2.3 Optical Fiber Configurations and Modes
49
Fig. 2.14 A conventional silica fiber has a circular solid core of refractive index n 1 surrounded by
a cladding with a refractive index n 2 < n 1 ; an elastic plastic buffer encapsulates the fiber
Although many different configurations of the optical waveguide have been
discussed in the literature, [4] the most widely accepted structure is the single solid
dielectric cylinder of radius a and index of refraction n 1 shown in Fig. 2.14. This
cylinder is known as the core of the fiber. The core is surrounded by a solid dielectric cladding, which has a refractive index n 2 that is less than n 1 . As described in
Sect. 2.3.5, the cladding material is carefully selected to enable light to propagate
efficiently along the core of the fiber. In addition, the cladding reduces scattering
loss that results from dielectric discontinuities at the core surface, it adds mechanical
strength to the fiber, and it protects the core from absorbing surface contaminants
with which it could come in contact.
In standard optical fibers the core material is a highly pure silica glass (SiO 2 )
compound and is surrounded by a glass cladding. Common core sizes are about 9
and 50 μm (micrometers). A standard cladding diameter is 125 μm. Higher-loss
plastic-core fibers with plastic claddings are also widely in use. In addition, most
fibers are encapsulated in an elastic, abrasion-resistant plastic buffer coating and an
outer strengthening jacket that have diameters of 250 μm and 900 μm, respectively.
These materials add further strength to the fiber and mechanically isolate or buffer
the fibers from small geometrical irregularities, distortions, or roughness of adjacent
surfaces. These perturbations could otherwise cause scattering losses induced by
random microscopic bends that can arise when the fibers are incorporated into cables
or supported by other structures.
Variations in the material composition of the core give rise to the two commonly
used fiber types shown in Fig. 2.15. In the first case, which is called a step-index
fiber, the refractive index of the core is uniform throughout and undergoes an abrupt
change (or step) at the cladding boundary. In the other case shown at the bottom of
Fig. 2.15, the core refractive index is made to vary as a function of the radial distance
from the center of the fiber. This type is a graded-index fiber.
Both the step-index and the graded-index fibers can be further divided into singlemode and multimode classes. As the name implies, a single-mode fiber sustains
only one mode of propagation, whereas multimode fibers contain many hundreds of
modes. A few typical sizes of single-and multimode fibers are given in Fig. 2.15 to
provide an idea of the dimensional scale. Multimode fibers offer several advantages
compared with single-mode fibers. As shown in Chap. 5, the larger core radii of
49
Fig. 2.14 A conventional silica fiber has a circular solid core of refractive index n 1 surrounded by
a cladding with a refractive index n 2 < n 1 ; an elastic plastic buffer encapsulates the fiber
Although many different configurations of the optical waveguide have been
discussed in the literature, [4] the most widely accepted structure is the single solid
dielectric cylinder of radius a and index of refraction n 1 shown in Fig. 2.14. This
cylinder is known as the core of the fiber. The core is surrounded by a solid dielectric cladding, which has a refractive index n 2 that is less than n 1 . As described in
Sect. 2.3.5, the cladding material is carefully selected to enable light to propagate
efficiently along the core of the fiber. In addition, the cladding reduces scattering
loss that results from dielectric discontinuities at the core surface, it adds mechanical
strength to the fiber, and it protects the core from absorbing surface contaminants
with which it could come in contact.
In standard optical fibers the core material is a highly pure silica glass (SiO 2 )
compound and is surrounded by a glass cladding. Common core sizes are about 9
and 50 μm (micrometers). A standard cladding diameter is 125 μm. Higher-loss
plastic-core fibers with plastic claddings are also widely in use. In addition, most
fibers are encapsulated in an elastic, abrasion-resistant plastic buffer coating and an
outer strengthening jacket that have diameters of 250 μm and 900 μm, respectively.
These materials add further strength to the fiber and mechanically isolate or buffer
the fibers from small geometrical irregularities, distortions, or roughness of adjacent
surfaces. These perturbations could otherwise cause scattering losses induced by
random microscopic bends that can arise when the fibers are incorporated into cables
or supported by other structures.
Variations in the material composition of the core give rise to the two commonly
used fiber types shown in Fig. 2.15. In the first case, which is called a step-index
fiber, the refractive index of the core is uniform throughout and undergoes an abrupt
change (or step) at the cladding boundary. In the other case shown at the bottom of
Fig. 2.15, the core refractive index is made to vary as a function of the radial distance
from the center of the fiber. This type is a graded-index fiber.
Both the step-index and the graded-index fibers can be further divided into singlemode and multimode classes. As the name implies, a single-mode fiber sustains
only one mode of propagation, whereas multimode fibers contain many hundreds of
modes. A few typical sizes of single-and multimode fibers are given in Fig. 2.15 to
provide an idea of the dimensional scale. Multimode fibers offer several advantages
compared with single-mode fibers. As shown in Chap. 5, the larger core radii of