Index Terms-Carrier injection, optical phase modulator, plasma dispersion effect, rib waveguides, silicon-on-insulator (SOI), silicon photonics.

Abstract-The fabrication restrictions that must be imposed on the geometry of optical waveguides to make them behave as single-mode devices are well known for relatively large waveguides, with shallow etch depth. However, the restrictions for small waveguides ( 1 m or less in cross section) are not well understood. Furthermore, it is usually a requirement that these waveguides are polarization independent, which further complicates the issues. This paper reports on the simulations of the conditions for both single-mode behavior and polarization independence, for small and deeply etched silicon-on-insulator (SOI) waveguides. The aim is to satisfy both conditions simultaneously. The results show that at larger waveguide widths, waveguide etch depth has little effect on the mode birefringence because the transverse-electric (TE) mode (horizontal-polarized mode) is well confined under the rib region. However, at smaller rib widths, the etch depth has a large influence on birefringence. An approximate equation relating the rib-waveguide width and etch depth to obtain polarization-independent operation is derived. It is possible to achieve single-mode operation at both polarizations while maintaining polarization independence for each of the waveguide heights used in this paper but may be difficult for other dimensions. For example, a 1-m SOI rib waveguide with an etch depth of 0.64 m and rib width of 0.52 m is predicted to exhibit such characteristics.

Silicon Photonics is experiencing a dramatic increase in interest due to emerging applications areas and several high profile successes in device and technology development. Despite early work dating back to the mid 1980s, dramatic progress has been made in recent years. Whilst many approaches to research have been developed, the striking difference between the work of the early to mid 1990s, and more recent work, is that the latter has been associated with a trend to reduce the cross sectional dimensions of the waveguides that form the devices. The question arises therefore as to whether one should move to very small strip waveguides (silicon wires) of the order of 250nm in height and a few hundred nanometres in width for improved device 2 performance but with little hope of polarisation independence, or to utilise slightly larger rib waveguides that offer more opportunity to control the polarisation dependence of the devices. In this paper we discuss devices suitable for one approach or the other and present designs associated both with strip and rib waveguides. In particular, we present designs of polarisation independent ring resonators with FSRs up to 12nm, we propose modulators for bandwidths in the 10s of GHz regime, and we present grating based couplers for rib and strip waveguides, and/or for wafer scale testing, as well as a novel means of developing Bragg gratings via ion implantation.

A flat spectral response has long been a requirement in photonic networking. In order to find a low cost alternative compared to some other technologies, a novel method is demonstrated to achieve such a response in silicon-on-insulator arrayed waveguide gratings (AWG) through free carrier absorption, implemented by ion implantation of dopant species. The AWG is designed using 1.5mum Si-overlayer on an SOI wafer utilising rib waveguides with a width of 1.1mum and an etch-depth of 0.88mum to facilitate the single-mode, birefringence-free operation. It is also essential to achieve a uniform dopant concentration throughout the guiding region to avoid any phase errors resulting from the free carriers. This can be achieved using multiple ion implantation steps. Both n and p type dopants are investigated and results showed significant reduction of doping length is achieved by using n-type dopant as compared to a p-type dopants. The broadened passband is measured to be 0.5nm, a 5 times broadening from the Gaussian peak.

Third order grating filters fabricated in small Silicon-on-Insulator rib waveguides are demonstrated. Variations in grating etch depth and duty cycle are considered, and a maximum experimental reflection of 42% is demonstrated for gratings of 1500 µm in length, with a grating period of approximately 689 nm and an etch depth of 200 nm. Agreement with modeling is shown to be good.