Page 57 - Optical Communications Essentials

Source: Optical Communications Essentials

Chapter
4

Optical Fibers

The optical fiber is a key part of a lightwave communication system. An optical
fiber is nominally a cylindrical dielectric waveguide that confines and guides
light waves along its axis. Except for certain specialty fibers, basically all fibers
used for telecommunication purposes have the same physical structure. The
variations in the material and the size of this structure dictate how a light sig-
nal is transmitted along different types of fiber and also influence how the fiber
responds to environmental perturbations, such as stress, bending, and temper-
ature variations. This chapter describes various fiber structures, physical char-
acteristics, operational properties, and applications.

4.1. Light Propagation in Fibers
Figure 4.1 shows the end-face cross section and a longitudinal cross section of a
standard optical fiber, which consists of a cylindrical glass core surrounded by a
glass cladding. The core has a refractive index n 1 , and the cladding has a refrac-
tive index n 2 . Surrounding these two layers is a polymer buffer coating that pro-
tects the fiber from mechanical and environmental effects. Traditionally the
core radius is designated by the letter a. In almost all cases, for telecommuni-
cation fibers the core and cladding are made of silica glass (SiO 2 ).
The refractive index of pure silica varies with wavelength, ranging from 1.453
at 850nm to 1.445 at 1550nm. By adding certain impurities such as germanium
or boron to the silica during the fiber manufacturing process, the index can be
changed slightly, usually as an increase in the core index. This is done so that
the refractive index n 2 of the cladding is slightly smaller than the index of the
core (that is, n 2 n 1 ), which is the condition required for light traveling in the
core to be totally internally reflected at the boundary with the cladding. The dif-
ference in the core and cladding indices also determines how light signals
behave as they travel along a fiber. Typically the index differences range from
0.2 to 3.0 percent depending on the desired behavior of the resulting fiber.

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