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.
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.
Silicon-on-Insulator (SOI) is a very interesting material system for highly integrated photonic circuits. The high
refractive index contrast allows photonic waveguides and waveguide components with submicron dimensions
to guide, bend and control light on a very small scale so that various functions can be integrated on a chip.
Moreover, SOI offers a flexible platform for integration with surface plasmon based components which in turn
allows for even higher levels of miniaturization. Key property of both waveguide types is the mode distribution
of the guided modes: a high portion of the light is concentrated outside of the core material, thus making them
suitable for sensitive detection of environmental changes.
We illustrate chemical and label-free molecular biosensing with SOI microring resonator components. In
these microring resonator sensors, the shift of the resonance wavelength is measured. A ring of radius 5 micron
is capable of detecting specific biomolecular interaction between the high affinity protein couple avidin/biotin
down to a few ng/ml avidin concentration. We describe the integration of surface plasmon waveguides with SOI
waveguides and discuss the principle of a highly sensitive and compact surface plasmon interferometric sensor
suitable for biosensing. The device is two orders of magnitude smaller than current integrated SPR sensors, and
has a highly customizable behavior. We obtain a theoretical limit of detection of 10-6 RIU for a component of
length 10 microns. We address material issues and transduction principles for these types of sensors.
Besides in chemical sensors, the SOI microring resonators can also be used in physical sensors. We demonstrate
a strain sensor in which the shift of the resonance wavelength is caused by mechanical strain. We have
experimentally characterized the strain sensors by performing a bending test