4 Fiber Optic Essentials
systems are used for emergency lighting, automotive lighting, traffic sig-
nals, signage, lighting sensors, and decorative lighting. Sometimes, their
only detector is the human eye. There are many popular entertainment
applications as well, including fiber optic decorations, artificial flowers,
and hand-held bundles of fiber which show different colored lights using
a white light source and a rotating color filter wheel.
As we can see, the field of fiber optics is very broad and touches many
different disciplines. A comprehensive review is beyond the scope of this
book; instead, we intend to describe the most common components used
in fiber optic systems, including enough working knowledge to actually
put these systems into practice (such as relevant equations and figures of
merit).
1.1 Optical Fiber Principles
As noted earlier, we can direct light from one point to another simply by
shining it through the air. We often visualize light in this way, as a bundle
of light rays traveling in a straight line; this is one of the most basic
approaches to light, called geometric optics. It is certainly possible to
send useful information in this way (imagine signal fires, smoke signals,
or ship-to-shore lights), but this requires an unobstructed straight line of
light, which is often not practical. Also, light beams tend to spread out as
they travel (imagine a flashlight spot, which grows larger as we move the
light further away from a wall). We could make light turn corners using
an arrangement of mirrors, but this is hardly comparable to the ease of
running an electrical wire from one place to another. Also, mirrors are
not perfect; whenever light reflects from a mirror, a small amount of light
is lost. Too many reflections will make the optical signal too weak for
us to detect. To fully take advantage of optical signaling, we need it to
be at least as easy to use as a regular electrical wire, and have the ability
to travel long distances without significant loss. These are the principal
advantages of an optical fiber.
Instead of using mirrors, optical fibers guide light with limited loss
by the process of total internal reflection. To understand this, we need
to know that light travels more slowly through transparent solids and
liquids than through a vacuum, and travels at different speeds through
different materials (of course, in a vacuum the speed of light is about
300,000,000 m/s). The relative speed of light in a material compared with
its speed in vacuum is called the refractive index, n, of the material.
1. Fiber, Cables, and Connectors 5
For example, if a certain kind of glass has a refractive index of 1.4, this
means that light will travel through this glass 1.4 times more slowly than
through vacuum. The bending of light rays when they pass from one
material into another is called refraction, and is caused by the change in
refractive index between the two materials. Refractive index is a useful
way to classify different types of optical materials; to give a few examples,
water has a refractive index of about 1.33, most glass is around 1.5–1.7,
and diamond is as high as 2.4. For now, we will ignore other factors that
might affect the refractive index, such as changes in temperature. We can
note, however, that refractive index will be different for different wave-
lengths of light (to take an extreme example, visible light cannot penetrate
your skin, but X-rays certainly can). At optical wavelengths, the variation
of refractive index with wavelength is given by the Sellmeier equations.
Total internal reflection is described by Snell’s Law, given by Equa-
tion 1.1, which relates the refractive index and the angle of incident light
rays. This equation is illustrated by Figure 1.1, which shows two slabs of
glass with a ray of light entering from the lower slab to the upper slab.
Here, n
1
is the index of refraction of the first medium,
1
is the angle
of incidence at the interface, n
2
is the index of refraction of the second
medium, and
2
is the angle in the second medium (also called the angle
of refraction). Snell’s Law states that
n
1
sin
1
= n
2
sin
2
(1.1)
Thus, we can see that a ray of light will be bent when it travels across
the interface. Note that as we increase the angle
1
, the ray bends until
n
2
n
1
θ
2
θ
1
Figure 1.1 Illustration of Snell’s Law showing how an incident light ray is bent as it
travels from a slab of glass with a high refractive index into one with a lower refractive
index, eventually leading to total internal reflection.

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