Low-loss and polarization-insensitive suspended fibre-to-waveguide mode size converter with a vertical multi-tip structure

This paper proposes a low-loss, polarization-independent mode size converter designed to achieve efficient coupling between standard single-mode optical fibres and on-chip silicon nanowire waveguides. The core structure comprises three tapered silicon waveguides positioned at the centre of a suspended SiO2 waveguide, arranged vertically and gradually varying in size along the horizontal direction. The multi-tip design in the vertical direction enables simultaneous expansion of the silicon waveguide mode size in both horizontal and vertical dimensions, while the suspended structure ensures effective mode expansion. By introducing a refractive index-matching liquid, the simulated coupling losses for TE and TM modes at a wavelength of 1550 nm are −0.35 dB/facet and −0.43 dB/facet, respectively. Additionally, this mode size converter features a 1 dB wavelength bandwidth exceeding 100 nm and a 1 dB alignment tolerance greater than ±2.8µm in both horizontal and vertical directions.