Page 170 - Optofluidics Fundamentals, Devices, and Applications

Optofluidic Photonic Crystal Fibers: Pr operties and Applications 145

F

Symmetric
S
mode field Starting polarization
state (heating)

Slow
Asymmetric
mode field
Polymer Fast
Silica Starting polarization
state (heating)
(b)
FIGURE 7-6 (left) A schematic of the selectively polymer fi lled grapefruit fi ber.
(right) The measured birefringence induced by the selectively polymer fi lled
grapefruit fi ber. (C. Kerbage, P. Steinvurzel, P. Reyes, et al., “Highly tunable
birefringent microstructured optical fi ber,” Opt. Lett., 27, 842–844 (2002).)

manually blocking some of the microstructure apertures on the fiber
face to allow filling of only the unblocked holes.
The previous examples of optofluidic grapefruit fibers have used
optofluidic tuning as an aid to fabrication, yet once the fluid monomer
phase is introduced and polymerized, the now-solid body is rendered
immobile. While tunability is still conferred thermally in these cases,
to fully take advantage of optofluidic tuning a mobile fluid phase is
required. Figure 7-7 shows exactly how this can be achieved using a
grapefruit fiber with a single inclusion filled with a mobile fluid whose
refractive index of 1.42 is slightly below that of the surrounding silica
[66]. Now mobile, this inclusion is moved controllably into and out of
a tapered region of the fiber, thus controlling the exposure of the modal
field to the fluid producing a variable amount of polarization phase
change, again through broken symmetry in the microstructure [66].
Figure 7-8 also shows this tunable polarization. The fluid (introduced
into the fiber by selective suction described above) is actuated by a
heater, that lies away from the tapered region. The heater expands the
air in the fiber microstructure that in turn pushes the fluid.
The use of fluids inside grapefruit fibers now allows fully tunable,
reversible, and reconfigurable in-fiber devices to be realized. Figure 7-8
shows another LPG in a grapefruit fiber but now with separate
low- and high-index fluids occupying contiguous spaces along the
grapefruit fiber microstructure [67]. Two heaters are now used; one
away from the LPG to move the microfluid bodies (the pump heater)
and the other on top of the LPG to change its resonance wavelength.
The pump heater affects the LPG spectral visibility by changing the
overlap of the low-index fluid and the LPG, thereby altering the modal
overlap with the fiber core. In combination, these heaters provide
complete control over the LPG spectral response, all in fiber.