Page 171 - Optofluidics Fundamentals, Devices, and Applications

146 Cha pte r Se v e n

S
Capillary
F
Fluid heater
Fluid Air channels

Taper Waist
Air channel
3
Analysis
Experiment
2
Phase change (π) 1

0

Poincaré sphere 0 10 20 30 40
ΔT (°C)
FIGURE 7-7 (top) A schematic of the selectively fl uid-fi lled grapefruit fi ber. (bottom)
The measured birefringence induced by the selectively fl uid-fi lled grapefruit fi ber.
(C. Kerbage, M. Sumetsky, and B. J. Eggleton, “Polarisation tuning by micro-fl uidic
motion in air-silica microstructured optical fi ber,” Electron. Lett., Copyright 2002 IEEE.)

The final evolution of optofluidic tuning inside grapefruit fibers is
to consider the fluid body as an optical element in its own right, rather
than as simply a way to modulate another structure in the fiber. To
this end, an LPG was fabricated out of the microfluid itself, residing
in the fiber microstructure [67]. Figure 7-9 shows a schematic of the
fluid LPG. The grapefruit fiber is tapered to allow access to the light
guided by the microstructure. Then fluid is suctioned into the micro-
structure as before; however, this time the fiber is rapidly, mechani-
cally oscillated in and out of the reservoir from which the microfluid
is drawn. This results in periodically spaced fluid and air bubbles that
form the elements of refractive index contrast in the fluid LPG. Since
the LPG is mobile, it can be moved away from the tapered region on
demand, effectively turning off the LPG resonance. Figure 7-9 also
shows that, upon applying an external heating source, the period of
the LPG can be dynamically and reconfigurably tuned, allowing com-
plete control over the wavelength of the LPG resonance.