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INTRODUCTION TO NETWORK CABLING
MODULE 2
FIBER OPTIC-BASED SYSTEM
Introduction to Network Cabling Fiber Optic-Based Systems
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Light and Optics
Goal 2
Students will have an understanding of the basics of light to include
speed, wavelength, frequency, intensity and attenuation. During this
module students will be working with light sensors, emitters and
optical power meters in support of module objectives.
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Light and Optics
Objectives:
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Define the characteristics of light to include: frequency and wavelength, speed, movement and amplitude
Identify the speed of light as 300,000kps
Identify that light slows down as it enters air or water
Identify that the metric system is used for measurements in optics
Define metric terms to include millimeter, micron and nanometer
Identify that scientists use scientific notation to work with very large or very small numbers
Convert numbers using scientific notation
Given a sine wave, determine a wavelength measurement Define the terms: frequency and wavelength
Describe the relationship between frequency and wavelength is inversely proportional
Give examples of devices that use wavelengths and frequencies
Define the Greek symbol, Lambda and what it represents
Define that energy in the Electromagnetic Spectrum is measured by frequency or wavelength
Identify the parts of the Electromagnetic Spectrum in terms of Radio, Visible, Non-Visible and Microwave
Compare wavelengths and identify which has the lower frequency
Identify that red is on one end of the visible spectrum and violet is on the other end
Define the acronym ROYGBIV as the colors of the spectrum
Given a color of light, identify its wavelength
Define light as both the visible and the non-visible
Use an infrared detector and check for the presence of non-visible light
Identify that white light contains components of all visible wavelengths
Take part in an experiment, observe and record the results of light with different wavelengths (4 SPOTS)
Define light movement in terms of Reflection and Refraction
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Light and Optics
Objectives (continued):
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Identify that most of what we see is the reflection of light
Describe how we perceive the color of an object
Identify incident angle, reflected angle and the normal line in terms of light reflection
Identify that when light refracts it changes speed and direction
Define Snell’s Law and give examples of how it applies to reflection and refraction
Diagram how light reflects off a mirror and indicate which angles provide the strongest reflections
Given an index of refraction chart, calculate the speed of light in a medium
Describe which direction a refracted light wave travels when interfacing with materials of different indexes
of refraction
Describe how a Fresnel lens works to refract and focus light
Define attenuation as loss of light over distance
Define that attenuation is a ratio, a function of distance and wavelength
Define that attenuation is measured in Decibels
Define that decibels are a log expression
Describe that 3 dB is a doubling of power
Given a decibel to power conversion chart, determine what percentage of power is lost per dB
Describe the operation of the power meter and light source
Observe cautions when using a power meter and light source
Use the subtractive or zero set method to measure the attenuation of an optical system
Use a power meter and plot the light intensity of four optical sources
Take part in an experiment, observe and record the results of an activity using light sources of different
wavelengths measured with an 850nm power meter
Introduction to Network Cabling Fiber Optic-Based Systems
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Light and Optics
Introduction
This course deals with the movement of light through strands of glass. These
strands are only slightly larger then a human hair and they provide us with the
ability to move large amounts of data. The use of these optical systems allows
the connected world to operate. Optical systems provide for our computer
networks, digital television and voice networks.
Figure 2.1.1
Introduction to Network Cabling Fiber Optic-Based Systems
Figure 2.1.2
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Light and Optics
Major corporations are laying thousands of miles of fiber in cities and even on the
floor of the ocean. These pulses of light represent the heartbeat of technology.
http://www.youtube.com/watch?v=cuxf6zBTO2g
Before going too deeply into Fiber Optic technology, it is important to understand the
basics of light. How fast does it move and at what wavelength? How does it bend
when it travels through different materials? What is the difference between reflection
and refraction? These are some of the questions that are addressed in this module.
Introduction to Network Cabling Fiber Optic-Based Systems
Figure 2.1.3
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Light and Optics
Light
An optic technician needs to understand some of the fundamental
characteristics of light and the fibers it moves through. These qualities
are important to understand when you work with optical systems.
Figure 2.2.1
The four characteristics of light are:
Speed
How fast is light moving and in what (air, water,
outer space, etc.)?
Wavelength and Frequency
Where does light fall in the electromagnetic
spectrum? What color is it? What is the distance
between peaks of the wave?
Movement
How does it travel or reflect? How does it interact
in a substance, like glass or water?
Amplitude
How bright or intense is the light? How is the
brightness reduced (fog, air, glass, etc.)?
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Light and Optics
Speed
Light is an electromagnetic wave that travels at a speed of 300,000
kilometers per second in a vacuum. That's 186,000 miles per second.
The speed of light is the maximum speed limit of the universe. Nothing
can travel faster than light!
How fast is 300,000 kilometers per
second?
Figure 2.3.1
It is 670 million miles per hour. If you
flew around the world in a jet, you
could make the trip in about 38 hours.
The space shuttle takes about an hour
and a half. A beam of light can do it in
1/7 of a second.
That's how fast.
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Light and Optics
Remember, this is the speed of light in a vacuum. As light passes
through air it slows down. As it passes through denser materials, like
water, it slows down even more. Light inside of a diamond slows down
to a mere 300 billion miles per hour or less than half the speed it
travels in air.
Figure 2.3.2
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Light and Optics
Light Math
Optics deals with small strands of glass and even smaller wavelengths of light. The typical size
of optical glass you will be working with is slightly larger than a human hair. The light that
travels through these light waveguides is even smaller.
In optics, the metric system is used. The basic unit of the metric system is the meter, which is
a little longer than 39 inches. The meter represents one ten-millionth of the distance from the
Equator to the North Pole.
Divide a meter by a thousand and you get a millimeter. Divide a meter by a million and you
get a micron.
How small is that? One of your hairs is about 75 microns thick. The fibers you will be working
with in this course are 125 microns in diameter.
The micron has a special symbol, the Greek letter mu: µm. Even smaller than that is the
nanometer, one-billionth of a meter.
A nanometer is extremely small and is one billionth or 1,000,000,000 of a meter. A human hair
is 10,000 times thicker than a nanometer. Wavelengths of light are measured in nanometers.
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Light and Optics
Converting one metric unit to another is easy because it's all decimals. All
you have to do is shift the decimal point to the right or left.
Prefix
Meaning
Number in 1 Meter
deci-
10
10 decimeters
centi-
100
100 centimenters
milli-
1,000
1,000 millimeters
micro-
1,000,000
1,000,000 micrometers
nano-
1,000,000,000
1,000,000,000 nanometers
Figure 2.4.1
NEW TERM
Metric System - A universal scientific system of measurement.
. Meter - The basic unit of the Metric System, about 39 inches (m).
. Millimeter - One thousandth of a meter (mm).
. Micron - One millionth of a meter. (symbol: µm) Also known as a micrometer.
. Nanometer - One billionth of a meter. Also called a nanon.
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Light and Optics
Scientific Notation
Look at the number below. On a separate sheet of paper, write this number down.
3,000,000,000,000,000,000,000,000,000,000,000,000,000,000
How long did it take you to write it? Check your work. Did you remember every single zero? It would be easy
enough to miss one.
Scientists work with large numbers like this, and very small numbers too. You can see that it would be hard to
calculate with numbers like these. So scientists have developed a technique called scientific notation. It's a way
of writing very large or very small numbers without using all the zeroes by manipulating decimal points.
- A number expressed in scientific notation contains two parts:
- A number greater than 1 but less than 10.
A power of 10 that multiplies that number. This power of 10 represents the new location of the shifted decimal
point.
Here's an example: the number above, written in scientific notation, would be:
3 X 1042. Forty-two is the number of places you'd have to move the decimal point to the left to turn “3” into the
number above.
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Light and Optics
Scientific Notation (continued)
We can make a formula for scientific notation like this: A X 10B
Here's how the formula works:
Take the number you want to convert and move the decimal place to the left until the number is between 1 and
10. This number is A in the formula.
Count the number of places the decimal point was moved and replace B with that number.
So: 420,000,000,000 = 4.2 X 1011 (Move the decimal point 11 places to the left.)
NEW TERM
Scientific Notation - The way a scientist can
quickly work with very large or very small
numbers.
Introduction to Network Cabling Fiber Optic-Based Systems
Figure 2.5.1
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Light and Optics
Scientific Notation (continued)
Scientific notation is used to make working with large numbers EASIER.
Where:
5.13 X 100 = 5.13 5.13 X 108 = 513,000,000
But what if it's a very small number? You use the same formula, but you move the
decimal point to the right and make "B" a negative number.
Like this: 0.000979 = 9.79 x 10-4 0.000000000045 = 4.5 x 10-11
Remember the base number must be between 1 and 10.
0.000000000000000002 = 2 X 10-18
Figure 2.6.1
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Light and Optics
Activity 2.1 Fundamentals of Light
Match the terms to their definitions
1.
2.
3.
4.
5.
A. frequency
B. amplitude
C. speed of light
D. micron
E. meter
C 300,000 kilometers per second
D one millionth of a meter
B intensity of light
E 39 inches
A color of light
Calculate the following using scientific notation:
6. Blue light has a wavelength of 0.00000045 meters. How would a scientist define that
number using scientific notation? X 10 -7
7. The speed of light is 186,000 miles per second. How would a scientist define that number
using scientific notation? X10 5
8. A typical Fiber Optic network operates at 0.000000085 meters. Convert that number using
scientific notation. X 10 -8
9. What would 6.8 X 1011 look like without scientific notation? 680000000000
10. What would 4.5 x 106 look like without scientific notation? 4500000
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Light and Optics
Waves
Light is a wave, so is sound and so is the energy sent to and from
your cell phone.
All of this wave energy is part of the electromagnetic
spectrum. The electromagnetic spectrum includes all
forms of electromagnetic waves from very long sound
waves to very small gamma rays. A bass wave from a
home theater can have a wavelength of over 50 feet
while a gamma ray is measured in nanometers.
Figure 2.8.1
Sound waves
allow us to enjoy
music.
Introduction to Network Cabling Fiber Optic-Based Systems
Figure 2.8.2
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Light and Optics
Waves (continued)
X-rays let us see problems in our bodies.
Figure 2.8.3
NEW TERM
Electromagnetic Spectrum – All of the
types of waves found in nature from sound
to gamma rays.
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Light and Optics
Wave Math
When measuring a wavelength, measure the distance where the wave repeats.
You may measure from the peak of a wave to the next peak. You may also
measure the points where the wave passes zero. In the diagram, A is the peak
to peak distance and B is the point where the wave passes zero. The length of
A is the same as the length of B. Either measurement is correct.
Figure 2.9.2
Figure 2.9.1
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Light and Optics
Frequency and Wavelength
In the diagram, compare the waves and you will notice that the more waves there are, the
less distance is between them.
Lower frequencies have fewer waves!
The length of the waves is longer!
Figure 2.10.1
Higher frequencies have more waves!
The length of the waves is shorter!
Figure 2.10.2
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Light and Optics
Frequency and Wavelength (continued)
Frequency is the rate that a wave changes. Wavelength is the distance between like parts
of a wave.
• The higher the frequency, the shorter the wavelength.
• The lower the frequency, the longer the wavelength.
The scientific term that expresses this
relationship is called: “inversely
proportional.” As one variable gets
larger, one gets smaller.
NEW TERM
Figure 2.10.3
Inversely Proportional - A relationship that
when one variable goes up the other goes
down.
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Light and Optics
Frequency and Wavelength (continued)
For example, the frequency of the electrical voltage in your home
is 60Hz. That means that it completes 60 cycles in one second.
A WiFi and a Blue Tooth operate at 2.4 giga hertz. Giga-means
one billion, so one Giga-Hertz means that the wave repeats one
billion times in one second.
Figure 2.11.1
Lambda, the Greek letter "λ" is
used to represent wavelength. The
power meter has a λ button that
allows the user to select 850nm,
1310nm or 1550nm for testing
different types of optical systems.
NEW TERM
Figure 2.11.2
Lambda - The Greek letter that represents
wavelength.
Figure 2.11.3
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Light and Optics
The Electromagnetic Spectrum
The various types of waves are arranged by wavelength.
Look at the diagram and notice that the spectrum runs from very long wavelengths
(sound waves) to very short wavelengths (gamma rays). Notice that the visible light
area is only a small part of the spectrum.
Visible light is the only part of the spectrum
we can see. White light is actually a
combination of many colors, each of which
has its own wavelength, ranging from violet
at the low end, to red at the high end. We
can not see ultraviolet waves but if
exposed to them for too long we can get
sunburn. We also can not see infrared light
at the other end of the visible spectrum.
Figure 2.12.1
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Light and Optics
Light Wavelengths
ROYGBIV
You may have learned Roy G. Biv as a way to describe
the visible spectrum.
R
O
Y
G
B
I
V
Red
Orange
Yellow
Green
Blue
Indigo
Violet
Introduction to Network Cabling Fiber Optic-Based Systems
Figure 2.13.1
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Light and Optics
Visible light
The different colors of light have different wavelengths. These wavelengths are very small and
range from 400 nanometers to 650 nanometers.
Figure 2.13.2
Red waves have a wavelength of 650nm.
They are about twice as long as violet
waves. Infrared or the black area on the
left of the scale is the area where fiber
optical systems operate.
Figure 2.13.3
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Light and Optics
Non-Visible Light
Energy whose wavelength is too long to see is "redder than red." Light with such long
wavelengths is called "infrared" light. The prefix "infra-" means lower than; so infra-red
means “lower than red.”
Infrared light travels more easily through glass. It is the type of light used in fiber optical
systems.
To see continuity, a technician uses
visible light. When Fiber Optic
systems are operating, they use nonvisible light.
The light source used in the course uses IR
light. The light source puts out very intense
light that can not be seen. Even though the
light from the light source can not be seen, it
is still present. Protect your vision and never
look into a light source.
Figure 2.14.1
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Light and Optics
Non-Visible Light (continued)
Warning
DO NOT LOOK INTO ANY FIBER OPTIC LIGHT SOURCE!
Some remote controls use IR light to send signals to televisions or AVR’s. If a
remote has to be pointed at the television or AVR then it uses IR light.
The C-Tech IR detector card tests for IR light.
NEW TERM
Figure 2.14.3
Figure 2.14.2
Introduction to Network Cabling Fiber Optic-Based Systems
Infrared light - Nonvisible light that is used in
Fiber Optic systems.
AVR - Audio Video
Receiver.
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Light and Optics
Watch the demonstration with the C-Tech IR detector, the IR
SPOT and the light source and power meter.
Explain the demonstration and how IR light is detected.
Show the movie and conduct the demonstration with
the equipment listed. Demonstrate non-visible light,
light source safety and line of sight for IR sources.
Non-Visible
Light_Power
Light_Intensity
SPOT
Attenuation
Z
Video
Video
Video
Video
Introduction to Network Cabling Fiber Optic-Based Systems
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Videos
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Light and Optics
Activity 2.4 Properties of Light
1. Describe the term inversely proportional.
As one property goes up the other goes down
2. In the diagram below which letter represents wavelength? Circle your answer.
3. When wavelength goes up, frequency goes down.
4. Hertz is a measurement used for:
a. speed
b. weight
c. wavelength
d. frequency
5. Light travels fastest in a vacuum.
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Light and Optics
Activity 2.4 Properties of Light (continued)
6. It is important to remember that the operational wavelengths used in Fiber Optic
communications are based on the light handling characteristics of:
a.the environment
b.the speed of light
c.the fiber
d.the laser
7. What does the Greek symbol "λ" mean? Lambda and or wavelength
8. What does the lambda button on the power meter do? Changeswavelengths
9. Give an example of a use of non-visible light. Remote controls, sources or any IR source
10. Which two of the following are a SPOT wavelength?
a.650
b.745
c.850
d.1310
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Light and Optics
Activity 2.3 Color and Wavelength
Observe the SPOT demonstration and fill in the color
and wavelength.
Color Wavelength
1.SPOT number 1______ __________
2.SPOT number 2______ __________
3.SPOT number 3______ __________
4.SPOT number 4______ __________
Have the students observe the demonstration or allow
them to examine SPOTS and log results.
Introduction to Network Cabling Fiber Optic-Based Systems
Video
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Light and Optics
Light Movement
Light rays always travel from the light source in straight lines, but the direction of those straight
lines can be changed as the light passes through or strikes various substances. This change in
direction of the light wave can occur in two patterns, either as reflection or refraction.
Reflection is when light is redirected after bouncing off a substance.
Refraction is when light enters a
substance and its speed and
direction changes.
NEW TERM
reflection – The description of light
when it bounces off of a surface.
Figure 2.18.2
refraction – The description of how
light slows and bends as it enters a
substance.
Introduction to Network Cabling Fiber Optic-Based Systems
Figure 2.18.1
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Light and Optics
Reflection
Most objects don’t produce their own visible light.
What we see most of the time is reflections of light off
objects. Objects reflect light differently. Most of the
light is absorbed by the object. Some of it is reflected.
An orange appears orange because all wavelengths of
light except orange are absorbed by the object.
Mirrors reflect all visible
wavelengths.
Figure 2.19.1
Figure 2.19.2
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Light and Optics
Reflection (continued)
The diagram above shows light
reflecting from a mirror. The
incident ray strikes the mirror and
reflects at the same angle. As
shown above, the angles of
incidence and reflection are the
same. The angles are measured
from the angle of the “normal
line.” The normal line is a line
perpendicular to the reflective
surface. The law of reflection
states that the angle of incidence
is always equal to the angle of
reflection.
Figure 2.20.1
NEW TERM
Angle of Incidence – The angle that light strikes a surface.
Angle of Reflection – The path that reflected light travels;
same as angle of incidence. Normal Line – A line that is
perpendicular to a surface.
Law of Reflection – This law states that the angle of
incidence is equal to the angle of reflection.
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Light and Optics
Critical Angle
All of the light is reflected back into
a substance if the angle of
incidence is less than the critical
angle. The critical angle is the
measurement of the angle of
incidence to the normal line.
The diagrams represent the
interface of air and water. The air
and water interface have a critical
angle of 49 degrees. As the angle of
incidence is increased all of the light
is reflected at the interface. This
phenomenon is known as total
internal reflection because almost
all of the light is reflected back into
the substance.
Figure 2.21.1
NEW TERM
Critical angle – The angle that light begins to
leave a substance at the interface. Below the
critical angle more of the light leaves the
substance.
Total Internal reflection – When the incident
angle allows most of the light to remain inside a
substance.
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Light and Optics
Refraction and the Speed of Light
When light refracts it changes speed and changes direction.
Light does not travel the same speed in all substances. Light in outer space travels at the speed of light.
When light enters the atmosphere, it slows down. If it was to travel through the ocean, it slows down
even more.
How light speeds up and
slows down is shown (left) in
the diagram of the raindrop.
As the light enters the
raindrop it slows down. As it
leaves the raindrop it goes
back to its original speed.
Figure 2.22.1
Figure 2.22.2
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Light and Optics
Index of Refraction
When light enters transparent materials, it
slows down as it travels through the material.
Technicians use the term “index of
refraction” to describe the speed of light in a
medium.
Index of refraction is the ratio of the speed of
light and the speed in a substance.
An index of refraction of 1 means the light is
traveling at the speed of light.
index of refraction = speed of light in a vacuum
speed in the substance
index of refraction of 2 means: 2 = 300,000/150,000
Medium
Index of refraction
Outer Space
Air
Ice
Water
Cranberry juice
Shampoo
Window glass
Ruby
Crystal
Diamond
1.00
1.00029
1.309
1.333
1.351
1.365
1.5
1.770
2.00
2.417
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Light and Optics
Index of Refraction (continued)
The chart above shows the index of refraction in different
substances. Light travels at its maximum speed in outer
space. Light moves slower in air and even slower in ice.
Using the chart, notice that light travels only half as fast in a
crystal, compared to space. In space, light travels at 300,000
kilometers per second. In a crystal it moves at 150,000.
Figure 2.23.1
NEW TERM
Index of Refraction – The ratio
of the speed of light in a
vacuum to its speed in a
substance.
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Light and Optics
Refraction and the Bending of Light
When white light goes into a prism, the light is
separated by wavelength because wavelengths
refract at different angles. The same color or
wavelength separation takes place in rainbows and
glass.
When a light ray enters a material with a
higher index of refraction, it bends towards
the normal.
Figure 2.24.1
Figure 2.24.2
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Light and Optics
Snell’s Law
Snell's law explains how light bends as it moves from one substance to another. According to
Snell’s Law, when light enters a substance with a higher index of refraction, the light bends
towards normal. When light enters a substance with a lower index of refraction, it bends away from
normal.
The diagram below represents a glass rod suspended in air. The air has an index of refraction of 1
and the glass has an index of refraction of 1.4. The normal line is a line shown in the diagram.
When light from the air enters the glass rod, it bends towards the normal. When light leaves the
glass and enters the air, it bends away from the normal.
Figure 2.25.1
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Light and Optics
Snell’s Law (continued)
Fishing birds take the bending of light
into account. Because of refraction the
fish appears closer than it really is.
Figure 2.25.2
NEW TERM
Snell's Law – The law that states
how light refracts in a substance.
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Light and Optics
The Fresnel Lens
The refraction of light is the basis of the Fresnel
(fray-NELL) lens. A Fresnel lens refracts the light
and creates a focused light pattern. It amplifies
the light as it captures and focuses it. The Fresnel
lens has allowed light houses to operate and send a
beam of concentrated light many miles out to sea.
Fresnel’s invention has saved many ships from
disaster.
Figure 2.26.1
Figure 2.26.2
Figure 2.26.3
Figure 2.26.4
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Light and Optics
Activity 2.5 Light Movement
A. index of refraction
F
B. 300,000 km per second
The slowing down and bending of light as it moves
through different media.
C Same as the angle of incidence.
C. reflected angle
E
F. refraction
An example of how light travels after it strikes an
object and bounces off.
G Describes how light bends towards or away from the
normal line.
A A number, that is always at least 1, that represents the
ratio of how fast light travels in different substances.
B The speed of light in a vacuum.
G. Snell’s Law
D
D. Fresnel lens
E. reflection
Focuses light using refraction.
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Light and Optics
Light Amplitude
The amplitude of light (scientists say intensity) is the brightness
of the light or the amplitude of the wave. Wavelength measures
the light wave from “side to side.” Amplitude measures the wave
from “top to bottom.” The “taller” the wave, the greater the
amplitude, the more intense the light.
Attenuation
Attenuation is the loss of light intensity over
distance. A light signal cannot travel forever. Just
as light from a flashlight or the headlights on a car
get dimmer the farther away they are, the farther
the signal travels the weaker it becomes.
Figure 2.28.1
Look at the picture to the right. Notice that
in fog the light does not travel as far as it
would if it was a clear night. The picture
demonstrates the attenuation of light.
Figure 2.28.2
NEW TERM
Attenuation – Loss of
signal strength.
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Light and Optics
Attenuation
There are three things to remember about attenuation and they are:
Attenuation is a ratio of the original light intensity to the intensity of the measured light.
When measuring light we measure how much the light is attenuated from the original light.
Attenuation is a function of distance. The farther light travels the more it is attenuated. It
may be possible to see a flashlight at 20 feet but impossible to see it at 20 miles.
Attenuation varies depending on the wavelength of the light and what it is moving through.
Attenuation varies with wavelength – some waves are not attenuated as much as others as
they travel through different mediums like air or glass. Fiber Optics uses infrared light
because there is less loss in the glass at that wavelength. Visible wavelengths or ultraviolet
wavelengths will not travel as far in glass as IR wavelengths
The photo to the left is of a simulated Fiber Optic cable. Note
that the light is weaker as it travels to the right.
The amplitude of the light is a function of distance, material and
light wavelength. What other things do you notice while looking
at the photo?
Figure 2.29.1
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Light and Optics
Attenuation of Sound
Changes in amplitude in a light signal are measured in decibels (dB). The decibel is a way of
measuring ranges of energy of any kind.
For example, humans are capable of hearing a wide range of sound from a faint whisper to a
loud noise. A single leaf falling to the ground is 3dB while thunder is 120dB. You would think
that would mean that the thunder is 40 times louder than the falling leaf, but that’s not how
decibels work. Thunder at 120dB is not 120 times louder than the falling leaf; it’s a trillion times
louder.
Figure 2.30.1
Figure 2.30.2
Introduction to Network Cabling Fiber Optic-Based Systems
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Light and Optics
Attenuation of Sound (continued)
An engineering rule-of-thumb states that for every 3dB of change the
power doubles or halves depending on whether the change is a positive
or negative one. For example, suppose a light bulb is 100 watts. Three dB
of attenuation will reduce it to 50 watts. A 3dB increase in the intensity of
the signal will boost it to 200 watts.
Introduction to Network Cabling Fiber Optic-Based Systems
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Light and Optics
Decibel to Power Conversion
dB Power Out as a % of Power In
1
79%
2
63%
3
50%
4
40%
5
32%
6
25%
7
20%
8
16%
9
12%
10
10%
11
8%
12
6.3%
13
5%
14
4%
15
3.2%
16
2.5%
17
2%
18
1.6%
19
1.3%
20
1%
21
0.3%
22
0.1%
23
0.01%
24
0.001%
% of Power Lost
21%
37%
50%
60%
68%
75%
80%
84%
88%
90%
92%
93.7%
95%
96%
96.8%
97.5%
98%
98.4%
98.7%
99%
99.7%
99.9%
99.99%
99.999%
Remarks
----1/2 the power
----1/4 the power
1/5 the power
1/6 the power
1/8 the power
1/10 the power
1/12 the power
1/16 the power
1/20 the power
1/25 the power
1/30 the power
1/40 the power
1/50 the power
1/60 the power
1/80 the power
1/100 the power
1/300 the power
1/1000 the power
1/10,000 the power
1/100,000 the power
Introduction to Network Cabling Fiber Optic-Based Systems
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Light and Optics
Decibel to Power Conversion (continued)
A fiber technician found a splice with 10dB of loss.
How much light is lost in the splice? 90% What is the loss? 1/10 the power
Figure 2.31.1
Introduction to Network Cabling Fiber Optic-Based Systems
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Light and Optics
The Power Meter and Light Source
The power meter and light source are used to measure
light. The light source provides the light and the power
meter measures the light at the other end of an optical
cable.
The following steps describe the controls of the light
source and power meter. It is a simple task to set these
units up and measure signal strength.
Optical Light Source
Figure 2.32.1
Provides Infrared light at 850nm.
The ON button turns the unit on and the ON light will come on.
Continuous or modulated Light sends steady or blinking light.
If the ON light is blinking the unit is in the modulated light or blinking mode. Push the button until the
ON light remains on steadily or continuously. Test with continuous light.
A low battery light on the unit indicates when the battery needs to be changed.
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Light and Optics
The Power Meter and Light Source (continued)
Warning
Do not look into the light source to see if it is working. Always
assume that it is working and that it is putting out intense IR light
that can not be seen. Even though you can not see it, this does not
mean that it is not there and that it can’t harm your eyes.
The optical light source has an ST type connector to connect to a
Fiber Optic system. When connecting the light source use the ST to
ST or ST to SC patch cord.
Figure 2.32.2
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Light and Optics
The Optical Power Meter
The purpose of the power meter is to measure the light from the light source
after it has passed through the Fiber Optic system. It offers a variety of
functions. It incorporates a power saving function to extend battery life. It
allows you to select between dB and dBm. Although you will only be testing
at 850nm, the power meter does have the ability to test 1310nm and
1550nm. There is also a "zero set" function, which allows a technician to
measure loss without having to perform any mathematical calculations.
Power On/Off – turns unit on or off.
dB or dBm – The display will indicate which of these modes the power meter
is in. In the dB mode the results are a ratio. In the dBm mode the power
meter measures the loss of light from a standard milliwatt measurement.
Figure 2.33.1
"λ" – the lambda button selects the wavelength to test. When testing, press
the button until the display indicates 850nm. The light source provides light at
a wavelength of 850nm. As you will learn in later modules the cables used in
this course are also optimized for 850nm.
Zero Set – this button zeros the meter display
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Light and Optics
Activity 2.6 Power Meter and Light Source Demonstration
Observe the demonstration and list the buttons on each device
and their function.
Show the demonstration and or demonstrate the functions of the
power meter and light source.
Introduction to Network Cabling Fiber Optic-Based Systems
Video
2
Light and Optics
Measuring Attenuation
If you take a ruler and measure something six
inches long, that’s a firm measurement. Six inches
is six inches no matter what the circumstances. A
measurement of 6dB always means a 6dB
difference up or down compared to something else.
Compared to what? Compared to a starting point
that you choose.
When you use a power meter, you take a first
measurement to use as your starting point. This is a
benchmark measurement. Then you use the meter
to get a second reading on the cable you wish to
test. Comparing these two measurements shows
how much more or less power this cable passes
than your benchmark. You don’t measure amplitude
or intensity directly; you always measure it
compared to a benchmark.
Introduction to Network Cabling Fiber Optic-Based Systems
Figure 2.35.1
2
Light and Optics
Testing Methods
There are two different methods you can use when testing and
they are the subtractive or zero set.
The subtractive method
This method is especially useful when comparing the
performance of one cable with another. The benchmark is
the reading for the first cable.
1.
2.
Turn on the power meter.
Attach the first cable to the power meter and to the
SPOT.
3. Turn on the SPOT and take the reading.
4. Replace the first cable with the second cable.
5. Take a reading for the second cable.
6. Subtract the first cable reading from the second
cable reading. If the answer is a positive number, the
second cable has more attenuation (lets less light
through)
than the first cable. If the answer is a negative number,
the second cable has less attenuation (lets more light
Introduction to Network Cabling Fiber Optic-Based Systems
through) than the first.
Figure 2.36.1
2
Light and Optics
The Zero-Set Method
This method is useful for comparing cables or comparing
different light sources. It sets up a common benchmark to
which everything is compared.
1.
2.
3.
4.
5.
6.
7.
Turn on the power meter.
Attach the cable and SPOT to the power meter.
Turn on the SPOT.
Zero out the power meter. This is the reference reading the “zero” condition.
Attach the next cable or SPOT and turn on the SPOT.
Take another reading.
This method imitates a direct measurement. All readings
are compared to the “zero” condition.
Figure 2.37.1
Introduction to Network Cabling Fiber Optic-Based Systems
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Light and Optics
Activity 2.7 Measuring Attenuation Demonstration
Watch the demonstration on testing methods and list the steps
for each method.
Video (S)
This demonstration tests the attenuation of two different patch
cables.
This Demonstration also measures using adapters and sets a
zero reference.
Video (Z)
Watch the demonstration or demonstrate measuring
attenuation with subtractive and additive methods.
Demonstrate how to connect and test cable systems
Introduction to Network Cabling Fiber Optic-Based Systems
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Light and Optics
Activity 2.8 Measuring the Attenuation of a Fiber Optic Cable
Using a power meter and light source measure the attenuation of a patch cord.
Use the checklist below and measure the attenuation of two Fiber Optic cables.
CABLE ONE
Type of Cable
Method Used
subtractive _____
zero set
_____
dB loss
_____
ST - ST, ____
SC - SC, ____
ST - SC, ____
Use the power to decibel conversion chart and indicate the approximate light loss in
percentage.
The approximate light loss at the end of the cable is about ___________ percent of the source.
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Light and Optics
Activity 2.8 Measuring the Attenuation of a Fiber Optic Cable
(continued)
CABLE TWO
Type of Cable
Method Used
subtractive _____
zero set
_____
dB loss
_____
ST - ST, ____
SC - SC, ____
ST - SC, ____
Use the power to decibel conversion chart and indicate the approximate light loss in percentage.
The approximate light loss at the end of the cable is about ___________ percent of the source.
Students take turns measuring the attenuation and logging results of the patch cords in
the student workstation.
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Light and Optics
Measuring Light
The SPOTs have an On/Off switch on the left side. They have a button on the front that
selects a steady or blinking light. The SPOTs have universal adapters and will accept
either SC or ST optical connector. Do not measure attenuation when the SPOT is in the
blinking mode. Test results will be unreliable.
There are four SPOTS.
•White
•Blue
•Red
•IR
To measure light intensity, follow these steps.
Connect the cable from the SPOT to the power meter (power meter and SPOT ON).
Select a wavelength (850nm).
Turn the SPOT on and measure the light intensity on the power meter.
Introduction to Network Cabling Fiber Optic-Based Systems
Figure 2.40.1
2
Light and Optics
Activity 2.9 Measuring Light Intensity
Measure and record light intensity of the four SPOTS.
Procedure:
1. Connect the cable from the SPOT to the power meter (power meter and SPOT ON).
2. Select a wavelength (850nm).
3. Establish a benchmark: zero the power meter using the blue spot as the light
source.
4. Measure the light intensity on the power meter for each of the four spots.
Complete this checklist
SPOT
Color
Wavelength
---------------------
---------------------
---------------------
---------------------
Power meter
Wavelength
dB or dBM
Modulate Yes/No
-------------------------------
-------------------------------
-------------------------------
-------------------------------
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Light and Optics
Activity 2.9 Measuring Light Intensity (continued)
Attenuation
Reading
------------- ------------- ------------- -------------
Rank the SPOTS from strongest to weakest
Strongest ------------- ------------- ------------- ------------- Weakest
Graph your readings using the graph on the next page:
Have students complete the checklist either by sharing the power
meters and SPOTs or breaking into teams. Have students plot the chart
on the following page. Encourage students to explain the resulting
curve.
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Light and Optics
Do the different SPOTS vary in
intensity across the spectrum?
Do you notice a pattern?
Can you explain the various
readings?
Refer to the chart on the next
page as needed.
Figure 2.42.1
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Light and Optics
The chart below is the response curve for the power meter at 850 nm.
The reason that the SPOT
readings vary is because the
power meter has its strongest
indication from the IR region.
Areas outside of this range are
attenuated, and measured by
the power meter.
Figure 2.43.1
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Light and Optics
Module Review
The four characteristics of light are:
•Speed
•Wavelength and Frequency
•Movement
•Amplitude
Light travels at 300,000 kilometers per second` in a vacuum. It slows down in air and
even more in water. The index of refraction is a number that indicates the speed of
light in a medium.
The electromagnetic spectrum is a scale that shows all of the waves in nature. Some
are extremely long waves and some are extremely short. Light, both visible and nonvisible is only a small part of the electromagnetic spectrum. Wavelength and
frequency are inversely proportional as one goes up the other goes down.
Non-visible light has a wavelength of 850nm or higher and shorter wavelengths of
light are visible.
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Light and Optics
Module Review (continued)
Remote controls and Fiber Optic systems use non-visible light.
Light reflects off of surfaces and refracts in other mediums. When light reflects
the angle of incidence equals the angle of reflection and if the angle of
incidence is greater than the critical angle light reflects back into the medium
from where it came.
When light enters a medium with a higher index of refraction it bends towards
the normal line. When light enters a medium with a lower index of refraction it
bends away from the normal line.
The amplitude of light is measured in dB and the loss of light is called
attenuation. There are three attributes of attenuation and they are attenuation
is a ratio, attenuation is a function of distance and attenuation varies by the
wavelength of light.
An attenuation of -3dB means halving of light.
Power meters and light sources are used to measure the attenuation of light.
The attenuation can be measured by either zero set or subtractive procedures
Introduction to Network Cabling Fiber Optic-Based Systems
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Light and Optics
New Terms
TERM
DEFINITION
Angle of Incidence
Angle of Reflection
The angle that light strikes a surface.
The path that reflected light travels – same as angle
of incidence.
Loss of signal strength.
Audio Video Receiver.
The angle that light begins to leave a substance at the
interface. Below the critical angle, more of the light leaves
the substance.
All of the types of waves found in nature from sound to
gamma rays.
Invented by Augustin Fresnel as a means to focus
refracted light.
The ratio of the speed of light in a vacuum to its speed in a
substance.
Non-visible light that is used in Fiber Optic systems.
Attenuation
AVR
Critical angle
Electromagnetic Spectrum
Fresnel Lens
Index of Refraction
Infrared Light
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Light and Optics
New Terms (continued)
TERM
DEFINITION
Law of Reflection
This law states that the angle of incidence is equal to
the angle of reflection.
The basic unit of the measurement of the metric
system, about 39 inches (m).
A universal scientific system of measurement.
One millionth of a meter, (symbol: µm). Also known as
a micrometer.
One thousandth of a meter (mm).
One billionth of a meter. Also called a nanon.
A line that is perpendicular to a surface.
The description of light when it bounces off of a surface.
The description of how light slows and bends as it
enters a substance.
The way a scientist can quickly work with very large or
very small numbers.
The law that states how light refracts in a substance.
When the incident angle allows most of the light to
remain inside a substance.
Meter
Metric System
Micron
Millimeter
Nanometer
Normal line
Reflection
Refraction
Scientific Notation
Snell's Law
Total Internal Reflection
Introduction to Network Cabling Fiber Optic-Based Systems
Introduction to Networking Fiber Optic-Based Systems
(Version 3.3)
© 1998-2012 by C-Tech Associates, Inc.
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regarded as affecting the validity of any of these or as an infringement on
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Light and Optics
QUESTIONS?
Module Test Time!
Introduction to Network Cabling Fiber Optic-Based Systems