# Edge Couplers in Silicon Photonic Integrated Circuits: A Review
## 4 Structural Transformations of Edge Couplers in Vertical Direction
### Edge Couplers Based on Cantilever Structure  
Several forms of vertical transformation of edge couplers have been depicted in previous sections including couplers assisted with multiple waveguides, couplers based on cascaded multi-stage tapers, couplers with index-matching cladding, and couplers based on three-dimensional tapers. Basically, these edge couplers have additionally designed structures upon the Si inverse taper. Typical SOI wafers use thick silicon as the substrate and grow a layer of silicon dioxide with a thickness of several microns as the buried oxide layer to isolate the device layer with the substrate. Intuitively, it is also applicable to design specific structures below the Si inverse taper. Cantilevers structures are commonly  
Figure 15: Schematics of (**a**) knife-edge taper; (**b**) conventional inverse taper; and (**c**) mode distribution inside the knife-edge taper for the TE mode at 1550 nm. Reproduced with permission from [99], Copyright AIP Publishing, 2013.  
used in micro-electro-mechanical systems (MEMS), and they also play important roles in edge coupler design [103; 104; 105; 106; 107]. The schematic of such a structure is demonstrated in Figure 16. Based on initial single Si inverse taper, the BOX layer and the substrate underneath the BOX layer are partly undercut to a certain thickness, and a suspended beam of Si taper cladded with SiO\({}_{2}\) is exposed. Several supporting beams are set to sustain the suspended arm to enable structure stability. A fiber is placed in the etched slot and aligned with the cleaved facet of the cantilever arm. Detailed process flow can be found in [103]. The converter is composed of suspended SiO\({}_{2}\) waveguide and overlapped Si taper. This novel design reaches approximately 2.0 dB coupling loss with a 5 \(\upmu\)m-MFD lensed fiber in the wide wavelength range from 1520 to 1600 nm. The 1-dB excess loss for alignment tolerance is about 1.7 \(\upmu\)m. The device has a rather large footprint with 550 \(\upmu\)m length. Overall, this cantilever edge coupler achieves fair coupling performance at the expense of too long size. More importantly, the complexity and difficulty of the fabrication process make this configuration less competitive when compared with other couplers that can achieve comparable performance under much easier fabrication. Nonetheless, the cantilever structure is a novel and clever design, actually. From one aspect, the undercut slot that remains after etching can further isolate the functional structure away from the Si substrate, which has a high refractive index, and mode leakage into the Si substrate can be significantly reduced. In some cases, refractive index-matching oil is filled in the slot below the suspended arm to further decrease the index mismatch between the fiber and the coupler [31]. From the other aspect, the remained cantilever arm has a similar formation with fiber [104]; namely, the suspended arm itself acts as an equivalent core, and the encircled intermediate in the etched slot acts as an equivalent cladding. This similarity in structure composition helps efficient coupling. The main obstacle to design a cantilever edge coupler is the complicated fabrication processes, since it is quite challenging to fabricate the cantilever structure. Multiple steps of lithography and etch are required to obtain the cantilever arm and deep trenches sequentially. In addition, pattern masks should be precisely designed and adjusted to control the dimension of the cantilever arm. These constraints make it hard for cantilever edge couplers involved in mass production. However, the delicate structure of cantilever edge couplers can help exploit higher device performance in research scenarios. It also has the potential to integrate cantilever edge couplers with the aforementioned MEMS, which may inspire novel photonic devices and integrated systems. In summary, it is intuitive  
Figure 16: (**a**) Typical structure of edge couplers with cantilever; (**b**) cross-section of the cantilever.