Page 167 - Optofluidics Fundamentals, Devices, and Applications

142 Cha pte r Se v e n

0

7,8 6
–4
Transmission (dB) completely Increasing 4 3
–3 mm –8 Fiber 5
channel

LPG –12 filled overlap 2 1
fiber –16
0
1500 1520 1540 1560
Wavelength (nm)
–300 μm
Fluid channel
FIGURE 7-4 (left) An optofl uidically tuned LPG fi ber that utilizes external optofl uidic
tuning to alter the transmission of the LPG. (right) Spectrum of the tuned LPG
demonstrating controllable extinction. (F. Cattaneo, K. Baldwin, Shu Yang, T.
Krupenkine, S. Ramachandran, J.A. Rogers, Digitally tunable microfl uidic optical
fi ber devices,” J. Microelectromech. Syst., Copyright 2003 IEEE.)

planar microfluidic channel [62]. The channel contains integrated
electrodes that are used to drive a fluid plug using electrowetting.
Normally, the LPG couples to cladding modes exposed to air sur-
rounding the fiber, but when the fluid plug is pumped onto the
surface of fiber, the optical response of the LPG changes. Another,
similar, device was demonstrated this time monitoring the back-
reflected modes of an FBG [63]. The same structure has been serial-
ized, providing a number of fluid intersections along the one LPG for
a greater degree of control by allowing discrete overlaps of the fluid
body with the fiber grating. These devices embody the advantages of
the semiplanar device: a high-quality photonic structure embedded in
a compact, planar microfluidic structure with advanced actuation tech-
nology leading to a high degree of compactness and functionality.
In the remainder of this chapter we discuss a variety of fiber-based
optofluidic devices complementary to those described earlier. We begin
with one of the earliest fiber-based optofluidic devices based upon
fluid inclusion inside the microstructure of a grapefruit fiber. These
inclusions serve to alter the local waveguiding properties of the fiber in
a controllable fashion to provide significantly enhanced functionality
over fiber devices alone, without compromising connectivity in SMF-
based light paths. We go on to discuss optofluidic tuning of transverse
PCFs. The transverse geometry renders the PCF microstructure into a
planar two-dimensional (2-D) photonic crystal. The introduction of
microfluidics into the microstructure allows for tuning the partial band-
gap in a dynamic and reconfigurable fashion. This tuning is applied to
make an optofluidic attenuator whose compactness and strong