Silicon Photonics is experiencing a dramatic increase in interest due to emerging application areas and several high profile successes in device and technology development. For both performance and cost reasons, there is a worldwide trend towards miniaturising silicon photonic waveguides. The shrinking of the device dimensions provides advantages in terms of cost and packing density, modulation bandwidth, improved performance in resonant structures, and an increase in optical power density within the devices. However, the size reduction comes at some costs in increased difficulty in maintaining single mode operation of the waveguides whilst controlling the polarisation properties of the device.
For some applications, it is essential that optical modulators and filters, as well as other components, operate in the single mode regime. It is also desirable that these components are polarisation insensitive. Design of the waveguide, which is the basic element of these devices, is therefore crucial. Due to the compatibility with single mode fibre dimensions as well as the possibility of control of polarisation, rib waveguides represent promising candidates for integration in SOI. The main limitation for rib waveguides is that the bend radius cannot always be sufficiently small to minimise the device footprint. Strip waveguides, on the other hand, are often considered as a good choice in this respect, as they allow very small bend radii resulting in a compact footprint. However, polarisation dependence of these waveguides can be significant. Therefore, both waveguide configurations are investigated in this paper, together with the implications for optical modulators.
Following from this discussion, we also consider optical modulators that operate in the depletion mode with the intrinsic bandwidth of several tens of gigahertz. We have previously reported a modulator based on a small rib waveguide with the height of < 500nm for high speed operation. However, in this paper we consider slightly larger designs to accommodate polarisation independence. Finally we discuss the characteristics of ring and racetrack resonators based on both rib and strip waveguides and methods of improving free spectral range whilst considering polarization effects. Both theoretical and experimental results are presented. The maximum free spectral range that we have demonstrated experimentally is ∼43nm.