Silicon Photonics: Fundamentals and Devices

10.6 3D Optical Waveguides

We now extend the theory for practical waveguides, which are basically three-dimensional (3D) in structure.

10.6.1 Practical Waveguiding Geometries

The symmetric and asymmetric slab waveguide structures discussed in Section 10.5.2 form a good starting point for a discussion of waveguiding properties in dielectric waveguides. A guiding layer of silica grown on silicon substrate having air as the upper cladding layer may serve as an example of the asymmetric guide. The silica guiding layer may have finite thickness but may extend to infinity in the other two dimensions. In accordance with the earlier discussions in this chapter, the RI profile makes step changes at the air–silica and silica–silicon interfaces.

A number of techniques that are employed presently for the fabrication of practical IO circuits, however, lead to significantly different types of guiding structures. When ions or molecules are driven into a substrate by diffusion or ion implantation, a guiding layer is produced at the surface of the substrate. However, the RI profile in the guide layer is far from being constant; rather, the RI decreases exponentially or nonlinearly from the value at the surface. Such structures are referred to as graded index waveguides.

Another important feature in practical waveguides is that optical energy is confined along both the x- and y-directions, unlike in the earlier case where confinement occurred only along the y-direction due to RI index steps. The ...