Page 161 - Optofluidics Fundamentals, Devices, and Applications

136 Cha pte r Se v e n

Fiber Tapering
Fiber tapering is a mature technique of postprocessing the shape of
optical fibers or, indeed, any capillary structure capable of being
heated to a plastic state [31]. Very simply, the fiber to be tapered is
heated along some length until it is softened, and then it is pulled in
opposite direction, thus applying tension to the plastic glass. This
results in the elongation of the fiber and the scaling of its internal
structure to create waveguides with customized optical properties
[32]. It is possible to fabricate tapers that continue to guide light
despite having dimensions significantly below the wavelength of
the said light [33,34]. Tapering also works on microstructured optical
fibers and, with the right parameters, the fiber structure is maintained
[32,35]. As such, fiber tapering is a simple and versatile process for
customizing silica fiber waveguides.

Bragg Gratings
Fiber Bragg gratings (FBGs) are periodic refractive index modulations
along the length of a fiber waveguide [36]. In silica telecommunications
fibers, the germanium-doped cores are photosensitive to ultraviolet
light: exposure causes a refractive index rise that is dependent on expo-
sure time, light intensity, and wavelength. So in order to fabricate an
FBG in SMF, all that needs to be done is expose the SMF core to a peri-
odic spatially varying ultraviolet light field. The discovery of the FBG
and the photosensitivity of germanium-doped silica arose from the
propagation of short-wavelength visible light through a fiber that,
upon reflection from the fiber ends, set up a standing wave, which
provided the necessary periodic intensity modulation [37]. Typical
modern FBGs are fabricated using a transverse beam of ultraviolet
laser light passed through a phase mask that generates two beams of
different diffraction orders. These beams then interfere creating the
necessary periodic intensity modulation for writing FBGs.
The characteristic response of an FBG whose period is sub-
wavelength is that it will resonantly reflect a given wavelength band
while transmitting all others [38]. As such, the FBG can be thought of
as an in-fiber wavelength-selective mirror. Two FBGs can be written
spaced apart in SMF, and this structure acts essentially as a Fabry-
Perot resonator [39]. Similarly, one continuous grating can have a
defect, or phase shift, introduced somewhere along its length and
also behave like a resonator [35]. The wavelength response of the FBG
depends upon the period of the grating. If the grating is mechanically
deformed or expanded through local heating, the resonant wave-
length of the FBG will change. This effect is the basis for many designs
of photonic environment sensors [36–38].
Long-Period Gratings
Another type of grating, the long-period grating (LPG), has a period
of several hundred wavelengths and resonantly couples light