Page 164 - Optofluidics Fundamentals, Devices, and Applications

Optofluidic Photonic Crystal Fibers: Pr operties and Applications 139

microstructured optical fibers and the potential for integration into
existing fiber-based specifications with minimal new development
using commercially available components. Smoothness of the void
surfaces in microstructured optical fibers is on the order of 100 pm [51].
As such, microstructured optical fibers provide an essentially atomi-
cally smooth environment for microfluidic flow and photon transport.
This roughness is to be compared with that of a typical planar optoflu-
idics material, silicon on insulator, undergoing an optimized fabrica-
tion process to remove roughness, for which the minimum roughness
achieved was 1.4 nm [52]. This is almost an order of magnitude above
that in a microstructured optical fiber. These smoothed surfaces enable
very low loss optical waveguiding. As an additional benefit, micro-
fluidic flow and interface behavior in these smooth structures are
guaranteed to conform to classical theoretical behavior. Microstruc-
tured optical fibers are commercially available in a wide variety of
designs, conforming to almost every conceivable photonic guidance
requirement. The sheer variety of these fibers, coupled with standard
fiber handling and probing techniques from communications technol-
ogies means that an almost limitless variety of device designs are pos-
sible using commercial components that are compatible with existing
SMF photonic hardware. Further, creating all-fiber optofluidic devices
typically involves laboratory postprocessing of commercial fibers using
simple optofluidic and photonic techniques, circumventing the need
for expensive fiber fabrication infrastructure.
Figure 7-2 shows an example of the all-fiber optofluidic fluid refrac-
tive index sensor [53], a measurement typical in optofluidic systems.
This design runs a channel between two separated FBGs, acting as a
Fabry-Perot resonator. Light propagation in the exposed core suffers a
phase delay that is again dependent upon the index of the surrounding
medium. Another design involves etching SMF [54] to expose the core
to the ambient fluid. Yet another design involves exposing an FBG
written into the core of an SMF, making the resonant wavelength
dependent upon the index of the surrounding medium. These two
designs provide a coarse and fine measurement of ambient refractive
index, respectively. The components used for this device are standard
optical communications equipment (SMF, holographically written UV
FBGs, and hydrofluoric acid etching) probed using photonics standard
tunable laser sources and spectral measuring equipment. Further, these
devices are all in SMF, automatically guaranteeing compatibility with
other SMF-based devices and networks. Similar work has been per-
formed elsewhere [55] using different methods to expose the fiber-
guided light to the fluid. The device shown in Fig. 7-2 is, in fact, a
hybrid optofluidic device; a planar substrate is used for the microflu-
idic layer and optical fibers for optical control.
Figure 7-3 shows another all-fiber optofluidic technology—the
selective filling of voids in a hollow-core PCF [56,57]. These devices
use methods relying on differential capillary force across all holes in