Page 162 - Optofluidics Fundamentals, Devices, and Applications

Optofluidic Photonic Crystal Fibers: Pr operties and Applications 137

forward into higher order core or cladding modes of the fiber [38]. In
conventional fibers, the dispersion of cladding modes, and as a con-
sequence the spectral response of LPGs, depends on the ambient
refractive index. Fiber LPGs are thus another useful structure for
fiber-based sensors [39]. In MOFs, the cladding modes LPGs couple
can have a large overlap with an MOF’s holes, so that the LPG spec-
tral response can be very sensitive to the content of the holes. This
enables all-in-fiber refractive index sensors, or microfluidic tunable
filters, and is further discussed as follows.
When the fiber incorporates one or more germanium-doped cores,
long-period gratings can be made using UV exposure in the same way
as Bragg gratings. However, LPGs can also be imprinted using a large
number of other techniques, discussed in Secs. 7-2 and 7-5.

7-1-3 Optofluidics: History and Development
Optofluidics is something of a derivative science, borrowing and
combining techniques and technologies from the more established
fields of microphotonics and microfluidics [40,41]. At first glance,
these two fields would appear highly disparate: one pertaining to
photon transport, usually associated with communications [6], and
the other relating to highly confined fluid flows and small-volume
chemical reactions [42]. However, closer comparison shows a wealth
of similarities in scale, structure, and transport phenomenon, and a
number of ways in which the nature of one may complement the
other. Devices designed using optofluidic principles have a number
of inherent advantages over photonic or microfluidic devices indi-
vidually. These advantages arise from controlled fluids (microfluidic
layer) that interact with controlled light (photonic layer). This inter-
action can then be used in a variety of ways: the optical properties
depending on the exact fluids at play, integrated micro-optofluidic
sensors are an obvious application, as are photonic devices (attenua-
tors, polarization controllers, dispersion compensators, and delay
lines) made tunable through the microfluidic layer.
Optofluidic design can be very generally applied to any photonic
structure with a void near the optical field in the device; given suitable
fluid properties and pressure gradients, fluids can be made to infil-
trate practically any photonic structure that is open to the outside.
This infused fluid interacts with the optical field in the device chang-
ing local optical properties. Fluids are also an inherently mobile phase.
This allows a localized region of certain optical properties to be propa-
gated to other parts of the device to allow modulation of the photonic
structure. Importantly, this modulation can be achieved with little
modification to the initial photonic structure upon which the optoflu-
idic device is based. The use of discrete bodies of fluid in optofluidics
also allows the air surrounding it to provide refractive index contrast
greater than that available in the surrounding solid structure.