Biophotonics_Concepts_to_Applications

the homogeneous wave equation in thefiber and the boundary condition at the
waveguide surfaces can propagate along thefiber.
Figure3.1illustrates the structure of a conventional opticalfiber. This structure
consists of a cylindrical silica-based glasscoreof refractive index n 1 , which is
surrounded by a glasscladdingthat has a slightly lower refractive index n 2 .An
external elastic polymer buffer coating protects thefiber from mechanical abrasions
and environmental effects such as water, oil, and chemicals that can erode the glass.
The refractive index of pure silica decreases with increasing wavelength, for
example, it ranges from 1.462 at 500 nm to 1.443 at 1600 nm. The index can be
changed slightly by adding impurities such as germanium oxide to the silica during
thefiber manufacturing process.
Variations in the material composition and the structure of the conventional
solid-corefiber determine how a light signal propagates along afiber and also
influence how thefiber performs in response to environmental perturbations, such
as stress, bending, and temperaturefluctuations. The two basicfiber types shown in
Fig.3.2are produced from variations in the material composition of the core. In the
step-indexfiberthe refractive index is uniform throughout the core and undergoes
an abrupt change (or step) at the cladding boundary. In agraded-indexfiberthe
core refractive index decreases as a function of the radial distance from the center of
thefiber.
The step-index and graded-index fibers can be further categorized into
single-mode and multimode classes. Asingle-modefiber(SMF) can support only
one mode of propagation, whereas many hundreds of modes can propagate in a
multimodefiber(MMF). Typical sizes of single-and multimodefibers are shown in
Fig.3.2to illustrate the dimensional scale. A MMF has several advantages com-
pared to a SMF. The larger core radius of a MMF makes it easier to launch optical
power into thefiber and to collect light from a biological sample. An advantage of
multimode graded-index fibers is that they have larger data rate transmission
capabilities than a comparably sized multimode step-indexfiber. Single-modefibers
are better suited for delivering a narrow light beam to a specific tissue area and also
are required for applications that involve coherence effects between propagating
light beams.
In practical conventionalfibers the core of radiusahas a refractive index n 1 ,
which is nominally equal to 1.48. The core is surrounded by a cladding of slightly
lower index n 2 , where n 2 =n 1 (1−Δ). The parameterΔis called thecore-cladding

Elastic protective Core n 1

buffer coating

Core diameter 2a

Cladding

n 2 < n 1

Fig. 3.1 Schematic of a
conventional silicafiber
structure

56 3 Optical Fibers for Biophotonics Applications