Fibre-chip coupling with silicon micro-lenses

Direct coupling from fibres to waveguides requires field matching to achieve high coupling efficiency. 3D on-chip tapers are well suited in principle, but practicability may depend on materials and geometrical structures used for the waveguide chip, especially when photonic crystal defect waveguides are involved. To adapt the spot size of the fibre mode to that of a waveguide with high index contrast, silicon micro-lenses can be used advantageously. Although positioning requires finer resolution with small foci this can be accomplished by state-of-the-art micro-positioning equipment. A theoretical description of the hybrid coupling concept and corresponding numerical results are presented, where the design covers the range from free-space ray-trace to FDTD at the chip-level. Experimental results for the characterisation of the optimised aspheric silicon micro-lenses are reported as well.