size converter assisted with multiple upper SiON waveguides for coupling with standard SMF is demonstrated in [75]. Simulations are comprehensively run including detailed discussion about fiber alignment tolerance, robustness to fabrication tolerances, and compatibility with standard microfabrication processes. This spot size converter can achieve a nominal low loss of 0.4 dB with a total length of 450 \(\upmu\)m. A 1-dB excess alignment tolerance with SMF is larger than 2 \(\upmu\)m while a 3-dB excess alignment tolerance is nearly 4 \(\upmu\)m. This novel design reduces the complexity of alignment and fixation of a SMF to photonics chips, but it is quite challenging for fabrication feasibility and further practical applications.