Download SET 3742 Light and Optical Fiber Interaction

Fakulti:
FAKULTI KEJURUTERAAN ELEKTRIK
Subject
: MAKMAL KEJ. ELEKTRIK
Subject Code : SET 3742
Review
Release Date
Last Amendment
Procedure Number
:1
: 2010
: 2010
: PK-UTM-FKE-(0)-10
SET 3742
FAKULTI KEJURUTERAAN ELEKTRIK
UNIVERSITI TEKNOLOGI MALAYSIA
SKUDAI, JOHOR
JABATAN KEJURUTERAAN PERHUBUNGAN (COMM)
OPTICAL COMMUNICATION LABORATORY
Experiment 2
Light and Optical Fiber Interaction
LIGHT AND OPTICAL FIBER INTERACTION
OBJECTIVE
1. To study the structure of optical fiber.
2. To study the principle of light traveling on optical fiber.
3. To study the effect of fiber losses on fiber optic communications.
EQUIPMENT REQUIRED
1.
Module KL-95001
2.
1 – meter optical fiber
3.
AC to DC Power Adapter
PREREQUISITES QUESTION
1.
What is meant by:
(a)
Step index fiber?
(b)
Graded index fiber?
2.
Give a simple explanation of mode.
3.
What is modal dispersion and how does it affect transmission in a fiber optic link?
4.
What is chromatic dispersion and how does it affect transmission in a fiber optic link?
5.
What are the most important criteria in the manufacture of an optical fiber cable?
PROCEDURE
Many industrial and home applications use light or optical sensors for various purposes. For
examples, security systems and lights that automatically go on when a room is dark. In this
experiment you will see how fiber optics can be used in various optical sensors.
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In Procedures A and B of this experiment we will use the light-gathering ability of optical fiber
to construct a simple, remote passive optical detection sensor. Light which is collected by the
fiber will be converted into an electrical signal by photodetector and then amplified. This
amplified signal will be converted to an audible signal by the speaker on the Lab Module. We
will output the amplified signals picked up by the passive sensor to the speaker for you to learn
some things about light that our eyes do not see.
Procedure C of this experiment involves converting the passive sensor to an “active” sensor.
The word “active” means that the sensor does not relay on ambient light or an external light
source to function. The sensor will produce its own light for use in the sensing process and
process it according to the application.
Procedure A.
1.
Using Module KL-95001. Hole the free end of the optical fiber about 10 cm away from any
operating fluorescent light. You probably will need to move the Module or need an
extension cord to complete this step. The noise you hear coming from the Speaker is the
sensor detecting the flurescent light turning on and off at 120 times a second.
2.
Answer the following questions,
(a) What happens to the sounds from the Speaker as you move the optical fiber closer to the
fluorescent light? Why?.
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(b) Were you surprised by the sounds from the Speaker when the optical fiber was pointed
at the fluorescent light? Why?
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(c) Describe the sounds from the Speaker when the optical fiber about 10 cm away from the
fluorescent light.
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3.
Obtain a television or computer screen and turn it on. Hold the optical fiber end as close as
you can get to the television screen. Move the fiber to different points on the screen while
listening for differences in the Speaker’s volume.
4.
Describe the amplitude and frequency of the sounds from the Speaker as you move the fiber
end to various points on the television screen. Compare the differences between the sounds
you hear on the television and computer screen.
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Procedure B
Many LED and LCDs found in household and consumer electronics are turned on and off at a high rate,
although they do not appear that way to our eyes. In the following steps you will see and hear, that our
eyes and mind can perceive only low frequencies of light intensity and high frequency light pulses are
“averaged” by our senses.
1.
Connect the Signal Generator Digital output to the input of an oscilloscope or frequency
counter.
2.
While observing the LED2, press and hold the Momentary Switch down and turn the Signal
Generator Frequency knob slowly CW. Observe how quickly your eyes do not discern the
LED2 turning on and off any more as you turn the knob.
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3.
Measure and record the frequency the LED to be blinking when you can just barely see it
turning on and off.
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4.
Could you hear the output of the Signal Generator through the speaker even though you
could not see the LED2 blinking when appropriately connected? What does that tell you
about the frequency response capability of our ears, compared to our eyes?
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Procedure C
In this portion of the activity you will re-configure the Lab Module to an active optical sensor. The
Transmitter TX1 and connected fiber of your Lab Module will provide the light source for the
sensor. Another fiber will collect the light originating from the TX1 and then couple this light down
the fiber to the photodetector located in the Receiver circuitry of the Lab Module. This sensor will
not need to depend upon ambient or external light to function. The “performance indictor” for the
system will be the speaker. If the speaker is relatively quiet, or if only a small amount of sound is
coming from it that means no light is entering the Receiver’s optic fiber. You will always hear a
very small amount of sound coming from the speaker. This is an intentional design feature, to
indicate that the demonstration unit is operating. In the following demonstrations, the two fibers
will be positioned to detect the presence of objects near the fiber ends.
1.
Connect the Signal Generator Digital output to the Transmitter input. Turn the Signal
Generator Frequency knob to about the 3 o’clock position to have 500Hz signal on Digital
output. Turn the Receiver Gain knob to the Max position.
2.
Tape the free end of each optical fiber to the top of two books of about the same thickness.
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3.
What happens to the sound from the speaker when the paper is placed between the TX1 and
RX1 fibers?
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4.
Does the existing light in the room – no matter whether the room lights are on or off – affect
the sensing of the paper moving between the two fibers?
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5.
Pass a piece of plastic or cellophane and a piece of black paper between the tips of two
fibers and listen for changes in the sound level from the Speaker.
6.
Answer the following questions:
a. Can you detect the presence of the plastic (from varying sounds from the Speakers) as
you did with the paper?
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b. Can you detect the presence of the black paper (from varying sounds from the Speaker)
as you did with the white paper?
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7.
Rearrange the optical fibers side by side on a single book, angled slightly toward each other.
8.
Hold a piece of white paper in front of the optical fiber tips about 25 mm away. Move the
paper closer and closer to the fiber tips until you locate the position at which the sound from
the Speaker is at which the sound from the Speaker is its maximum. If you do not hear any
sound for any position of the paper, position the fiber tips closer and at slightly different
angles to each other.
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9.
Move the paper closer to the fiber tips until the paper touches both ends.
10.
Graph the intensity (loudness) of the sounds from the Speaker as the distance of the white
paper from the fiber tips varies.
11.
Now pass the piece of plastic and the piece of black paper in the front of the two fibers to
determine if you can detect its presence.
12.
Answer the following questions:
a. Could this sensor be used in a manufacturing company? Perhaps for counting objects
which pass a point on an assembly line. How?
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b. How is this sensor configuration in 2 different than the one in 8? Is it the opposite – that
is, does the volume increase when objects are present?
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