2.8 Photonic Crystal Fiber Concepts
79
Fig. 2.27 Cross-sectional
end view of the structure of
one type of photonic
bandgap fiber
Air core
Buffer coating
Cladding with
embedded holes
Pitch Λ
Hole
diameter d
is analogous to the role of a periodic crystalline lattice in a semiconductor, which
blocks electrons from occupying a bandgap region. In a traditional PBG fiber the
hollow core acts as a defect in the photonic bandgap structure, which creates a region
in which the light can propagate. Whereas modes at all wavelengths can propagate
along index-guiding fibers, the guided light in a PBG fiber is allowed in only a
relatively narrow wavelength region with a width of approximately 100–200 nm.
Figure 2.27 shows a two-dimensional cross-sectional end view of the structure
of an example PBG fiber. Here a large central hollow core is formed by removing
material in the center of the fiber from an area that can be occupied by seven air holes.
Such a structure is called an air-guiding or a hollow-core PBG fiber, and allows about
98% of the power in the guided modes to propagate in the air hole regions. Analogous
to an index-guiding fiber, the holes in the cladding region have a diameter d and a
pitch . Such hollow-core fibers can have a very low nonlinearity and a high damage
threshold. Thus PBG fibers can be used for dispersive pulse compression with high
optical intensities. In addition, fiber optic sensors or variable power attenuators can
be constructed by filling PBG fibers that have larger core holes with gases or liquids.
2.9 Optical Fiber Cables
In any practical application of optical waveguide technology, the fibers need to be
incorporated in some type of cable structure [49–53]. The cable structure will vary
greatly, depending on whether the cable is to be pulled into underground or intrabuilding ducts, buried directly in the ground, installed on outdoor poles, or submerged
underwater. Different cable designs are required for each type of application, but
certain fundamental cable design principles will apply in every case. The objectives
79
Fig. 2.27 Cross-sectional
end view of the structure of
one type of photonic
bandgap fiber
Air core
Buffer coating
Cladding with
embedded holes
Pitch Λ
Hole
diameter d
is analogous to the role of a periodic crystalline lattice in a semiconductor, which
blocks electrons from occupying a bandgap region. In a traditional PBG fiber the
hollow core acts as a defect in the photonic bandgap structure, which creates a region
in which the light can propagate. Whereas modes at all wavelengths can propagate
along index-guiding fibers, the guided light in a PBG fiber is allowed in only a
relatively narrow wavelength region with a width of approximately 100–200 nm.
Figure 2.27 shows a two-dimensional cross-sectional end view of the structure
of an example PBG fiber. Here a large central hollow core is formed by removing
material in the center of the fiber from an area that can be occupied by seven air holes.
Such a structure is called an air-guiding or a hollow-core PBG fiber, and allows about
98% of the power in the guided modes to propagate in the air hole regions. Analogous
to an index-guiding fiber, the holes in the cladding region have a diameter d and a
pitch . Such hollow-core fibers can have a very low nonlinearity and a high damage
threshold. Thus PBG fibers can be used for dispersive pulse compression with high
optical intensities. In addition, fiber optic sensors or variable power attenuators can
be constructed by filling PBG fibers that have larger core holes with gases or liquids.
2.9 Optical Fiber Cables
In any practical application of optical waveguide technology, the fibers need to be
incorporated in some type of cable structure [49–53]. The cable structure will vary
greatly, depending on whether the cable is to be pulled into underground or intrabuilding ducts, buried directly in the ground, installed on outdoor poles, or submerged
underwater. Different cable designs are required for each type of application, but
certain fundamental cable design principles will apply in every case. The objectives