Page 257 - Optofluidics Fundamentals, Devices, and Applications

Bio-Inspir ed Fluidic Lenses for Imaging and Integrated Optics 231

The simplest nonimaging opto-fluidic lens takes the form of a
modified waveguide. The end facet(s) will be curved to yield a focus-
ing or defocusing effect. By controlling the curvature, the refractive
index difference, and the dispersion of the substrate and the optical
fluid, the effective focal length can be tailored to suit the needs of the
system. Glebov et al. have created arrays of such curved waveguide
facets in conjunction with index-matching fluid for use in dense
wavelength division multiplexing (DWDM) networks, where arrayed
beam collimation and focusing is required [81].
While such curved-facet waveguides provide the ability to add
some light beam manipulation into planar optical designs, their capa-
bilities can be rather limited, compromising the performance of many
miniaturized systems relative to their macro counterparts. The lens
positions and lens power have been restricted by the location and
dimensions of the waveguide ends. A more fully functional miniatur-
ized analog to a benchtop optical system would include “freestand-
ing” lenses that can be any aperture size or any curvature needed,
and can further be positioned at any location needed to create the
desired system.
The “freestanding” fluid-filled two-dimensional lens is an analog
to the air-pocket lens, allowing freely positionable lenses with the
further ability to choose the refractive index difference and the lens
dispersion qualities. These benefits, offered by the use of fluid, allow
lens power and chromatic dispersion to be chosen by design. Filling
is readily accomplished via capillary action or with the aid of a vac-
uum chamber. Two-dimensional fluid-filled optics, such as prisms,
y-couplers, and lensed waveguide facets, were demonstrated by Kou
et al. [87] in 2004. Godin et al. demonstrated freestanding fluid-filled
lenses in 2006 [86,93,94], allowing for systems comprising multiple
lenses to be realized. Such lenses can readily be designed with highly
custom profiles, such as aspheric or parabolic curvatures. This pro-
vides a significant advantage over bulk systems, where creating such
unique curvatures becomes costly and labor intensive.
While the use of lens fluid helps to reduce the refractive index
difference and thereby reduce reflective losses, the need for optically
smooth sidewalls nevertheless remains. For mold-replicated devices,
smooth sidewalls start with a smooth mold. Smoother molds can be
obtained by using improved etching techniques, such as cryogenic
etching, or improved photolithography processes (when creating
polymer-based molds). Etched molds allow for post-etch smoothing
processes, such as anisotropic wet etching [82]. To keep improving the
performance of two-dimensional optical systems, increasing attention
needs to be paid to sidewall smoothness.
Two-dimensional optics lack the ability to contain and control
light in one dimension, generally out of the plane of the chip or
device. For very short optical paths, vertical losses may not be cata-
strophic for some applications. For applications such as optical