Polarisation independent silicon-on-insulator slot waveguides

The minimisation of birefringence, or polarisation mode dispersion, is vital for simplifying and miniaturising photonic components. In this work, we present a systematic study of the slot waveguide geometries required for having zero birefringence (ZB). We show that the rail widths required for ZB are more strongly dependent on the height of the waveguide than on the slot separation. After which, we demonstrate that the ZB geometry is significantly affected by the slanting of the waveguide walls. This paper proceeds to show that within the range studied, one can fix the height, slot, slant angle, and bend radius, and still achieve ZB by varying the widths of both of the rails. Given a fabrication tolerance of 5 nm, we show that a coherence length on the order of a hundred microns can be achieved. We finish by showing that for straight and bent ZB waveguides, having symmetric rails is preferable due to higher tolerances and lower sensitivity to bending. Since any arbitrarily shaped slot waveguide is a combination of both single mode straight and bent waveguides, we have a toolbox from which we can achieve ZB for any given slot and height.