Fiber Optic Essentials

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.