Page 190 - Optofluidics Fundamentals, Devices, and Applications

Optofluidic Photonic Crystal Fibers: Pr operties and Applications 165

fluid-filled bandgap fibers [129], as has multiorder dispersion engi-
neering assisted by microfluidics for optimal four-wave mixing [136].
In both cases, the control over the dispersion required to achieve
phase matching is rather stringent, but realistic using tunable fluid-
filled PCFs. Recent development in the understanding of supercon-
tinuum generation also shows that a close control over dispersion
properties can increase dispersive wave generation and hence
improve the power density in the blue part of the spectrum [137,138].
Fluid-filled PCFs further offer the possibility to locally tune the dis-
persion along the length of the fiber. This could be used for soliton
compression, to gain further control over supercontinuum genera-
tion [139], or even for harnessing optical rogue waves [140,141]. Using
liquid cores to generate supercontinuum over longer wavelength
ranges has also been suggested [90,130]. Finally, PCFs in general have
been proposed for creating ultraflat dispersion fibers, or for disper-
sion compensation in telecommunications links [142–146]. However,
many of the PCF designs suggested for these purposes require unre-
alistically stringent fabrication tolerances. By adding the tunability
that fluids provide in PCFs, fabricating these devices could become
more realistic [147].
Fluid-filled PCFs used as tunable spectral filters also offer unique
characteristics that are bound to be used in future work. We have
already discussed the tunable band and notch filters that can be
achieved combining PCFs, their bandgaps, LPGs, and microfluidics.
Tunable short-pass filters can also be made, using the refractive index
dependence of the fundamental core mode cutoff within a PCF taper
[32,148]. However, it is perhaps the fact that these filters can be dis-
tributed along an appropriately designed fiber that will attract the
most interesting applications and further work. Indeed, gain-doped
solid-core PBGFs inherently suppress unwanted optical emission in
fiber amplifiers and lasers [149]. Making this effect tunable through
microfluidics should allow unprecedented control over amplifier
noise and enable higher power all-fiber tunable lasers.
We briefly mentioned in the preceding section that the nonlinear-
ity of the fluids infiltrated in PCFs offers new possibilities that have
only just started to be explored [90,131–133]. PCFs, with their 2-D
periodic arrangement of very long holes, indeed offer a unique plat-
form for the experimental analysis of 2-D discrete nonlinear dynam-
ics: when filled with high-index fluids, each hole of the PCF becomes
an individual waveguide, so that the fiber becomes an almost ideal
array of coupled waveguides. Other work [132] has shown that the
thermo-optical nonlinearity of the fluids creates nonlocal, nonlinear
coupling between waveguides, and has demonstrated nonlocal gap
solitons in such a geometry [132]. It has also been shown that nonlin-
ear localization in space and time (space-time solitons, or light “bul-
lets”) can occur in the fluid-filled PCF geometry [133], a phenomenon
that cannot exist in continuous nonlinear media. Fluid-filled PCFs