Low loss mode size converter from 0.3 µm square Si wire waveguides to singlemode fibres

A novel integrated mode size converter for single-mode Si wire waveguides is presented. The mode size converter is constructed with two-dimensional tapered Si waveguides and overlaid high-index polymer waveguides. We calculated the proposed mode size converter characteristics, and fabricated 1.09 mm length Si wire waveguides with the converters. The measured loss of the mode size converter was 0.8 dB per conversion, and the total insertion loss through the sample with an Si wire waveguide was 3.5 dB.