Chapter 1: Optics
The purpose of this chapter will be to discuss the physical principle of how fiber optics
works and is used in telecommunications. Fiber optics is extremely small diameter glass
that carries light which contains voice and data. Each fiber optical cable is coated with a
plastic type coating and multiple strands of fiber are placed together to form a cable.
Other materials are added to the cable to give it strength. Each strand of fiber can carry a
much larger amount of data then copper wire and thus we say it has a much greater
bandwidth (more data per period of time). Each strand of fiber is connected to a solid
state laser that takes the electrical signal and converts it to an optical signal and at the
other end of the cable it is connected to a receiving device which converts the light into
an electrical signal. If the fiber is long it may have an amplification station which
strengthens the signal. This may be repeated numerous times depending on length of the
fiber. The data is coded in binary (see discussion at the end of Chapter 3).
Objectives:






Explain the basic properties of light
Demonstrate an understanding of the principles of reflection
Demonstrate an understanding of the principles of refraction
Explain how total internal reflection occurs
Explain how total internal reflection is used in fiber optics
Explain the basic principles of a LASER
Properties of Light
The basic rule of optics states that light travels in a straight line unless it strikes an object
which causes it to be bent or absorbed, if it is bent it will then continue in a new straight
path, the exception to this is when diffraction occurs (light passes through a narrow
opening). The two basic ways in which bending occurs in optics is by reflection and
refraction. Before we can fully understand the bending in fiber optics we must
understand the basic principles of reflection and refraction. Light travels at a constant
speed and this is the fastest speed which can be obtained. When light travels into another
optical medium it will slow. The speed of light in a vacuum is 186,000 miles per second
or 3 x 108 meters per second. At this speed light takes about one and a half seconds to go
from the earth to the moon. Light can be thought of as traveling in waves or it can also
be thought of as a particle. Some principles of optics only will work for a particle model
of light and other areas of optics will only work for an electromagnetic wave model.
Many of the basic principles work for either model. The particles of light are called
photons. Different photons have different amounts of energy. Different colors of light
can be though of being composed of different colored photons. A photon is a massless
particle composed of pure energy and thus the amount of energy is directly related to the
color of light. From the wave model discussion the different wavelengths of light (the
distance from one part of a wave to the next corresponding part of the wave, see Chapter
3) will make up the different colors of light. This duality of light is a model that makes
the understanding of the principles of light easier and for our purposes will not make a
difference in our discussions.
Electromagnetic Spectrum
The electromagnetic spectrum is a family of light like radiation that has both a particle
and wave nature. This family includes: gamma rays, X-rays, ultraviolet, light, infrared,
microwaves and radio waves. The list was from most energetic with the shortest
wavelength first to the least energetic with the longest wavelengths. The properties of
light previously discussed are also true for members of this family; the speed is still 3 x
108 m/s, reflection and refraction still occurs. Light will reflect off a glass mirror and a
gamma ray will pass through the mirror, therefore the actions are the same as previously
discussed but the substances will change that will allow for the interaction. All these
forms of radiation are invisible to the human eye.
Gamma Rays are a form of nuclear radiation and normally discussed in nuclear physics.
The gamma ray is created in the nucleus of an atom. Some substances in nature are
naturally radioactive and as they go through radioactive decay they will release gamma
rays to lower the total energy of the atomic nucleus. Gamma rays can penetrate many
difference substances; lead is normally used to stop the gamma rays because of its high
density. Gamma rays are used in some cancer treatments.
X-rays are a form of atomic radiation which is emitted by the electrons that orbit the
nucleus of an atom. X-rays can penetrate many substances, but can be blocked by thin
sheet of lead. X-rays are used in the medical field to look at hard substances (bones) in
the human body; they are also used in some types of radiation therapy for cancer patients.
Ultraviolet Radiation is used in the sorting of mail to insure that postage has been
attached to the envelope. It is used to make certain kinds of paint glow, what is
sometimes referred to as black light. If you observe a black light and you see a purple
color that is visible radiation not the ultraviolet radiation. It is used to sterilize medical
equipment to eliminate bacteria. It can cause damage to the eye if the retina of the eye is
exposed for length periods of time. Certain photographic films will detect ultraviolet
waves and they will be converted into a violet or blue color on the film, which causes the
sky to appear bluer than it really is.
Light has been discussed previously and will not be repeated at this location.
Infrared Radiation is normally discussed as two different types, one being near infrared
and the other thermal infrared. The near infrared is that part of the electromagnetic
spectrum with wavelengths just longer then visible red light. Near Infrared is detected by
some satellites and aircraft to look for diseases in plants such as corn blight. The thermal
infrared is the area of the spectrum that is known as heat. You experience thermal
infrared when you walk near a heat source and you feel the heat on your body; your body
is detecting the thermal infrared. Thermal infrared is used detect heat source and is used
to detect cancer cells; especially breast cancer. It is also used to detect when parts of a
machine is wearing out and has too much friction; such as the bearings in a railroad car.
Microwaves are used in many ways in our everyday lives from telecommunication to
cooking. If you have a satellite dish on your home then it is receiving microwaves, long
distance telephone communication uses microwaves and of course cooking with a
microwave oven. A microwave oven causes the molecules in the substance to vibrate
thus creating friction which causes the substance to heat up. The more dense the
substance usually the faster it will heat.
Radio Waves are used for many different types of communications and the name
sometimes causes people to believe it is only referring to radio. These wavelengths are
used in cell phones, cordless phones, wireless networking, radar, television, CB’s and of
course AM and FM radio. Radio waves are the longest least energetic of all the members
of the electromagnetic spectrum.
Reflection
When light reflects by striking a mirror
or other reflective surface the principle
is very simple. The angle of the
incoming light ray is equal to the angle
of the light after reflection. A normal
is a line that is perpendicular to the
surface of the object (mirror).
Therefore the gray line represents a
mirror the normal (green line) is
perpendicular to the surface of the
mirror. The Law of Reflection states
that angle of incident (incoming ray) is
equal to the angle of reflection (the
outgoing ray). The angle is measured
between the incident ray and the
normal and the reflected ray and the
normal.
Figure 1.1 Reflection
of incidence=of reflection
(0.1)
When light strikes a curved mirror the rules are the same. You can think of a mirror as
being composed of multiple straight sections which are extremely small. The reflection
occurs in the same way. Each little straight section has the incident ray equaling the
reflected ray. The normal for each of these sections will be the radius of the curve for
spherical shaped mirror, such as those used for security in department stores. This area
of physics is called geometric optics and how the images are formed in a security mirror
or a make mirror is interesting to understand but do not lead to the direct principle of
understanding fiber optical cable.
Refraction
Refraction occurs when light travels
from one substance into another
substance. If the light enters the
other substance perpendicular to the
surface then only the speed is
changed. If you are going from an
optically less dense medium to an
optical denser medium then the light
will slow. So if light is going from
air into glass the light travels faster
in the air then it does in the glass.
The opposite is true if you go from
glass into air that light increases in
speed. Light has a maximum speed,
i.e. the speed of light, and the
Figure 1.2 Refraction
medium with the least optical dense
medium is a vacuum. If the light
does not strike the surface perpendicular to the surface then the angle of the light ray also
changes. If we are going from less dense to more dense then the light ray will bend
toward the normal and if we are going from denser to less dense it will bend away from
the normal. The law which applies to refraction is known as Snell’s Law and it is as
follows.
ni sin i  nr sin r
(0.2)
ni is the index of refraction for the first optical medium, nr is the index of refraction of the
second optical medium. The index of refraction of air is approximately 1. I is the angle
of incident which the angle between the normal and the incident ray. r is the angle of
refraction which is measured between the normal and the refracted ray. We will not
mathematically solve this relationship. The sine function is a trigonometry function, the
sine of 90 degrees is 1 and the sine of 0 degrees is 0. The angle for this type of problem
cannot exceed 90 degrees.
If we have a ray of light trying to exit the
denser medium into the less dense medium
then as previously noted it will bend away
from the normal toward the surface of the
medium.
Figure 1.3 Refraction
Refraction Simulation
An online experiment will be used to assist your understanding this concept. You will
move the light source around observing both the incident and refracted or reflected rays.
Note the angles of these rays will be stated. To move the light source simply click and
drag the source and the incident ray will move.
You will need to complete the following table and answer the two questions below the
table. Submit your answers for the two questions as attachment when you submit Chapter
1's homework assignment.
Make the substance water above the horizontal line and make the substance below water
(you can play with other combinations). Do several examples of going from air to water
and record you results (measurements given). Move the light source below the water and
repeat.
Complete the following table.
Angle of incidence Angle of refraction (or Reflected or refracted
reflection)
1. At what angle did reflection occur?
2. What was the critical angle in the online experiment when refraction no longer
occurred and reflection began? Click the following hyperlink to reach the online
experiment
http://www.jefferson.kctcs.edu/kteam/ph171_4066/Chapter1/RefractionSimulator
Files/default.htm
Refraction on a Curved Surface
The surface of the fiber optical cable
must also be polished to a smooth
curved surface so that light properly
enters the cable the shape of the front
surface must be convex. A convex
surface is higher in the center and
shallower at the edges. The light that
enters the convex surface is focused
toward the center of the fiber, thus
eliminating much of initial losses. The
curved surface can be thought of as a
number of straight surfaces just as we
discussed in the mirrors previously.
Total Internal Reflection
If we assume the less dense medium is air we can develop the critical case when the
equation is no longer possible to be solved. Therefore using equation (0.2) and if ni = 1.5
and nr = 1 (air) and the largest angle that r can be is 90o (this is when the light refracts
along the surface of the interface) and the sin of 90o is 1. Therefore if we substitute into
equation (0.2) we get the following:
1.5sin i  1
1
1.5
 1 
i  sin 1  
 1.5 
sin i 
(0.3)
i  41.8deg
Therefore if angle of incidence exceeds 41.8 degrees for these mediums then the
equations of Snell’s no longer can be solved. At this point the interface of the medium
becomes a reflective surface and no longer a refractive surface. This is known as total
internal reflection and is the principle in which fiber optics works on. You will not be
required to do any calculations using the sine function, it is shown to you to confirm
the observations you have made visually in the online laboratory. Repeat the online
lab if you need to explore this principle again.
For additional information about "Total Internal Reflection" visited the links provided
below:
http://laser.physics.sunysb.edu/~wise/wise187/2001/reports/andrea/report.html
http://theory.uwinnipeg.ca/mod_tech/node114.html
Fiber Optics
The optical fiber has a higher dense then the air surrounding it. Therefore if the angle of
incident is greater than the critical angle total internal reflection will occur. Therefore, as
the light strikes the walls of the surface they are reflected instead of being refracted and
the light continues down the fiber. Note the
higher the ratio of optical density of the
mediums the small the angle becomes to have
total internal reflection. Thus light still has the
same physical properties and continues to travel
in a straight line, but is bent continuously as it
reflects of the outer wall of the fiber. Therefore,
the fiber can be bent at different angles as is
required for proper installation. For additional
information about "Fiber Optics" visited the
links provided below:
http://www.timbercon.com/Total-Internal-Reflection.html
http://electronics.howstuffworks.com/fiber-optic6.htm
http://encarta.msn.com/encyclopedia_761566545/Fiber_Optics.html
The surface of the fiber optical cable must also be polished to a smooth curved surface so
that light properly enters the cable the shape of the front surface must be convex. A
convex surface is higher in the center and shallower at the edges. The light that enters the
convex surface is focused toward the center of the fiber, thus eliminating much of initial
losses. The curved surface can be thought of as a number of straight surfaces just as we
discussed in the mirrors previously.
Lasers
The optical information that is placed in the fiber cable is done by a solid state laser diode
in general. We will not go into the depth of understanding how laser diodes function, but
will discuss the principles of a laser and how it is different from regular light.
There are many types of lasers and many different uses for lasers. In this chapter we will
discuss what a laser produces and how we can transmit information by using a laser.
LASER stands for light amplification by stimulated emission of radiation. A laser
produces a single wavelength of coherent light. If we think of a light bulb it produces
most all the colors in the rainbow, using a prism we can observe the colors of light in the
light bulb. A laser in general will produce only a single wavelength of light (a singe
color), but just by having a single color of light does not create a laser. If you take a light
bulb and pass the light through a color filter you will get a single color of light, this is not
a laser. The laser light must be coherent. One of the ways to think of light is that it is
composed of transverse waves (transverse waves will be discussed in Chapter 3; a
transverse wave is like a water wave). If we have a single color of light then we will
have all the waves having the same length (wavelength). Yet to be coherent light all the
waves must start at the same time (all the peaks of the waves must be aligned and all the
troughs must be aligned). A laser creates coherent light rays of a single wavelength.
How the laser produces it light from an equation understanding is above the level of this
class mathematically and since we are not discussing physical optics and the atomic
spectrum in this class we will not discuss the actual principles of lasing. If you wish to
explore more detail on this subject go to the following website:
http://www.middlebury.edu/~PHManual/Photos/heliumneon/fig2.html .
There are several different types of lasers and we use some of these lasers in our
everyday life. What are some examples of the use of a laser that you can name? Some
lasers use gases, other use liquids and some use solids. You probably have seen a laser
pointer; this type of laser is a solid state device and is similar to the laser used in
telecommunications.
If the laser is turned on and off rapidly then a code of information can be transmitted by
the device much as Morse code was used with the telegraph a century ago. This code can
be composed of on and off bits of information; all letters and numbers can be transmitted
using this coding. All images can be made to be represented by numbers and the
numbers converted into this system. This system is known as binary and is discussed in
the Chapter 3 when discuss how a CD player works.
The laser is connected to the end of the fiber optical cable and transmits the information
to a receiving station at the other end of the fiber. The electronic equipment then decodes
the signal and sends the information on to the appropriate computer or other
telecommunication device. If the distance is great a repeater may be employed. A
repeater is a receiver and another sending device.