Page 189 - Optofluidics Fundamentals, Devices, and Applications

164 Cha pte r Se v e n

propagation from a Ti:sapphire laser dominated by self-phase modu-
lation (SPM) [126], we can observe nonlinear broadening [126], soliton
propagation [126], or dispersive wave radiation [127,128] depending
on whether the dispersion is normal, anomalous, or zero at the launch
wavelength. The modal dispersion of fluidic PBGFs is also of interest
for phase-matched nonlinear processes; the fact that the modal disper-
sion profile is similar in each transmission band can enable wideband
parametric amplification across different bandgaps [129], where tem-
perature tuning may enhance the wavelength range or conversion
efficiency. Finally, whereas the experiments described here rely on the
optical nonlinearity of the silica core, the nonlinear properties of the
fluid itself offer another rich area of investigation [90,131], especially
with regard to the PBGF cladding modes [132–133].

7-6 Future Directions

7-6-1 Photonic Devices
Many of the device geometries discussed in this chapter are academic
proofs of concepts, somewhat remote from real-world applications. It
is hard to imagine that a thermally driven microfluidic optical switch
with a 2-s response time can compete against the many other, much
faster optical switching technologies that are already commercially
available. Speed, however, need not be an issue: in principle, a fluid
can be driven up to the speed of sound. As seen in some of the exam-
ples given earlier, the distances over which a fluid needs to be dis-
placed to achieve an optical response can be as short as a few microns.
This limits the reconfiguration time to a few nanoseconds, which in
many instances is more than enough. We nevertheless believe the
greatest potential for microfluidic PCF devices lies not in devices
needing ultrafast reconfiguration times, but rather in the possibility
to tune optical properties such as dispersion or filter characteristics
over wide ranges within milliseconds.
In particular, one of the most promising aspects of microfluidic
optical fibers is the demonstrated ability to tune their dispersion
properties over wide ranges—be it through the thermal or electrical
tuning of bandgaps in fluid or liquid crystal filled photonic bandgap
fibers, or through adjusting the refractive index in the holes of an
index-guiding fiber. While this tunable dispersion has already been
used to demonstrate some nonlinear phenomena at unusual wave-
lengths, as discussed in Sec. 7-5, many more applications can be fore-
seen. It has been numerically predicted that some PCF geometries
should allow second and third harmonic generation within the fun-
damental mode (as opposed to harmonic generation into a higher
order mode), allowing very large conversion efficiencies [135]. All-in-
fiber degenerate four-wave mixing for efficient wavelength conversion
from 532 nm to shorter wavelengths has also been suggested using