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Version: January 2013 1
Lesson Subject Area(s) Physics & Physical Science
Associated Unit N/A
Lesson Title Letters from Hogwarts
Header Picture of water beads.
Grade Level 11 (10-12)
Lesson # 1 of 1
Lesson Dependency
Time Required 40-50 Minutes
Summary
This lesson is meant to serve as a bridge between the lessons about how light behaves and how those laws are applied in an engineering sense. Students will be given a brief review over light, the laws of refraction (Snell’s Law, Critical Angle, and Total Internal Reflection), and a brief history of fiber optics. Emphasis will then shift to how those Snell’s Law is applied to fiber optics and how engineers make it work on a large scale. After the lesson is complete, the students will participate is a fiber optics demo where they send signals through a polymer rod.
Engineering Connection
Often in the classroom, students lose sight of why they are learning the material. With this lesson, the connection between the classroom and the engineering world is reinforced by showing how the simple idea of refractions has spawned a global market of fiber optics. Despite the simple principles behind fiber optics, its success did not happen overnight. Instead many areas of technological advancement (processing techniques, packaging, solid state physics for example) had to come together to make global communicate by fiber optics viable.
Engineering Category = 1 Choose the category that best describes this lesson’s amount/depth of engineering content: 1. Relating science and/or math concept(s) to engineering 2. Engineering analysis or partial design 3. Engineering design process
Keywords Snell’s Law, Fiber Optics, Light, Morse Code
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Version: January 2013 2
Educational Standards
Version: January 2013 3
National and State
Choose standards from http://asn.jesandco.org/resources/ASNJurisdiction or browse educational standards on TeachEngineering.
State/national science/math/technology (provide source, year, number[s] and text):
Chapter 112. Texas Essential Knowledge and Skills for Science Subchapter C. High School
Statutory Authority: The provisions of this Subchapter C issued under the Texas Education Code, §§7.102(c)(4), 28.002, and 28.025, unless otherwise noted.
§112.31. Implementation of Texas Essential Knowledge and Skills for Science, High School, Beginning with School Year 2010-2011.
The provisions of §§112.32-112.39 of this subchapter shall be implemented by school districts beginning with the 2010-2011 school year.
Source: The provisions of this §112.31 adopted to be effective August 4, 2009, 34 TexReg 5063; amended to be effective August 24, 2010, 35 TexReg 7230.
§112.39. Physics, Beginning with School Year 2010-2011 (One Credit).
(7) Science concepts. The student knows the characteristics and behavior of waves. The student is expected to:
(D) investigate behaviors of waves, including reflection, refraction, diffraction, interference, resonance, and the Doppler effect;
Source: The provisions of this §112.39 adopted to be effective August 4, 2009, 34 TexReg 5063.
http://www.teachengineering.org/browse_standards.php Year 2009
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National Science Education Standards: Science [1995]
Current Standard
• Content Standard E: Science and Technology (Grades K - 12)
Standard's Subset
- key:
Link to ALL information for a standard
Standard has one or more explicit curriculum alignments
Version: January 2013 4
Content Standard E: As a result of activities in grades 9-12, all students should develop Abilities of technological design Understandings about science and technology (Grades 9 - 12)
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ITEEA Educational Standard(s)
ITEEA (provide standard number, grade band, benchmark letter and text):
Standard 17. Students will develop an understanding of and be able to select and use information and communication technologies. (Grades K – 12)
L. Information and communication technologies include the inputs, processes, and outputs associated with sending and receiving information. (Grades 9 - 12)
M. Information and communication systems allow information to be transferred from human to human, human to machine, machine to human, and machine to machine. (Grades 9 - 12)
N. Information and communication systems can be used to inform, persuade, entertain, control, manage, and educate. (Grades 9 - 12)
O. Communication systems are made up of source, encoder, transmitter, receiver, decoder, storage, retrieval, and destination. (Grades 9 - 12)
P. There are many ways to communicate information, such as graphic and electronic means. (Grades 9 - 12)
Q. Technological knowledge and processes are communicated using symbols, measurement, conventions, icons, graphic images, and languages that incorporate a variety of visual, auditory, and tactile stimuli. (Grades 9 - 12)
Pre-Requisite Knowledge
While it is encouraged to already have covered Snell’s law at the time of this lesson, this lesson is self-sufficient if that is not the case.
Learning Objectives After this lesson, students should be able to: Apply Snell’s Law to an experimental set-up
Apply Snell’s Law and its derivatives (Critical Angle/Total Internal Reflection) to fiber optics
Version: January 2013 5
Be able to identify various components of a simple fiber optics system
Be able to give the human analogies to fiber optics components (Eye=Photodiode, Brain=Converter, etc)
Introduction / Motivation
After centuries of relying on old owl technology, Hogwarts has finally to upgrade their communication system to fiber optics. Here we will explore what is the fundamental basis for fiber optics.
Teacher Notes:
There is a power point to follow for this lesson that includes the “script.”
As an attention grabber demo, purchase dehydrated water gems and soak in a large beaker (Make sure the students do not see them in the unhydrated state yet). The water gems are a hydrophilic polymer that will absorb water. As the beads absorb water their index of refraction will begin to more closely resemble that of water until you can no long see the beads.
Take the beaker (will fully hydrated bead in it) and place in front of the class. Ask the students how many beads are in the beaker. Most will probably say zero. Take your hand and pull some of the beads out showing that the beaker is in fact full of beads. Show them the dehydrated bead and explain to them that it is a hydrophilic polymer that absorbs water. Drop a few dehydrated beads in the beaker to show that they can be seen in the water. Ask why you can see the dehydrated bead but not the hydrated bead (The index of refraction is different). Ask them what happens to the index of refraction as the bead absorbs more water (does it more closely resemble water?) Emphasis that this is due to how light behaves in different materials.
After the demo, explain that you wanted to take such concepts from the classroom and apply them to real world. A prime example of that is the development of fiber optics which relies on total internal reflections to work. To emphasis the concepts you are going to do a fiber optics activity using a plastic rod (hold up the rod) and morse code but in order to get to that point you need to discuss the history of FOs and how the classroom material is applied. You are now ready to Segway in to the power point. Comments can be found in the notes section of each slide that will help guide the lesson.
Lesson Background & Concepts for Teachers
Snell’s law and it applications are basic stuff for a physics teacher. However, the principles and engineering behind fiber optics are not. The power point does a good job of guiding the lesson but will not give you comprehensive knowledge of the subject. When I created this lesson I found the following book chapter helpful
http://media.techtarget.com/searchNetworking/downloads/FiberOptic_ch3.pdf
Version: January 2013 6
Vocabulary / Definitions Word Definition
Electromagnetic Wave A form of energy emitted and absorbed by charged particles which exhibits wave-like behavior as it travels through space
Refraction The change in direct of a wave due to the transition from one medium to another
Reflection The complement to the incident wave that is returned off the medium Critical Angle The angle at which total internal reflection takes place
Total Internal Reflection The situation when no light is able to diffract out of a medium due to the critical angle being reached
Attenuation The process of losing the intensity of a signal through such events as absorption, bending losses, and ablation of the material
Transmitter In the context of fiber optics, a transmitter is the device that creates the light signal to be sent over the fiber optic cables
Receiver The device, typically a photodiode, that receives the sent signal from the signal
Converter The device that converts the digital light signal into a readable output.
Chromatic Dispersion The refractive index is a function of wave length and as a result any signal that is a combination of wavelengths will refract to different degrees resulting in the separation of each individual wavelength
Bending Losses When the angle of bending is significant enough to destroy total internal reflection
Ablation of Materials When the material is damaged in such a way that total internal reflection is not maintained
Scattering A general term referring to the interaction of waves with an object of the appropriate size scale (a house will scatter radio waves, electrons will scatter x-rays
Associated Activities
Sending a Message
Lesson Closure
Assessment Please see the power point presentation for the assessing the students during the lesson. Questions and expected comments are listed in the notes.
For the post assessment, please see the worksheet for the accompanying activity, “Sending a Message.”
Lesson Extension Activities
Additional Multimedia Support Fiber Optics PowerPoint
Version: January 2013 7
References http://media.techtarget.com/searchNetworking/downloads/FiberOptic_ch3.pdf
Attachments Power Point
Worksheet
Other
Redirect URL
Contributors Brian Rohde, Don McGowan
Supporting Program
NSF GK-12 University of Houston,
Acknowledgements
Students in 1st, 2nd, 3rd, 4th, and 7th period Pre AP Physics at Friendswood ISD
Introduction to Light and Fiber Optics
Outline for this unit
• Brief discussion and history relating to light
• Brief History of Fiber Optics
• Snell’s Law
• Total Internal Reflection
• TIRs application to fiber optics
• Types of Fibers
• How to send signals with fibers optics
• Activity
What is Light?
What is Light?
Particle (Newton, 1600s) versus Waves (Huygens, 1678)
Double Slit Experiment (Thomas Young, 1803)
Single Photon Experiment (G.I. Taylor, 1909)
Wave‐Particle Duality consisting
of a Photon
How do Humans See Light?The retina contains two types of photoreceptors…Rods and ConesRods (~120 million) do most of the work at night time while Cones (6‐7 million) have the ability to sense Red (64%), Green (32%), and Blue(2%)
• Rods are not simulated by red wavelengths and are much more sensitive than cones
• Cone Stimulation
Evolution of Fiber
• 1880 – Alexander Bell invents the photo‐phone
• 1930 – Patents on tubing
• 1950 – Patent for two‐layer glass wave‐guide
• 1960 – Laser first used as light source
• 1965 – High loss of light discovered
• 1970s – Refining of manufacturing process
• 1980s – Optic Fiber technology becomes backboneof long distance telephone networks.
Snell's Law of Refraction
• Snell's Law ‐ The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant.
• n1 sin1 = n2 sin2
• n = index of refraction• = angle • 1,2 = substance 1 & 2
• Note: n1 sin1 = n2 sin2 = n3 sin3 = …when additional layers exist and there is no critical angle.
refr
inc
refr
inc
refr
inc
vv
sin
sin
We can classify materials by their “index of refraction” defined as
the ratio of
vcn
speed of light in a vacuum, cspeed of light through medium, v
i.e.ncv
inc
refr
refr
inc
refr
inc
refr
inc
refr
inc
nn
nn
ncnc
vv
/1
/1
/
/
sin
sin
Snells’ Law
Note then that:
So often Snell’s Law is also written as: n1sin1 = n2sin2
reversingthe
indices!
Index of Refraction
• Index of refraction is unitless and typically greater than one. It is a measure of how light is impeded by that substance.
n = c
vs
c = speed of light = 3 X 108 m/s
n = index of refraction nair = 1.0003 nglass = 1.5
vs = velocity of light in a substance = m/s
A ray of light is shown entering a glass prism, bendingdown (toward the normal) as it enters.
As the ray re‐enters the air through the opposite face of the prism
(1) it bends up.(2) it passes through without bending.(3) it bends further down.
A ray of light is shown entering a glass prism, bendingdown (toward the normal) as it enters.
As the ray re‐enters the air through the opposite face of the prism
(1) it bends up.(2) it passes through without bending.(3) it bends further down.
Air-into-glass: lightbends toward normal
Glass-into-air:light bends away from normal
Critical AngleCritical Angle (<c)– The term used to describe an angle of incidence that produces an angle of refraction of 90o.
• When the angle of refraction reaches 90o, the refracted ray will lie along the boundary between two media (such as water and air).
Terms
• Critical angle is the angle at which no refraction occurs only total internal reflection occurs.
• Total internal reflection occurs at any angle past the critical angle. No light refracts when passing from a more dense to less dense media at these angles.
Critical angle
Critical Angle
• The critical angle (incident angle) occurs based on the following formula:
ni sin c = nr sin 90˚
sin c = nr where ni › nrni
Total Internal ReflectionTotal Internal Reflection – When the angle of incidence is larger than the critical angle, there is no refraction from one medium to another. All of the light is reflected back to the first medium.
• Only occurs when light travels from a medium in which the speed of light is lower to a medium in which the speed of light is higher.
Refraction, Critical angle, Total Internal Reflection
How Optical Fibers WorkHow Optical Fibers Work
Light is carried along fiber optic cables by repeated total internal reflection.
Total internal reflection occurs when the light hits the wall of the central core at an angle greater than the critical angle.
fiber optic cables are made of three layers...
The ray of light enters the central core of the optical fiber at an angle greater than the critical angle so it is internally reflected.
To ensure the ray is internally reflected the cladding is made from a material which has a lower refractive index than the central core.
How do optical fibers guide light? By total internal reflection
2
1
claddingn2 < 1.45
coren1 = 1.45
2
1
1
2
sin
sin
nn
In above, light beam escapes. Now consider 2 > 90o. Light cannot escape:
1> c
claddingn2 < 1.45
coren1 = 1.45
1
21
2
1
sin
sin
)90sin(
nn
nn
c
c
o
E.g. for n2 = 1.3, c = 63.7o
Optical Fibers in communication
Single optical fiber
Cable containing thousands of copper wires
Optical fibers allow more data to be transmitted more quickly.
Types of fibers
• Single‐mode fibers have small cores (about 3.5 x 10‐4 inches or 9 microns in diameter) and transmit infrared laser light (wavelength = 1,300 to 1,550 nanometers).
• Multi‐mode fibers have larger cores (about 2.5 x 10‐3 inches or 62.5 microns in diameter) and transmit infrared light (wavelength = 850 to 1,300 nm) from light‐emitting diodes(LEDs).
• Some optical fibers can be made from plastic. These fibers have a large core (0.04 inches or 1 mm diameter) and transmit visible red light (wavelength = 650 nm) from LEDs.
• fibers used in communication have highly transparent cores so very little light is absorbed.
• The central core is very narrow to prevent multipath dispersion.
If the core is too wide then the light traveling along the axis will travel a shorter distance than light which has undergone repeated TIRs. So the pulse of light would become longer than it should be. (if it is sent over large distances the pulse can merge with the next pulse).
• The cladding around the central core also prevents signals crossing over if fibers were to touch, so therefore improving security.
• Also monochromatic (single wavelength) light is used to prevent spectral dispersion.
Step Index fiber
Refractive index
Distance through cross section of fiber
nglass = 1.5
ncladding = 1.2
nair = 1
Important !!
The graph below shows how the refractive index varies with distance through the cross section of the fiber.
INDEX PROFILE
The boundary between the core and cladding may either be abrupt, in step‐index fiber, or gradual, in graded‐index fiber
Fiber Optic Types
26
• multimode step‐index fiber
– the reflective walls of the fiber move the light pulses to the receiver
• multimode graded‐index fiber
– acts to refract the light toward the center of the fiber by variations in the density
• single mode fiber
– the light is guided down the center of an extremely narrow core
Sending A Message
• You call you relative in Europe….how does your message get there.
• Hardware– Transmitter, Cable, Amplifier/Repeater, Receiver (Photodiode)
• Sound to Electrical to optical to electrical signal to Sound conversion
Optical Communications
• Digital transmission: light pulse = 1, no light = 0• Pulsing frequencies very fast ~ 10 GHz
time
≈
Amplitude
TbBaseband signal
≈
time
Amplitude Carrier Envelope
ASK signal
0
1
0
1
SOURCE: ALCATEL
Submarine Cables in North East Asia
• Transmission
• Refraction
• Reflection
• Scattering
• Bending losses
Signal Loss and Attenuation (TE)
Morse Code
• Developed by Samuel Morse for sending electrical telegraphs in 1836
• Consists of dots, dashes and spaces– Resembles the modulated intensity method used in current fiber optics
Why am I mentioning this?
• You guys are going to send messages using Morse code and a large diameter plastic fiber
• Groups of 2
• One group sends message to another
Guidelines
• Determine the critical angle
• Form a message that is 10 to 30 Characters long
• Establish with the receiving team what will count as a DOT, a DASH, and a S P A C E.
• DO NOT TELL THEM WHAT YOU SENT
– We are on the HONOR system
Laser Safety
• Do NOT shine the laser in to anyone’s eye!!!
If it comes up we can discuss how fiber optic cables are made…..
How Are Optical Fibers Made?
• Making optical fibers requires the following steps:
– Making a preform glass cylinder– Drawing the fibers from the preform– Testing the fibers
Making the Preform Blank
• The glass for the preform is made by a process called modified chemical vapor deposition (MCVD).
Making the Preform Blank
• In MCVD, oxygen is bubbled through solutions of silicon chloride (SiCl4), germanium chloride (GeCl4) and/or other chemicals.
• The precise mixture governs the various physical and optical properties (index of refraction, coefficient of expansion, melting point, etc.).
• The gas vapors are then conducted to the inside of a synthetic silica or quartz tube (cladding) in a special lathe. As the lathe turns, a torch is moved up and down the outside of the tube.
Making the Preform Blank
• The extreme heat from the torch causes two things to happen:
– The silicon and germanium react with oxygen, forming silicon dioxide (SiO2) and germanium dioxide (GeO2).
– The silicon dioxide and germanium dioxide deposit on the inside of the tube and fuse together to form glass
Making the Preform Blank
• The lathe turns continuously to make an even coating and consistent blank.
• The purity of the glass is maintained by using corrosion‐resistant plastic in the gas delivery system (valve blocks, pipes, seals) and by precisely controlling the flow and composition of the mixture.
• The process of making the preform blank is highly automated and takes several hours. After the preform blank cools, it is tested for quality control.
Drawing Fibers from the Preform Blank
• Once the preform blank has been tested, it gets loaded into a fiber drawing tower.
• Diagram of a fiber drawing tower used to draw optical glass fibers from a preform blank.
• The blank gets lowered into a graphite furnace (3,452 to 3,992 degrees Fahrenheit or 1,900 to 2,200 degrees Celsius) and the tip gets melted until a molten glob falls down by gravity. As it drops, it cools and forms a thread.
Drawing Fibers from the Preform Blank
• The operator threads the strand through a series of coating cups (buffer coatings) and ultraviolet light curing ovens onto a tractor‐controlled spool.
• The tractor mechanism slowly pulls the fiber from the heated preform blank and is precisely controlled by using a laser micrometer to measure the diameter of the fiber and feed the information back to the tractor mechanism.
• Fibers are pulled from the blank at a rate of 33 to 66 ft/s (10 to 20 m/s) and the finished product is wound onto the spool. It is not uncommon for spools to contain more than 1.4 miles (2.2 km) of optical fiber.
