Page 54 - Optical Communications Essentials

The Behavior of Light

44 Chapter Three

TABLE 3.1. Some Common Birefringent Crystals and Their
Ordinary and Extraordinary Indices of Refraction
Crystal name Symbol n o n e
Calcite CaCO 3 1.658 1.486
Lithium niobate LiNbO 3 2.286 2.200
Rutile TiO 2 2.616 2.903
Yttrium vanadate YVO 4 1.945 2.149

the e ray component is deflected at a slight angle so it follows a different path
through the material.
Table 3.1 lists the ordinary index n o and the extraordinary index n e of some
common birefringent crystals that are used in optical communication compo-
nents. As will be described in later chapters, they have the following applications:

■ Calcite is used for polarization control and in beam splitters.
■ Lithium niobate is used for light signal modulation.
■ Rutile is used in optical isolators and circulators.
■ Yttrium vanadate is used in optical isolators, circulators, and beam displacers.

3.7. Summary

Some optical phenomena can be explained using a wave theory whereas in other
cases light behaves as though it is composed of miniature particles called pho-
tons. The wave nature explains how light travels through an optical fiber and
how it can be coupled between two adjacent fibers, but the particle theory is
needed to explain how optical sources generate light and how photodetectors
change an optical signal to an electric signal.
8
In free space a light wave travels at a speed c 3 10 m/s, but it slows down
by a factor n 1 when entering a material, where the parameter n is the index
of refraction (or refractive index) of the material. Values of the refractive index
for materials related to optical communications are 1.00 for air and between
1.45 and 1.50 for various glass compounds. Thus light travels at about
8
2 10 m/s in a glass optical fiber.
When a light ray encounters a boundary separating two media that have dif-
ferent refractive indices, part of the ray is reflected back into the first medium
and the remainder is bent (or refracted) as it enters the second material. As will
be discussed in later chapters, these concepts play a major role in describing the
amount of optical power that can be injected into a fiber and how light waves
travel along a fiber.
The polarization characteristics of light waves are important in examining
the behavior of components such as optical isolators and filters. Polarization-
sensitive devices include light signal modulators, polarization filters, Faraday

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