Page 213 - Optofluidics Fundamentals, Devices, and Applications

188 Cha pte r Ei g h t

a microfluidic device was demonstrated [51,85]. Trade-offs between
focusing and angle of view were analyzed [108]. Fast actuation with
a response time of milliseconds was achieved using piezoelectric
actuators [109]. Compact varifocal micro-lens with integrated ther-
mal actuator and sensor was implemented [110]; a set of integrated
cylindrical lenses with independent biaxial tuning was constructed
[69]; variable focus was also achieved by replacing liquids with dif-
ferent refractive indices [111]; chemical actuation of pH-sensitive
hydrogel was also employed to design a varifocal lens [80]; hydro-
dynamical interface between two liquids flowing in a curved micro-
channel was exploited to construct a variable focus lens [77];
additional designs and actuation techniques have been continuously
reported [48,51,84,88,89,112,113].
Lenses mentioned in this section exhibit aspherical surfaces lead-
ing to optical aberrations, which were analyzed for a variety of con-
figurations [51,60,85,114–116]. These aberrations may be diminished
by using compound structures [117,118], introduction of an aperture
[51], arbitrary profile membranes with variable thickness [48], com-
posite membranes [119], and two different lens curvatures [86]. On
the other hand, these aberrations can be useful to compensate aberra-
tions of other optical components.
Along with imaging and aberrations control, the liquid-filled
lenses were employed for biaxial beam shaping and optical signal
processing. A set of two orthogonal cylindrical lenses [69] for beam
shaping is reviewed here in greater details. The device consists of
three chambers separated by two PDMS membranes and filled with
liquids of different refractive indices (see Fig. 8-4a). Differences in the
pressures applied to the three chambers result in bending of the mem-
branes. Under the proper design the shape of the bending membrane
is nearly cylindrical. Therefore, the shape of the beam becomes highly
elliptical with an aspect ratio of approximately 10 (see Fig. 8-4f
through 8-4h). Comparison between the beam intensity profiles in
the figure suggests that focusing along one direction has no appre-
ciable effect on the other. Therefore, variations of differential pres-
sures are directly translated into changes in the focal distances with
practically no cross talk.
Pneumatically Actuated Membrane-Based Lens
Pneumatically actuated optofluidic lenses are lenses in which a poly-
mer with varying shape is used as an optical element for focusing.
They are based on the same concept used in liquid-filled lenses—
there is a chamber with a soft wall, which bends under applied pres-
sure. In the liquid-filled lens the fluid mimics the behavior of a lens.
Pneumatically actuated polymeric lens [71] contains no liquid and
the focusing is achieved solely by the differences between the optical
path lengths lr() through the membrane across the aperture (see
Sec. 8-2-1). In the liquid-filled lens, it is the bending profile ur() alone