Fiber Optic Speed of
Light Apparatus
Instruction Manual
IndustrIal FIber OptIcs
*
Copyright © 2018
Previous Printings 2001, 2004, 2010, 2011
by Industrial Fiber Optics, Inc.
12 0225 IF-SL Rev H
Printed in the United States of America
* * *
All rights reserved. No part of this publication may be reproduced,
stored in a retrieval system, or transmitted in any form or by any means
(electronic, mechanical, photocopying, recording, or otherwise)
without prior written permission from Industrial Fiber Optics.
* * * * *
IndustrIal FIber OptIcs
1725 West 1st street
tempe, AZ 85281-7622
UsA
– i –
TABLE OF CONTENTS
INTRODUCTION.......................................................................................................... 1
BACKGROUND INFORMATION........................................................................... 2
The Speed of Light – A Centuries-Long Quest.................................................................. 2
Speed Zones Occur When Light Meets Matter.................................................................. 3
Fiber Optics: Light Meets its Master at Last...................................................................... 4
Optical Fiber: The Workhorse Today.................................................................................. 5
There is Always a New Frontier.......................................................................................... 7
ACTIVITY.......................................................................................................................... 8
Materials Required for the Project...................................................................................... 8
Equipment Setup................................................................................................................ 9
Calibration.......................................................................................................................... 10
Measurement..................................................................................................................... 11
Speed of Light Calculation................................................................................................. 11
APPENDIX....................................................................................................................... 13
Kit Assembly...................................................................................................................... 13
Fiber Preparation............................................................................................................... 19
References......................................................................................................................... 20
Glossary.............................................................................................................................. 22
Warranty Information
This kit was carefully inspected before leaving the factory. Industrial Fiber Optics products are
warranted against missing parts and defects in materials for 90 days. Since soldering and incorrect
assembly can damage electrical components, no warranty can be made after assembly has begun.
If any parts become damaged, replacements may be obtained from most radio/electronics supply
shops. Refer to the parts list on page 14 of this manual for identification.
Industrial Fiber Optics recognizes that responsible service to our customers is the basis of our
continued operation. We welcome and solicit your feedback about our products and how they
might be modified to best suit your needs.
– ii –
– 1 –
INTRODUCTION
Welcome to the fascinating world of fiber optics technology! We believe you will find the instructions and
exercises in this science curriculum not only educational, but challenging and enjoyable as well.
This manual is an essential component of the Speed of Light Apparatus — which is designed for use in
science, physics, industrial technology and vocational education classrooms, grades 10-12. Physics fans and
other science-oriented individuals (including those who are just intellectually curious about how things
work!) may also obtain the apparatus for studies in home or workshop.
Your particular apparatus may have been purchased either fully assembled at our factory, or in kit form
which you can assemble on your own. Easy-to-follow instructions for this kit are contained in the Assembly
portion of this manual.
The manual is intended to guide both instructors and students through a basic introduction to the principles
of fiber optics and the measurement of the speed of light. The apparatus includes one experiment, along
with descriptions of components, a glossary of terms commonly encountered in fiber optics, and a list of
optional reading references which discuss fiber optics theory and development.
Everything is here for you to get in on the ground floor of fiber optics, to develop an understanding of
emerging technology in this vast new field of potential applications, and to ready yourself for more
advanced studies.
When you are ready to move on to more sophisticated discussions, look for our other study materials and
kits specifically designed to make learning both fun and rewarding.
– 2 –
BACKGROUND INFORMATION
The miracles of modern-day fiber optic technology didn’t simply show up on someone’s doorstep. Centuries
of curiosity, experimentation, frustration and perspiration passed before pure science won out over
superstition and guesswork. A small Italian gentleman was among the first to seek enlightenment about the
speed of light . . .
The Speed of Light – A Centuries-Long Quest
Scientists, and probably even more casual thinkers, began speculating about the speed of light centuries ago.
When you consider that early civilizations often made gods and goddesses out of suns, planets and moons,
it isn’t hard to understand their fascination and wonder about one of the most powerful forms of energy in
their world.
One of the first, that we know of, to ask himself, “Hmmm....wonder how fast that stuff travels?” was
Galileo Galilei (1564-1642), Italian astronomer and physicist....the same fellow who dropped objects off the
now-Leaning Tower of Pisa trying to calculate their rates of fall.
Galileo scored with the tower business, in calculating the fall rates of items as varied as feathers and
cannonballs. Light was another matter, however, and he knew he had to attempt his measurements of light
speed over a much greater distance if he hoped to get a whatever-per-second grip on that speeding
luminous energy.
He decided that hilltop-to-hilltop was a good starting point, and that’s where he began, with himself on one
hill and his trusty assistant on another, well after the sun had set. Each man carried an oil-burning lamp with
a cover, so he could shield the light until needed.
Galileo uncovered his lamp briefly, sending a flash of
light over to his assistant. The assistant replied, with his
own quick flash of light back to the boss. The exchanges
continued that night, and many nights more, from hilltops
increasingly further apart.
Galileo had hoped to measure the time which elapsed from
the moment the lamp shields were uncovered until the light
was perceived by his eye. Rotten luck. He finally decided
(rightfully) that he and his assistant weren’t physically up
to the challenge of grappling with an item we now know
to travel at 186,000 miles per second. (Incorrectly) he
determined that light travels at infinite speed.
Figure 1. Galileo Galilei (1564-1642).
– 3 –
The next recorded method of measuring light was that of Frenchman Armande H. L. Fizeau (1819-1896)
in 1849. Try to visualize his elaborate, apparently unwieldy array of mirrors, lenses, and a huge rotating
cogwheel with 720 teeth, designed to measure the speed of light over a distance of 5.39, count ’em, miles.
But the thing worked. Pretty much. Fizeau and his apparatus computed light speed at 194,000 miles per
second. Not bad, when you consider the scientific tools available at that time, plus the difficulty of the
experiment.
In the years that followed, investigators steadily improved on Fizeau’s equipment and methods of
observation. Most notable was the work of American physicist, Albert Michelson, (1852-1931) who
replaced Fizeau’s 720-tooth wheel with an eight-sided mirror. (He also increased the measurement distance
to 44 miles.)
In 1926, Michelson got a grip on light, computed at 299,796 kilometers per second — or 186,284 miles
per second.
The quest for precision, and the continued refinement of the measurement, continued until, today, really
only hard-core mathematicians quibble about the number of decimal places we should assign to the speed
of light.
The speed of light in a vacuum is now listed as 299,792.4562 kilometers per second. For the purposes of
everyday discussion we generally suffice with a figure of 300 million meters per second, or 186,000 miles
per second.
Speed Zones Occur When Light Meets Matter
Up to this point, we’ve talked primarily about light traveling through a vacuum, such as outer space. When
light passes through some medium other than a vacuum (such as a gas, solid, or liquid), it slows down. But
often not by much.
• At sea level, light speed through air is only about 70 kilometers less per second than it is in a
vacuum. (At higher altitudes, where the air is less dense, and where light is impeded less by solid
airborne particles, light speed increases.) For most practical purposes we can rate light speed in air
versus light speed in a vacuum the same.
• In water, however, things get relatively sluggish. The speed of light is about 25 percent less than in
a vacuum.
• In glass, light has even a tougher time. Its speed drops by about 33 percent, compared to its rate in
a vacuum.
To put these relationships into some form of mathematical perspective we utilize the term “refractive index”
(or “index of refraction”) — which refers to the ratio between the speed of light in a vacuum and its speed
in some other medium. Here’s the equation:
Light speed in avacuum
light speed in anothermedium
Re=ffractiveindex
– 4 –
Or, massaging things down into more manageable symbols:
Substituting the numerical values of light speed in air, water, and glass we would obtain the following
refractive indices:
For air: n = 1.000
For water: n = 1.333
For glass: n = 1.50
If we went to five decimal places instead of three, we would find the refractive index for air is
actually 1.00029.
Precise? Definitely much more so than in the days of Fizeau, not to mention Galileo. You’ll soon learn
how we can measure the speed of light with accuracy that would have astounded those well-intentioned
gentlemen with their lamps, cogwheels, and mirrors.
Fiber Optics: Light Meets its Master at Last
The modern-day technology of fiber optics got its start back in the days when tinkerers and scientists were
trying their best to bend light around corners. It isn’t exactly clear why anyone would want to do that, but a
lot of people, even a hundred years ago, were unwilling to accept that light travel was confined to
straight lines.
They tried hundreds of schemes
— many involving intricate
arrangements of mirrors —
but no one scored big until
1870 when English physicist,
John Tyndall (1820-1893),
demonstrated a most impressive
principle to members of the
British Royal Society.
With a simple apparatus whose
major components included a
light source, a slender stream
of falling water, and a bucket,
Tyndall demonstrated that light
could be guided inside a most
definitely un-straight arc of water.
His equipment is represented in
Figure 2.
Figure 2. John Tyndall’s demonstration: Guiding light in water.
c
v
n=
Light
Source
Water
Light rays
gradually leak out
1026.eps
– 5 –
Good show! Good show! Tyndall depicted for the first time what now occurs millions of times a day in our high-tech
communications industry: Light is guided inside some curving medium, and very little is allowed to escape until
we’re ready to let it go. He used a stream of water. We use slender fibers of glass and plastic.
Sixty-five years after Tyndall’s water, light, and bucket demo, Norman R. French, a scientist with American Telephone
and Telegraph (AT&T), conceived of and demonstrated the use of light-guiding fibers as a communications device.
Proof of the pudding: He was granted a patent for an “optical telephone system” which transmitted voice signals on
beams of light through a network of “light pipes”.
By the early 1960s, scientists (most working for the phone companies) were right on the doorstep of creating the first
fiber optic telephone networks. Three components, in particular, were proving to be increasingly compatible:
• Small, compact Light Emitting Diodes (LEDs)
• Lasers
• Glass optical fibers
Researchers from throughout the world put those items through assorted scientific hoops day after day, year after
year, until they arrived at the basis for the largest and most efficient fiber optic telephone communications in
existence today.
In 1976, the Bell System installed a trial fiber optic telephone system, and one year later had a commercial
system in operation near Chicago. The phenomenal reliability of that system was in large part responsible for
the rapid acceptance of fiber optic networks today. Bell’s “acceptable” percentage of “outages” or out-of-
operation-due-to-failure-time for its existing wire and microwave carrier system was .02 percent. The new
fiber optic set-up, even while the bugs were still being removed, had a downtime average of .0001 percent.
Small wonder that engineers and financiers alike started thinking: “Fiber optics...hmmm.....sound good....
sound very good.”
Optical Fiber: The Workhorse Today
Optic fibers are usually made of plastic or
glass. Plastic fibers cost less than glass but
they lose light more readily. Both consist of
two essential layers of transparent optical
materials, core and cladding.
Viewed from one end, as in Figure 3,
the fiber materials appear in cross section
as concentric circles, with a common
center, or axis, thus the term coaxial. The
outer layer is called cladding; the inner
layer, which carries the light, is called the
core. What makes this little combination
workable is the concept we discussed
earlier, called the refractive index. The
cladding of an optical fiber has a lower refractive index than that of the core, and that simple difference
helps trap light rays within the core, rather than allowing them to wander off into space.
Figure 3. Cross-section of a typical optical cable.
1309.eps
Core
Cladding
Buffer
Strength
members
Jacket
– 6 –
When light enters an optical fiber, it travels in a straight line until it strikes the boundary between core and
cladding. At that point, light is deflected back into the core and proceeds once again in a straight line until
it strikes the core/cladding boundary again. The process repeats itself time and again, and it permits light to
negotiate curves in the fiber by sort of ricocheting its way around the turns.
Whether we’re talking glass or plastic, however, one principle works the same for both, and that’s a little
jewel known as Snell’s Law – the thing that explains the ricochet-around-the-turns process we
described above.
The amount that light bends depends partly on two materials having different refractive indices (as in
optical fiber) but it also depends on the angle at which the light ray strikes the boundary between the two
materials. That “incoming” light ray is known as the incident ray, and the “outgoing” light ray which
bounces off the surface is known as the reflected ray. Mr. Snell (his full name was Willebrod Snell van
Royen), a Dutch mathematician, birth date unknown, explained in 1621 how light is bent, by explaining
the relationship between angles of incidence and angles of transmission. Figure 4 depicts these two angles.
NOTE: both angles are measured from a line drawn perpendicular to the surface, at the point where the
incident ray strikes.
Here’s the equation:
The and are the respective refractive
indices of the two materials involved
(in an optical fiber these would be
the refractive indices of the core and
cladding). is the angle of incidence;
is the angle of transmission.
Snell’s Law isn’t limited to the bending of
light, though. It also helps explain how
light can be reflected from a surface. That’s
handy for us, because if light couldn’t be
reflected, our optical fiber would be just
so much excess hose, rather than the
communications wonder it is.
When an incident light ray strikes the boundary surface between our two materials at a 90-degree angle,
most of that light is going to penetrate the materials and keep on going. The same will be true if the incident
angle is very small — say on the order of a few degrees away from the perpendicular.
h1• sin q1=h2• sin q2
Figure 4. Ray of light traveling through
two materials
θ
1
θ
2
η
1
η
2
Θ
2
Medium 1: air (n1)
Incident ray
Θ1
Medium 2: water (n2)
Refracted
ray
Normal lineReflected ray
1000.eps
– 7 –
However, as Snell’s law tells us, there is a point — otherwise known as the critical angle (when the
incident light ray is leaning well away from the perpendicular) — when the incoming light will be totally
reflected off the boundary between the two materials, as in our optical fiber. Stated another way: If light
traveling through one material encounters another material having a lower index of refraction, and if that
light strikes the boundary between the materials at a low-enough, or “glancing” angle, it cannot escape
from the first material. This is where we get the term total internal reflection, which explains how light is
guided in an optical fiber.
There is Always a New Frontier
We’ve come quite a distance in time and technology since the days when Galileo grappled with the
awesome speed of luminous energy. Today, knowing the speed of light isn’t so important to us as our ability
to control the way in which light travels. Through the marvels of fiber optics, we have achieved partial
control, but we are only on the threshold of new discovery. Just as people in John Tyndall’s day wanted
to throw a few curves into the flight path of light, you can bet that people today (perhaps even you!) are
thinking of ways to disprove old beliefs, to overcome the challenges of the unknown, and to master the
mysteries of science with innovation and determination.
– 8 –
ACTIVITY
Using the Speed of Light Apparatus and a dual-channel oscilloscope you will be able to measure the
speed of light in a transparent medium.* In this case, the medium is plastic optical fiber.
The transmitter portion of the apparatus generates more than 500,000 light pulses per second, which
appear as a cone of red light leaving the red LED. When one end of the fiber is connected to an LED and
the other end to a detector or receiver, our red light will make a circular trip through the fiber. The time
required for these red pulses to travel through a 20-meter-long fiber is about 100 billionths of a second, or
100 nanoseconds.
A bit on the quick side? Rather. However, with the aid of an oscilloscope, this becomes a straightforward,
practical, and visually rewarding demonstration of how an elusive natural phenomenon such as light can be
captured (briefly) and measured by people who put their minds to work.
Materials Required for the Project:
Speed of Light Module, which includes:
• Speed of Light Apparatus *
• 110 VAC-to-DC power adapter - use only the one provided
• 15 centimeters of plastic fiber†
• 20 meters of plastic fiber†
Required but not included:
• Dual-channel oscilloscope, 20-MHz bandwidth or greater
• (time-base measurement capability of 0.1 micro-seconds or less)
• 2 oscilloscope probes
* If you purchased your Speed of Light Apparatus as a kit, you must first assemble it, following the
instructions in the Assembly portion of this manual. If the kit was assembled at the factory, you may
proceed with Equipment Setup following.
† Check the ends of the fiber to see if they are polished and flat. If not, go to the Fiber Preparation section
and polish the ends of the fiber cable before going further.
– 9 –
Equipment Setup
1. Turn on the oscilloscope.
2. On the oscilloscope make the following settings:
• Set the Horizontal Mode Switch on A
• Set the Triggering Mode on Auto
• Set the Trigger Source Switch on Channel 1
• Set Triggering on Positive Slope
• Set the voltage control of input Channel 1 on 1-volt per division
• Set the voltage control of input Channel 2 on 0.5-volt per division
• Set the input coupling of both channels on AC
• Set the Timebase on 50 nanoseconds per division
• Set Verticle Mode to ALT.
3. Connect the probe of Channel 1 to the (blue) test point marked “Reference” on the Speed of Light
Apparatus.
4. Connect the Ground lead of Channel 1’s probe to the ground test point just below the “Reference”
test point. (It is not essential to use probes with ground leads, but the waveforms on the oscilloscope
screen will have better shapes from which measurements can be made.) If your probes do not have
ground leads, use the ground on the oscilloscope.
5. Connect the probe of Channel 2 to the “Delay” (blue) test point on the apparatus.
6. Connect the ground lead from Channel 2’s probe to the ground test point just below the “Delay”
(white) test point.
7. Move Channel 2’s input selector to the “ground” position.
8. Plug the connector of the 110 VAC-to-DC power adapter into the receptacle on the left side of the
apparatus. Plug the AC adapter into a 110 VAC 60 Hz receptacle.
9. As soon as the AC adapter supplies power to the apparatus, the yellow LED should light up. D3, the
fiber optic LED, should also be visible if you lean down to look sideways into the front of the blue
fiber optic housing.
10. Turn the “Calibration Delay” knob on the apparatus to the 12 o’clock position.
11. Loosen the fiber optic cinch nuts on the fiber optic LED D3 and detector D8.
12. Select the 15-cm length of plastic fiber and insert one end of it into LED D3 until it is seated, then
lightly tighten the fiber optic cinch nut.
13. Insert the other end of the optical fiber into detector D8 until seated, then tighten its fiber optic
locking nut.
– 10 –
Calibration
It is important to calibrate this apparatus to ensure accurate results for the remainder of the activity. The
calibration will be done with 15 cm of optical fiber installed to simulate a distance of zero. Admittedly, 15
cm is not zero range, but the time delay in 15 cm of fiber is less than one nanosecond — not enough to
affect the accuracy of this apparatus and test equipment. If at any time your results differ from those we
describe, you should go back and double-check your measurements.
1. A pulse should now be observed as Channel
1 on the oscilloscope CRT screen. It should
be approximately 3.5 volts in amplitude and
35 nanoseconds in pulse width. The pulse
width is measured at 50% amplitude of the
pulse. See Figure 5. This is the calibration
pulse which will serve as a reference pulse for
subsequent measurements.
2. Turn the input selector of Channel 2 from
ground to AC coupling.
3. A second pulse from 1 to 1.5 volts in
amplitude and 75 nanoseconds wide should
also now be visible. This is the pulse received
through the 15 cm fiber optic cable.
4. Using the vertical positioning knobs, align the
bases of Channel 1 and Channel 2 traces with
the second grid line above the bottom of the
CRT screen.
5. Using the horizontal positioning knob, align
Channel 1 with the second grid line from the
left of the CRT screen.
6. Rotate the “Calibration Delay” knob on the
apparatus until the peak of the received pulse
coincides with the peak of the reference pulse
as shown in Figure 6.
7. Read just the oscilloscope’s sweeptime/
timebase scale to 20 nanoseconds per
division. (This scale may be available only by
using the 10X magnification knob on the oscilloscope. Consult the oscilloscope operator’s manual if
you are uncertain about how to locate the proper scale.)
8. Fine-tune the “Calibration Delay” Adjustment knob on the apparatus to achieve best overlap/
coincidence of the reference and received pulses.
9. Carefully loosen the fiber cinch nuts on LED D3 and Detector D8 and remove the 15 cm length
of fiber.
Figure 5. Photograph of the oscilloscope
screen with the “Reference” pulse
displayed as Channel 1.
Figure 6. Correct alignment of Channel 1 and 2
oscilloscope traces for proper Speed
of Light Apparatus calibration.
– 11 –
Measurement
Finally . . . we are going to make our speed of light measurement! Using the oscilloscope we will measure
the time required for the red LED light pulses to travel through 20 meters of plastic fiber.
1. Insert one end of the 20-meter plastic fiber gently but firmly, into LED D3 until the fiber is seated.
Lightly finger-tighten the fiber optic cinch nut on the LED.
2. Insert the other end of the fiber into
Detector D8 and lightly tighten the fiber
optic cinch nut.
3. Observe the display on the CRT screen
of the oscilloscope. The received pulse
should now have moved to the right of the
reference pulse, with a reduced amplitude of
approximately 50%. See Figure 7.
4. Very carefully measure the time difference
between the reference pulse and the
delay pulse, in nanoseconds. Make the
measurement from the second vertical
graticule used in your initial calibration to the
peak of the relocated received pulse (that is,
the received pulse generated when using the
20-meter optical fiber).
5. Write down the result of your measurements from Step 4. (It should be between 90 and 110
nanoseconds.)
Speed of Light Calculation
We now have just about all the information we need to calculate the speed of light. Because the light in our
demonstration is traveling through a plastic fiber, which is not a vacuum, we need to know the index of
refraction of the plastic fiber to compensate for the slower light speed.
As you will recall (or at least, we hope you recall, heh, heh) from the preceding Background Information
sections here, the index of refraction of a particular medium equals the speed of light in a vacuum divided
by the speed of light in the medium. (n = c/v).
“Ahhh,” you’re probably saying to yourself, “It’s all coming back to me now.” Sure.
Figure 7. Oscilloscope display showing
approximate positions of the
reference pulse and the received
pulse delayed through 20 meters of
ber.
– 12 –
Today we won’t attempt to measure the index of refraction of the plastic fiber. That’s a project for another
day. For now, we’ll rely on the manufacturer’s specifications, which say our fiber’s refractive index is 1.5.
Use the familiar equation:
where:
c = the speed of light in a vacuum
n = the index of refraction of the plastic fiber
l = the length of the plastic fiber, in meters
t = the travel time of light through the 20 – meter fiber, in nanoseconds
Now take the measurement number you wrote down and substitute it for ‘t’ in the equation. If the results
of your measurement resembled 100 nanoseconds, your numbers should look something like this:
In other words — the speed of light.
Just to make sure you understand why
you feel proud about yourself (rightfully
so) for having arrived at the right
numbers, run back through the preceding
Background Information sections.
If you begin with people’s initial
curiosity about the speed of light, more
than 500 years ago, you probably can
appreciate how far that curiosity — and
determination, and scientific dedication,
and sudden inspiration — have changed
our world. The demonstration you
completed so simply with your Speed
of Light Apparatus was founded upon
the endless questions and, eventually,
scientific explorations, of thousands of
adventurous people who dared to ask:
“Why?”
c
nl
t
=
•
– 13 –
KIT ASSEMBLY
If you obtained your Industrial Fiber Optics Speed of Light Apparatus in kit form, rather than already
assembled at the factory, these instructions will explain how to get everything together and ready for
operation.
Tools and equipment required for assembly
• 25-watt soldering iron
• Needle-nose pliers
• Single-edge razor blade or sharp knife
• Small standard blade screwdriver
• Small Phillips screwdriver
• Small adjustable wrench
• Solder*
• Water, glycerin or light oil
• Wire cutters
* Solid-core solder is recommended for assembly. If rosin-core solder is used you may wish to purchase flux
remover to make cleaning the board easier after you have finished soldering.
Parts Identication
All the components required for this apparatus are listed in Table 1. Compare the contents of the kit to the
list in the table before you start assembly, to ensure all components are present.
1. Identify all the resistors, using the color codes in Table 1.
2. This kit contains six axial leaded ceramic capacitors. The values of these capacitors may be denoted
by a three digit code, or by color bands similar to those on the resistors.
3. Two of the capacitors have values of .047 uF. They are identified by the code 473, or color bands of
yellow, violet, orange, white.
4. The other four capacitors have values of 150 pF. They are identified by the code 151, or color bands
of brown, green, brown, silver.
5. Identify all the resistors, using the color codes in Table 1.
6. Locate diodes D2, D4, D5, D6 and D7. They are the red glass components (with a black cathode
marking on one end) and wire leads protruding from both ends.
7. Identify the remaining components shown in Table 1.
– 14 –
Table 1. Speed of Light Apparatus Parts List.
P/N Value Description Color Code
Q1 2N2369 NPN switching transistor
Q2 2N5179 NPN wide bandwidth transistor
Q3 2N5179 NPN wide bandwidth transistor
Q4 2N5179 NPN wide bandwidth transistor
D1 Yellow LED
D2 1N914/1N4148 Switching diode
D3 IF-E96E Fiber optic red LED Blue housing,orange dot or P/N
D4 1N914/1N4148 Switching diode
D5 1N914/1N4148 Switching diode
D6 1N914/1N4148 Switching diode
D7 1N914/1N4148 Switching diode
D8 IF-D91 Fiber optic photo diode Black housing, orange dot
C1 1 µf 50 V Electrolytic capacitor
C2 10 µf 16 V Electrolytic capacitor
C3 150 pf Axial ceramic capacitor Brown green brown silver or 151
C4 150 pf Axial ceramic capacitor Brown green brown silver or 151
C5 .047 µf Axial ceramic capacitor Yellow violet orange silver 473
C6 150 pf Axial ceramic capacitor Brown green brown silver 151
C7 150 pf Axial ceramic capacitor Brown green brown silver 151
C8 .047 µf Axial ceramic capacitor Yellow violet orange white 473
R1 1 k 1/4 watt carbon lm resistor Brown black red
R2 2.2 k 1/4 watt carbon lm resistor Red red red
R3 270 ohms 1/4 watt carbon lm resistor Red violet brown
R4 2.2 k 1/4 watt carbon lm resistor Red red red
R5 47 ohms 1/4 watt carbon lm resistor Yellow violet black
R6 2.2 k 1/4 watt carbon lm resistor Red red red
R7 2.2 k 1/4 watt carbon lm resistor Red red red
R8 10 k Linear potentiometer Not applicable
R9 47 ohms 1/4 watt carbon lm resistor Yellow violet black
R10 220 ohms 1/4 watt carbon lm resistor Red red brown
R11 10 k 1/4 watt carbon lm resistor Brown black orange
R12 22 ohms 1/4 watt carbon lm resistor Red red black
R13 1 k 1/4 watt carbon lm resistor Brown black red
R14 4.7 k 1/4 watt carbon lm resistor Yellow violet red
R15 2.2 k 1/4 watt carbon lm resistor Red red red
R16 10 k 1/4 watt carbon lm resistor Brown black orange
U1 7805 Linear voltage regulator
U2 TLC555CP CMOS timer
U3 74LS221 TTL dual monostable
U4 DB101 Bridge rectier
H1 2.1 mm power jack
H2 Test point, white (2)
H3 Test point, blue (2)
H4 Printed wiring board
H5 Six inches 24 gauge wire
H6 Chassis
H7 110 VAC-to-DC power adapter
H8 Knob
H9 2-56 machine screw with nut (2)
H10 4-40 machine screw with nut
H11 #4 3/8” sheet metal screws (6)
H12 Rubber feet (4)
H13 Polishing paper
F1 20-meter ber
F2 15-centimeter ber
– 15 –
Figure 8. Printed wiring board legend showing the correct parts placement for assembly of the
Speed of Light Apparatus.
– 16 –
Figure 9. Circuit schematic of the Speed of Light Apparatus.
120
VAC 60 Hz
J1
1µF
C1 10µF
C2
1N914
D6
47
R9 220
R10
10k
R11
22
R12
1k
R13
R14
4.7k
2.2k
R15 10k
R16
2N5179
Q4
2N5179
Q2
1N914
D7
C7
150 PF
2N5179
Q3
C8
.047µF
2N2369
Q1
2.2k
R2
270
R3
150 PF
C3
150PF
C4
2.2k
R4 1N914
D2
.047µF
C5
C6
150 PF
2.2 k
R6
1
3
2
U1
7805
1k
R1
3
6
2
48
1
TLC555
U2
Q
15
2
B
A
13
16 11
1
74LS221
AU3
1N914
D5
1N914
D4
47
R5
2.2 k
R7
10 k
R8
Q
12
9
3
10 7
6
8
U3B
74LS221
U4
DB101
AC-DC
ADAPTER
12VDC @ 100 mA
REFERENCE
GND
DELAY
GND
D3
IF-E96E
D8
IF-D91
1000 m fiber
optic cable
D1
Yellow
1116.eps
– 17 –
Assembly Procedure
All components are to be mounted on the side of the blue printed wiring board with the white printing
except for R8. All soldering is to be completed on the opposite side. Avoid applying prolonged heat to any
part of the board or component to prevent damage. After soldering each component, trim its lead length
flush with the solder.
A copy of the printed wiring board’s legend is shown in Figure 8. Use it to properly place the components
on the board. A copy of the circuit schematic is also included as Figure 9.
1. Identify pin 1 of U2 and U3 (the lower left pin of the integrated circuit [IC], when viewed from
above). Insert the ICs into designated spots marked on the printed circuit board, with pin 1 to your
lower left. The lettering on the ICs will face the same direction as the markings on the board. Solder
in place.
2. Identify the plus and minus signs on U4 and match them to the same signs on the printed
circuit board.
3. Insert resistors R1 through R7, and R9 through R16, one at a time into the printed wiring board
and solder. Do not install the potentiometer R8 yet.
4. Orientation of diodes D2, and D4 through D7 is critical. Line up the “bar” markings on the diode
with the bar on the printed circuit board diode positional indicators. Insert and solder.
5. C1 and C2 are capacitors which are sensitive to the direction in which they are installed. Identify
the wire lead marked with minuses (- - -), then the round pad on the printed wiring board
corresponding to that capacitor. Match up the two, insert the leads properly into the board, and
solder each capacitor into place.
6. There is no positive/negative orientation of capacitors C3 through C8. Identify each, insert leads
through the board and solder in place.
7. Q1 is the only 3-lead metal “can” transistor in your kit. Align the metal tab protruding from the can
with the white ink tab on the printed wiring board and solder.
8. Q2, Q3, and Q4 are 4-lead metal can transistors. Align the metal tab protruding from the can with
the white ink tab on the printed wiring board, then solder all leads into place.
9. Locate the square pad within the area on the wiring board designated for placement of D1 (the
yellow LED). Insert the shortest leg of D1 (the cathode) into the square pad on the printed wiring
board, and solder into place.
10. U1 will be installed with the flat metal tab contacting the board. Use needle-nose pliers to grip
the three leads approximately 0.3 inch (7.5 mm) out from the square black base and bend them
downward at a 90° angle to penetrate the board. Insert the 4-40 screw through the metal tab and
the wiring board, and fasten it in place using the nut provided. Solder the three leads onto the
board.
11. Install the two white test points H2 at the locations on the wiring board marked as “GND”
connections, then solder.
– 18 –
12. Install the two blue test points that are marked as “Reference” and “Delay” and solder to the
wiring board.
13. Clean the printed circuit board with soap and warm water to remove solder residue. Soapy water
will not harm the components as long as electrical power is not being applied — in which case you
don’t want to get anywhere near water anyway, for safety’s sake. If you used a rosin core solder,
clean the board with flux remover before washing in soap and water. Rinse thoroughly. Shake the
board to remove water from under the ICs. Wipe everything dry with paper towels and let it air-dry
for 30 minutes.
14. Insert D3 and D8 in designated areas on the printed circuit board and secure in place with the 2-56
screws and nuts provided. Solder each lead.
15. Remove the nut and washer from the potentiometer R8. Insert the shaft of the potentiometer
through the bottom of the board, aligning the tab on the pot with the hole on the board. Use the
washer and nut to tighten R8 securely to the board.
16. Turn the board over to expose its bottom, with the fiber optic emitter D3 and detector D8 pointing
away from you. Solder a 5-cm (1.5”) 24-gauge jumper wire from the left pin of the pot to the solder
pad just underneath it. Solder another jumper of the same length between the pot’s middle and right
pins, then to the open solder pad that is connected to R7.
17. Place the power supply jack onto the printed wiring board and solder all three of its leads into place.
Apply heat and solder sparingly to prevent the solder from flowing down into the jack.
18. The printed wiring board is now assembled and ready for installation in the chassis provided. Using
the six #4 sheet metal screws provided, fasten the printed circuit board onto the raised supports of
the chassis. Do not over tighten the screws or you could crack the chassis.
19. Remove the adhesive backing from the four rubber feet provided, and attach one to each corner of
the chassis base.
Proceed to the “Fiber Preparation” section to finish the ends of the fiber cable.
– 19 –
FIBER PREPARATION
Cutting and polishing the ends of plastic fiber cable is easy. In addition to the items contained in your kit,
you need only a polishing liquid (water, glycerin, or light oil), a sharp knife or single-edge razor blade, and a
wire stripper. The 600-grit paper included in the kit for polishing fiber can be reused by rinsing it under tap
water to remove any polishing residue. To polish the two fiber optic cables, complete Steps 1 and 2 below,
for both ends of each fiber.
1. Cut 1 to 2 mm off the end of the
fiber cable with the razor blade or
sharp knife, trying to get as square
a cut as possible.
2. Wet the 600-grit polishing paper
with water or light oil and place
the paper, abrasive side facing up,
on any hard flat surface. Polish
the end of the fiber by moving
it in a “Figure 8” pattern against
the paper, while holding the fiber
perpendicular to the polishing
surface. Supporting the fiber
against a flat perpendicular object
during polishing will help support
the flexible fiber and result in a
better/flatter termination. After
20 Figure 8s, examine the end
of the fiber. (A microscope or
magnifying glass is helpful, but not
required.) If the end is cloudy, not
flat, or has scratches, repeat
Step 2.
Your Speed of Light Apparatus is now ready for operation. Turn to the ACTIVITY Section of this manual
for step-by-step instructions on how to operate the apparatus.
Figure 10. Figure-9 pattern used for polishing the
ber ends.
– 20 –
REFERENCES
Following is a list of books, magazines, and other publications which may be useful in the study of fiber
optics. If the title does not mention fiber optics, please still consider it a worthwhile source of information,
since fiber optic systems span multiple technologies.
Books
Introductory
Fiber Optics: A Bright New Way to Communicate, Billings, Dodd, Mead & Company, New York, NY 1986
The Rewiring of America: The Fiber Revolution, David Chaffee, Academic Press, Inc., Orlando, FL
32887 1988
Understanding Fiber Optics, Second Edition, Hecht, Howard W. Sams, 201 West 103rd Street,
Indianapolis, IN 46290 1989
Fiber Optics Communications, Experiments & Projects, Boyd, Howard W. Sams, 4300 West 62nd Street,
Indianapolis, IN 1982
Technician’s Guide to Fiber Optics, Sterling, AMP Incorporated, Harrisburg, PA 17105, 1987 (Paperback),
Delmar Publishers, 2 Computer Drive West, Box 15-015, Albany, New York 12212-9985
(Hardbound version)
Fiber Optics Handbook, Second Edition, Hentschel, Hewlett Packard, 1988
Advanced
Fiber Optic Communications, Fourth Edition, Palais, Prentice-Hall Publishing, 1998
Optical Fiber Transmission, Basch, Howard W. Sams, 4300 West 62nd Street, Indianapolis, IN 1986
College Level
An Introduction to Optical Fibers, Cherin, McGraw-Hill Book Company, 1983
Fiber Optic Handbook for Engineers and Scientists, Allard, McGraw-Hill Publishing, New York, NY 1990
Fiber Optics, James C. Daly, CRC Press, Inc., 2000 Corporate Blvd, N.W., Boca Raton, FL 1986
Fiber Optics in Communication Systems, Elion and Elion, Marcel Dekker, Inc. 1978
Pulse Code Formats for Fiber Optic Communications, Morris, Marcel Dekker, Inc. 1983
Optical Fibre Sensing and Signal Processing, Culshaw, Peter Peregrinus LTD., 1984
Principles of Optical Fiber Measurements, Marcuse, Academic Press, 1974
Semiconductor Devices for Optical Communications, Kressel, Springer-Verlag, Inc., 1980
– 21 –
Safety
Safety with Lasers and Other Optical Sources, Stiney and Wolbarsht, Plenum Press, 1980
Safe Use of Lasers, ANSI Standard Z136.1, LIA, 12424 Research Parkway, Suite 130, Orlando, FL 32826
Safe Use of Optical Fiber Communications Systems Utilizing Laser Diodes & LED Sources, ANSI Standard
Z136.2, LIA, 12424 Research Parkway, Suite 130, Orlando, FL 32826
A User’s Manual for Optical Waveguide Communications, Gallawa, U.S. Department of Commerce
Monthly Publications
The first two items are journals which are available to members of the respective professional societies.
The last four are trade magazines available free of charge to qualified readers.
Applied Optics, Optical Society of America, 1816 Jefferson Place, NW, Washington, DC 20036
Optical Engineering, SPIE, P. O. Box 10, Bellingham, WA 98227
Fiberoptic Product News, Gordon Publications, Inc., Box 1952, Dover, NJ 07801
Laser Focus World, PenWell Publishing Co., 1421 S. Sheridan, Tulsa, OK 74112
Lightwave Magazine, PenWell Publishing Co., 1421 S. Sheridan, Tulsa, OK 74112
Photonics Spectra, The Optical Publishing Co., Berkshire Common, Pittsfield, MA 01202-4949
Fiber Optic Buyer’s Guides
Fiberoptic Product News Buying Guide, Gordon Publications, Inc., Box 1952, Dover, NJ 07801
Lightwave Buyer’s Guide, PenWell Publishing Co., 1421 S. Sheridan, Tulsa, OK 74112
Organizations
Optical Society of America, 1816 Jefferson Place, NW., Washington, DC 20036
Society of Photo-Optical Instrumentation Engineers (SPIE), P. O. Box 10, Bellingham, WA 98227
Laser Institute of America, 12424 Research Parkway, Suite 130, Orlando, FL 32826
– 22 –
GLOSSARY
Absorption: In an optical fiber, the loss of optical power resulting from conversion of that power into
heat. See also: Scattering.
Acceptance Angle: The angle within which an optical fiber will accept light for transmission along its
core. This angle is measured from the centerline of the core.
Angle of Incidence: The angle formed between a ray of light striking a surface and a line drawn
perpendicular to that surface at the point of incidence (the point at which the light ray strikes the surface).
Angstrom: ( Å ) A unit of length often used to characterize light. An Angstrom is equal to 0.1 nm or
10-10 meters. The word is often spelled out as Angstrom(s) because the special symbol is not available on
typewriters and older printers.
Attenuation: Reduction of signal magnitude, or loss, normally measured in decibels. Fiber attenuation
normally is measured per unit length in decibels per kilometer.
Cable: A single fiber or a bundle, sometimes including strengthening strands of opaque material covered
by a protective jacket.
Cladding: The layer of glass or other transparent material surrounding the light-carrying core of an optical
fiber. It has a lower refractive index than the core. Coatings may be applied over the cladding.
Connector: A device mounted at the end of a fiber optic cable, light source, receiver or housing that
mates to a similar device to couple light optically into and out of optical fibers. A connector joins two fiber
ends or one fiber end and a light source or detector.
Core: The central part of an optical fiber that carries the light.
Coupler: A device which connects three or more fiber ends, dividing one input between two or more
outputs or combining two or more inputs into one output.
Critical Angle: The smallest angle of incidence at which light will undergo total internal reflection.
Detector: A device that generates an electrical signal when illuminated by light or infrared radiation. The
most common in fiber optics are photodiodes, photodarlingtons and phototransistors.
Fall time: Typically specified as the time required for a signal to fall from 90 percent to 10 percent of its
original negative or positive amplitude. See also: Rise time.
Fiber: The optical wave guide, or light-carrying core or conductor. Generally refers to the combination of
optical core and cladding. See also: Core; Cladding.
Fiber buffer: A buffering material that is used to protect an optical fiber’s core and cladding from
physical damage.
Fiber optics: A branch of optical technology that deals with the transmission of radiant energy through
optical waveguides generally made of glass or plastic.
– 23 –
Infrared: Wavelengths of light longer than 750 nm and shorter than 1 mm. Infrared radiation cannot be
seen, but can be felt as heat. Transmission of light through glass fibers is best in the region of 800 -1600 nm
and through plastic fiber in the region of 640 - 900 nm.
Jacket: A layer of material surrounding an optical fiber but not bonded to it.
LED: Light-emitting diode.
Light: Strictly speaking, electromagnetic radiation visible to the human eye. Commonly, the term is applied
to electromagnetic radiation with properties similar to those of visible light, including the invisible near-
infrared radiation used in fiber optic systems. See also: Infrared.
Light Emitting Diode (LED): A P-N junction semiconductor device that emits incoherent optical
radiation when biased in the forward direction.
Numerical Aperture: (NA) The numerical aperture of an optical fiber defines the characteristic of a
fiber in terms of its acceptance of impinging light. The larger the numerical aperture the greater the ability of
a fiber to accept light. See also: Acceptance angle; Critical angle.
Optoelectronics: The field of electronics that deals with LEDs, lasers, and photodetectors, or any other
electronic devices that produce, respond to, or utilize optical radiation.
Photodetector: A light detector.
Photon: A unit of electromagnetic radiation. Light can be viewed as either a wave or a series of photons.
Phototransistor: A transistor that detects light and amplifies the resulting electrical signal. Light falling
on the base-emitter junction generates a current, which is amplified internally. Simple, but slow.
Refractive index: The ratio of the speed of light in a vacuum to the speed of light in a material.
Abbreviated “n.”
Receiver: A device that detects an optical signal and converts it into an electrical form usable by other
devices. See also: Transmitter.
Responsivity: The ratio of detector output to input, usually specified in amperes/watt (or microamperes
per microwatt) for photodiodes, photodarlingtons and phototransistors.
Rise time: The time required for an output to rise from a low level to peak value. Typically specified as
the time to rise from 10 percent to 90 percent of its final steady state value. See also: Fall time.
Scattering: The changes in direction of light confined within an optical fiber, occurring due to
imperfections in the core and cladding. Scattering causes no changes in the wavelengths of radiation. See
also: Absorption.
Splice: A permanent junction between two optical fiber ends.
Step-index fiber: An optical fiber in which the refractive index changes abruptly at the boundary
between core and cladding.
Transmitter: A device that converts an electrical signal into an optical signal for transmission through a
fiber cable. See also: Receiver.
