We present the design and fabrication of a refractive polymer wedge that allows perfectly vertical coupling of
light into a silicon waveguide, which is of interest for flip-chip bonding of vertical cavity emitting light sources
on a silicon integrated circuit. The structure includes a conventional diffractive grating coupler that requires
off-normal incidence to avoid second order Bragg reflections. The polymer wedge is thus used to refract vertically
impinging light into an off-normal wave that couples into the underlying grating. The fabrication involves two
steps: mold fabrication and imprint replication. Firstly negative wedge-shaped craters are etched into a quartz
mold by Focused-ion-beam milling. Secondly the mold is used to imprint a UV-curable polymer onto a silicon chip
containing waveguides and grating couplers, and so replicating the wedges. The characterization setup consisted
of a fiber-to-fiber transmission measurement of a silicon waveguide equipped with a pair of grating couplers and
polymer wedges. The obtained fiber coupling efficiency was equal to the efficiency of regular grating couplers
and fiber positioned at an off-normal angle. The proposed fabrication method enables low cost integration of
vertical cavity emitting light sources on silicon integrated photonic circuits.
The interfacing of an optical fiber and a photonic integrated circuit becomes more complex on a high refractive index
contrast waveguide platform due to the large mismatch in mode size between the optical fiber mode and the waveguide
modes in the integrated circuit. In this paper we review our work in the field of diffractive grating structures, in order to
realize a high efficiency, polarization independent, large bandwidth optical interface with high index contrast
waveguides fabricated on the silicon-on-insulator platform.
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