Page 159 - Optofluidics Fundamentals, Devices, and Applications

134 Cha pte r Se v e n

crystal fiber (PCF) tuned using microfluidics. We review optofluidics
in a variety of MOFs, including grapefruit fibers, transverse PCFs,
fluid-filled PCFs transformed into bandgap-guiding fibers, and fiber-
based ultracompact optofluidic interferometers.

7-1 Introduction

7-1-1 Optical Fibers
Since optical fibers are the defining characteristic of the optoflu-
idic devices discussed here, a variety of relevant optical fiber
designs warrant discussion. The ubiquitous silica telecommunica-
tions single-mode fiber (SMF) [6] is used for optical transport,
while microstructured optical fibers (MOFs) [9–14], including
photonic crystal fibers (PCFs) [15–19] are used for both optical and
microfluidic transport and manipulation. In this section, both vari-
eties will be discussed, with emphasis on the MOFs that form the
optofluidic devices.
The most prevalent optical fiber design is the silica telecommuni-
cations SMF [6]. After many years of research and development, the
SMF is deployed across the world, providing the physical backbone
of modern communications. A consequence of this widespread use is
that SMF and its associated light source and detection technologies
provide high-quality, low-loss, modular means of generating, trans-
porting, and analyzing light. This platform forms the basis for inves-
tigation into fiber-based optofluidic devices. SMFs are composed of
highly pure fused silica whose circular core is a region, 8 μm in diam-
eter silica doped using heavy ions during the fabrication process [20].
This doping provides a refractive index contrast against the sur-
rounding silica cladding that is 125 μm in diameter and itself is
protected by a variety of polymer coatings. Typically, these fibers
support a single propagating mode at 1550 nm with a multimode
cutoff wavelength of around 1100 nm. The single mode propagates
with very low loss, approximately 0.2 dB/km [21].
Figure 7-1 shows several more recent developments in fiber
waveguides: the MOF. MOFs are characterized by hollow inclusions
that run the length of the fiber. These come in many forms, from fibers
with a single hole forming a hollow core [22] to fibers with inclusions
that surround an SMF-like core, known as a grapefruit fiber [23]. A
special class of MOF is the PCF. These fibers consist of a periodic
array of inclusions surrounding a core that may be either solid [16] or
hollow [18,24]. These fibers display a wealth of optical phenomena
[25–30]; however, it is the cladding/core inclusions comprising the
fiber microstructure that are of interest in fiber-based optofluidics.
These inclusions form a natural home for microfluids, and their over-
lap with the guided mode of the fiber allows them to influence the