Tailoring the Response of Silicon Photonics Devices

Abstract

Shrinking waveguide dimensions in silicon photonics results in a series of performance enhancements, but at some cost. We analyse the waveguide geometry in optical modulators and filters to address some issues associated with this trend.Silicon photonics has experienced rapid development for the last three years and several significant results have been reported, demonstrating the viability of the technology [e.g., 1, 2, 3, 4]. One of the recent trends in silicon photonics has been the reduction of waveguide dimensions. This reduction facilitates tighter bending radii and therefore a smaller device footprint which in turn, significantly reduces the cost. Furthermore, technical performance of many silicon photonic devices is enhanced. However, there are also some issues with this trend to smaller dimensions, notably increased propagation losses, increased polarisation dependence and difficulty in coupling to/from optical fibres. These issues can be overcome with a careful design of the waveguide and device geometry.We have previously shown that by reducing waveguide dimensions, multi-GHz bandwidth optical modulators can be achieved by utilising a horizontal pn junction in a waveguide with an overall height of 450 nm [5]. Although the waveguide structure of these previous devices has been designed to approach birefringence-free propagation, the fact that the pn junction is horizontal introduces a difference between the phase shifts for TE and TM polarisations, and hence the device itself is not polarisation independent.In order to improve the polarisation performance to approach polarisation independence of the modulator, we propose here a modulator with a pn junction that is a V-shape structure, as shown in Figure 1. The V-shape is based on the natural etch angle of silicon (54.7 degrees). The operation of the modulator is similar to that presented in [5] in that it is based upon a pn junction. If the rib waveguide is polarisation independent, by varying the height of the V-shaped pn junction in the rib, it is now possible to achieve an identical phase shift for TE and TM polarisations. Highly doped siliconSilicon dioxide Highly doped siliconHighly doped silicon