This paper studies two different approaches for evanescent wave optical sensing: an horizontal one and a vertical one. In horizontal waveguides, the evanescent wave is distributed on the upper cladding. While in a vertical configuration, the evanescent wave is distributed on the left and right sides of the waveguide. In an horizontal configuration the evanescent wave can be also on both sides of the waveguide in order to increase the optical energy for sensing if the substrate under the waveguide is locally removed. However, in this configuration to achieve sensitive devices, the layers have to be freestanding and thin [1] limiting practical implementations of such approaches. Furthermore, very few materials can be defined as tall and thin in the case of a vertical configuration, as the deposition techniques often used (PECVD/LPCVD) are meant for films in the couple of micron range. In the following we will investigate the properties of the materials used but also the fabrication feasibility for both configurations.@en

In this Letter, a slope transfer method to fabricate vertical waveguide couplers is proposed. This method utilises wet etched Si as a mask, and takes advantage of dry etching selectivity between Si and SiC, to successfully transfer the profile from the Si master into SiC. By adopting this method, a <2° slope is achieved. Such a taper can bring the coupling efficiency in SiC waveguides to 80% (around 1 dB loss) or better from around 10% (10 dB loss) without taper. It further increases the alignment tolerance at the same time, which ensures the successful development of a plug-and-play solution for optical sensing. This is the first reported taper made in SiC.

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