Page 252 - Optofluidics Fundamentals, Devices, and Applications

Bio-Inspir ed Fluidic Lenses for Imaging and Integrated Optics 227

as described in Sec. 2-1-3. Stretching the membrane before mounting
it to the aluminum fluid chamber prevents wrinkles. The amount of
prestretch is defined as
ΔR × 100(%)
R
0
where ΔR is the radial displacement by the applied tensile stress and
R is the initial radius in the relaxed state. Fluid is then injected into
0
the membrane-mounted chamber till the membrane is deformed to
the desired shape. The amount of fluid injection is controlled by a
syringe pump and the fluid pressure is monitored by a pressure sen-
sor. To fabricate lenses using the inflated (or deflated) membrane as a
mold, uncured PDMS is poured onto the membrane. After curing, the
profile of the membrane of the liquid-filled mold is transferred to the
cured PDMS. Demolding the cured lens from the membrane is rather
easy because cross-linking does not occur between the second PDMS
and the already-cured membrane. As a result, focus-fixed lenses of
complementary surface profiles to the liquid-filled mold are pro-
duced. The fabrication process of a focus-fixed plano-convex PDMS
lens out of a concave fluid-filled mold is illustrated in Fig. 9-22.

(a)

(b)

(c)
Al block Fluid

PDMS membrane PDMS lens
FIGURE 9-22 Process fl ow of molding a convex lens: (a) achieve the desired
curvature by controlling the fl uid volume and/or pressure; (b) pour PDMS
prepolymer onto the membrane and cure PDMS; and (c) demold PDMS lens
from the liquid-fi lled mold. (Adapted from Sung Hwan Cho, Frank S. Tsai,
Robert Vasko, Jeff Vasko, and Yu-Hwa Lo, “Fluid-fi lled tunable mold for
polymer lens,” IEEE Conference on Lasers and Electro-Optics, 2008 and
2008 Conference on Quantum Electronics and Laser Science, Copyright
2008 IEEE.)