Spot size mode converter for efficient coupling to SiN waveguides

To miniaturize optical passive components or to have optical interconnects replace the current copper/low k interconnects for clock distribution, super high index contrast optics are needed because they allow optical waveguides with small bending radius, ie. < 50um. Silicon nitride core on oxide cladding has loss of <0.1dB/180degrees for 20um bending radius. However, coupling loss from the fiber to SiN waveguides, with 0.7umx0.7um cross section for single mode, is very large, > 20dB. To reduce the coupling loss, our approach is to have a double-core architecture, where fiber is first coupled to fiber matched waveguide, and then coupling from fiber match waveguide to SiN waveguide through a spot size mode converter. We have found the mode converter loss is reduced by 8dB by reducing the tip of the taper from 0.35um to 0.15um. In this paper, we are reported results of tips with less than 0.1um. We also describe the fabrication technology that enables us to make such fine tip with smooth surfaces.