Page 168 - Optofluidics Fundamentals, Devices, and Applications

Optofluidic Photonic Crystal Fibers: Pr operties and Applications 143

attenuation is enabled through the combination of optofluidics and
photonic crystals. After this, we discuss another fiber-based opto-
fluidic component—the ultracompact microfluidic interferometer.
This device uses SMFs for optical transport and a tapered square fiber
to hold a fluid meniscus in the beam path. This refractive index con-
trast between the fluid and the surrounding area provides a phase
delay within the beam providing interferometric modulation. The use
of optofluidics again enables the compactness and reconfiguration of
the device. The last devices discussed are optofluidic photonic bandgap
fibers based on PCFs that utilize optofluidic tuning to change their
optical guidance mechanisms. We conclude with a discussion of poten-
tial future directions for fiber-based optofluidics.

7-2 Grapefruit-Fiber Optofluidic Devices
Figure 7-5 shows a cross section of a silica grapefruit fiber [9], so
named for their microstructure shaped like the flesh of the afore-
mentioned citrus fruit [64]. This fiber has a series of symmetric air
inclusions surrounding a core of germanium-doped silica identical
to a telecommunications SMF. This design makes the grapefruit
fiber ideal for optofluidic application; the holes in the microstruc-
ture provide access to the guided light field for fluids infused therein
while the doped core structure enables easy optical coupling to SMF
and holographic writing of grating structures. Optofluidic tuning in
these fibers enables a slew of in-fiber, reconfigurable components.
Indeed, grapefruit fibers were among the first designs of optical
fibers to be optofluidically tuned.
Figure 7-6 shows a schematic of the initial optofluidic grapefruit
fiber used in combination with an LPG [9]. A liquid monomer was
drawn into the fiber microstructure and cured in place using UV
light. The resulting polymer has a tunable refractive index that
decreases with increasing ambient temperature. When unmodified,
the core of the grapefruit fiber is sufficiently far from the holes that
the presence of polymer does not affect light propagation in the core.
However, if the fiber is tapered, the core mode can be made to interact
with the filled holes [31]. By tapering the grapefruit fiber and then
filling its holes with polymer, an in-fiber variable attenuator can be
made. As the polymer’s refractive index is decreased, the modal field
becomes less confined by the core in the tapered region, and increas-
ingly leaks into the cladding. This controlled leakage allows for con-
trolled attenuation inside the fiber.
More generally, shifting the polymer refractive index creates a
tunable optical environment inside the fiber. This kind of local tun-
ability can be used to influence other structures inside the fibers, such
as holographically written long-period gratings (LPGs) in the fiber
core. In that case, as the polymer in the fiber microstructure surround-
ing the LPG is heated, the refractive index of the polymer decreases