Page 216 - Optofluidics Fundamentals, Devices, and Applications

Adaptive Optofluidic Devices 191

lenses to reduce spherical aberrations. Such lenses allow construction
of large apertures not available in other types of fluidic lenses along
with millisecond transition times. Pneumatically actuated lens can be
driven at 500 Hz with 4 diopter variation in the refractive power [71].
Focal length variations with millisecond response time are shown in
Fig. 8-5c. Considerable focusing power can be obtained at low pres-
sures (see Fig. 8-5b). Therefore, these lenses can be used for fast longi-
tudinal scanning in three-dimensional imaging.
The advantages of such lens as opposed to the liquid-filled lens
become obvious. First, the optical performance of a pneumatically
actuated lens is not compromised by the bubbles that form in the
liquid. Second, apertures much larger than in the other types of flu-
idic lenses are readily available. Finally, the lens carries no additional
mass of the liquid, so the response time is faster, and the impact of
mechanical shocks and gravitational distortions is diminished. One
drawback is the lower focusing power (i.e., dioptres) than in the other
types of fluidic lenses.

8-2-3 Composite Membrane Devices
Thin flexible membranes made of a silicon elastomer PDMS have
been intensively used in microfabricated devices to construct pressure-
actuated valves [39,143,144], check valves [145,146], and adaptive
lenses [71,95,97,142]. The character of deformation of a plain mem-
brane under pressure is defined by the shape of the frame it is attached
to, allowing for a very limited tunability. Here we focus on composite
membranes with pieces of rigid epoxy grafted inside PDMS. The
dimensions and positions of the epoxy pieces are defined with a high
precision by UV-lithography, allowing high control of its mode of
deformation under applied pressure.
Composite structures in this context are a combination of two or
more materials, each of which retains its own elastic properties. Cer-
tain combinations of several materials allow deformation to be highly
adjustable and easily tailored. This feature was successfully employed
to build tunable optical devices [119]. The pattern of epoxy grafted
into the membrane defines its mode of deformation under pressure.
Planar architecture allows standard easy and precise soft-photolithog-
raphy fabrication techniques to be used for adaptive optical devices.
It was shown in Sec. 8-1-2 how a blazed grating can be effectively
tuned by replacing the liquid on top. Manipulation of liquids with
different refractive indices allows tunable beam deflection. Another
type of tunable gratings is based on composite membrane technol-
ogy, which allows grating geometry tuning with mechanical actua-
tion [119]. The membrane’s mode of deformation is made highly
adjustable. The proposed technique was demonstrated in two types
of devices: stretcher and rotator (see Fig. 8-6a, and 8-6b). In the stretcher,
the grafted pieces of epoxy focus the pressure-induced extension of the