Optical Fiber Communication
Scientech 2502
Product Tutorials
Ver. 1.1
Designed & Manufactured byAn ISO 9001:2008 company
Scientech Technologies Pvt. Ltd.
94, Electronic Complex, Pardesipura, Indore - 452 010 India,
+ 91-731 4211100, : info@scientech.bz , : www.ScientechWorld.com
Optical Fiber Communication Scientech 2502
Optical Fiber Communication
Scientech 2502
Table of Contents
Safety Instructions
4
Introduction
5
Features
6
Technical Specifications
7
Introduction to Fiber Optic
9
Theory of Fiber Optic
8
Advantages of Fiber Optic System
16
Characteristics of Optical Fiber
18
Experiments
•
•
•
•
•
•
•
•
•
•
•
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Experiment 1
Setting up Fiber Optic analog link
44
Experiment 2
Setting up Fiber Optic digital link
46
Experiment 3
Intensity Modulation System using analog input signal
48
Experiment 4
Intensity Modulation System using digital input signal
50
Experiment 5
Frequency Modulation System
53
Experiment 6
Pulse Width Modulation System
55
Experiment 7
Study of Propagation loss in Optical Fiber
57
Experiment 8
Study of Bending Loss
59
Experiment 9
Measurement of Optical Power using Optical power meter
61
Experiment 10
Measurement of Propagation Loss using OPM
63
Experiment 11
Measurement of Numerical Aperture
65
Experiment 12
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Optical Fiber Communication Scientech 2502
•
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•
•
•
•
•
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•
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•
•
•
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Characteristics of E-O converter using OPM
67
Experiment 13
V-I Characteristics of Photo LED (E - O converter)
69
Experiment 13A
V-I Characteristics of Photo Detector (E - O converter)
71
Experiment 14
Characteristics of Fiber Optic Communication Link
73
Experiment 15
Setting up of FO voice link using Intensity Modulation
75
Experiment 16
Setting up of FO voice link using FM
77
Experiment 17
Setting up of FO voice link using PWM
79
Experiment 18
Study of switched fault in Intensity Modulation System
82
Experiment 19
Study of switched fault in FM
84
Experiment 20
Study of switched fault in PWM
86
Experiment 21
Computer Communication using RS232 interface
78
Experiment 22
BIT rate measurement
90
Experiment 23
Sensitivity
91
Experiment 24
Determination of Power Margin (Power Budget)
92
Experiment 25
To Measure Bit Error Rate
93
•
Experiment 26
To Study and Observation of Eye Pattern
Frequently Asked Questions
96
101
Glossary
109
Warranty
112
List of Accessories
112
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Optical Fiber Communication Scientech 2502
Safety Instructions
Read the following safety instructions carefully before operating the product.
To avoid any personal injury, or damage to the product, or any products connected to
it;
Do not operate the instrument if you suspect any damage within.
The instrument should be serviced by qualified personnel only.
For your Safety:
Use proper Mains cord
: Use only the mains cord designed for this product.
Ensure that the mains cord is suitable for your
country.
Ground the Instrument
: This product is grounded through the protective earth
conductor of the mains cord. To avoid electric shock
the grounding conductor must be connected to the
earth ground. Before making connections to the input
terminals, ensure that the instrument is properly
grounded.
Observe Terminal Ratings : To avoid fire or shock hazards, observe all ratings and
marks on the instrument.
Use only the proper Fuse
: Use the fuse type and rating specified for this product.
Use in proper Atmosphere : Please refer to operating conditions given in the
manual.
•
Do not operate in wet / damp conditions.
•
Do not operate in an explosive atmosphere.
•
Keep the product dust free, clean and dry.
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Optical Fiber Communication Scientech 2502
Introduction
Scientech TechBooks are compact and user friendly learning platforms to provide a
modern, portable, comprehensive and practical way to learn Technology. Each
TechBook is provided with detailed Multimedia learning material which covers basic
theory, step by step procedure to conduct the experiment and other useful information.
Scientech 2502 Fiber Optic Communication TechBook demonstrate full Duplex
method of transmitting information from one place to another by sending pulses of
light through an Optical fiber the light forms electromagnetic wave that is modulated
to carry information. Fiber Optics Communication Scientech 2502 is Advanced Fiber
Optic TechBook is basically designed to learn the communication techniques in Fiber
Optics.
The TechBook demonstrates properties of Fiber Optics Transmitter & Receiver,
characteristics of Fiber Optics Cable, different types of Modulation / Demodulation
techniques and PC to PC communication via fiber link using RS232 interface. It can
also be used to demonstrate various Digital Communication Techniques via Fiber
Optic link using Scientech Digital Communication TechBooks.
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Optical Fiber Communication Scientech 2502
Features
•
Full Duplex Analog & Digital Trans-receiver
•
Single Module covering large number of experiments including
experiments with Optical Power Meter
•
660 nm & 950 nm (Optional Laser Source) channel with Transmitter &
Receiver
•
AM-FM-PWM modulation / demodulation
•
PC-PC comm. with RS232 ports & software
•
On board Function Generator
•
Crystal Controlled Clock
•
Functional Blocks indicated on-board mimic
•
Input-output & test points provided on board
•
On board voice link
•
Built in DC Power Supply
•
Numerical Aperture measurement jig and mandrel for bending loss
measurement
•
Switched faults on Transmitter & Receiver
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Optical Fiber Communication Scientech 2502
Technical Specifications
Transmitter
:
2 in number, fiber optic LED having peak
wave length of emission 660 nm & 950 nm
(Optical Laser Source)
Receiver
:
2 in number, fiber optic photo (-) detector
Modulation Techniques
:
1. direct amplitude modulation & demodulation
2. Frequency modulation / demodulation
3. Pulse width modulation / demodulation
Drivers
:
Analog &
separately
digital
for
both
channels
Comparator
:
2 Nos.
AC Amplifiers
:
2 Nos.
Clock
:
Crystal controlled - 4.096 MHz
PLL Detector
:
1 No
Filters
:
2 Nos, 4th order Butter worth Filter (3.4
KHz cut- off frequency)
Analog Band Width
:
350 KHz
Digital Band Width
:
2.5 MHz
Function Generator
:
1. 1 KHz Sine wave (Amplitude adjustable)
2. 1 KHz Square wave (TTL)
Voice Link
:
Fiber Optics voice link using microphone &
speaker (built in)
PC-PC Communication
:
Using two
interface.
RS 232 Port
:
9 Pin D- type connector
Baud Rate
:
19200 baud
Switched Faults
:
Four in transmitter & four in receiver
Fiber Optic Cable
:
Connector type standard SMA (sub
miniature assembly) duly polished fiber at
both end for maximum transmission &
perfect round spot for numerical aperture
measurement
Cable type
:
Step indexed multimode PMMA plastic
cable
Core Refractive Index
:
1.492
Clad Refractive Index
:
1.406
channels
through
RS232
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Optical Fiber Communication Scientech 2502
Numerical Aperture
:
Better than 0.5
Acceptance angle
:
Better than 60 degrees
Fiber Diameter
:
1000 microns
Outer Diameter
:
2.2 mm
Fiber Length
:
0.5 m & 1 m
Inter connections
:
2 mm sockets
Test Points
:
50 nos.
Power Supply
:
110-220V, ± 10%, 50 / 60 Hz
Power Consumption
:
4.5 VA (approximately)
Interconnections
:
4 mm Banana sockets
Test points
:
50 Nos.
Dimensions (mm)
:
W 326 x H 52 x D 525
Weight
:
2.5 Kg. (approximately)
Product Tutorials
:
Online (Theory, procedure, reference results,
etc).
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Optical Fiber Communication Scientech 2502
Theory
Introduction to Optical Fiber:
Communication can be broadly defined as the transfer of information from one point
to another. Before Fiber optics came along, the primary means of real time data
communication was electrical in nature. It was accomplished by using copper wire or
by modulating information on to an electromagnetic wave which acts as a carrier for
the information signal. All these methods have one problem in common the
communication had to be over a straight line path. Fiber optics provides an alternative
means of sending information over significant distances using light energy. Light is
utilized for communication has major advantages because it can be modulated at
significant higher frequencies than electrical signals. That is till 1870, when an Irish
physicist John Tyndall carried out a simple experiment. He filled a container with
water and shone light into it. In the darkened room he pulled the bung from the
opposite end of the container. The light shone out, of course but in which direction?
The light followed the curved path of water. The light was guided and a new science
was born. This was due to a property of light called refraction.
Understanding how Fiber optics are made and function for uses in everyday life is an
intriguing work of art combined with science. Fiber optics has been fabricated from
materials that transmit light and are made from a bundle of very thin glass or plastic
Fibers enclosed in a tube. One end is at a source of light and the other end is a camera
lens, used to channel light and images around the bends and corners. Fiber optics has
a highly transparent core of glass, or plastic encircled by a covering called "cladding".
Light is stimulated through a source on one end of the Fiber optic and as the light
travels through the tube, the cladding is there to keep it all inside. A bundle of Fiber
optics may be bent or twisted without distorting the image, as the cladding is designed
to reflect these lighting images from inside the surface. This Fiber optic light source
can carry light over mass distances, ranging from a few inches to over 100 miles.
There are two kinds of Fiber optics. The single-mode Fiber optic is used for high
speed and long distance transmissions because they have extremely tiny cores and
they accept light only along the axis of the Fibers. Tiny lasers send light directly into
the Fiber optic where there are low-loss connectors used to join the Fibers within the
system without substantially degrading the light signal. Then there are multi-mode
which have much larger cores and accept light from a variety of angles and can use
more types of light sources. Multi-mode Fiber optics also uses less expensive
connectors, but they cannot be used over long distances as with the single-mode Fiber
optics.
Fiber optics has a large variety of uses. Most common and widely used in
communication systems, Fiber optic communication systems have a variety of
features that make it superior to the systems that use the traditional copper cables. The
uses of fiber optics with these systems use a larger information-carrying capacity
where they are not hassled with electrical interference and require fewer amplifiers
then the copper cable systems. Fiber optic communication systems are installed in
large networks of Fiber optic bundles all around the world and even under the oceans.
Many Fiber optic testers are available to provide you with the best Fiber optic
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Optical Fiber Communication Scientech 2502
equipment. In Fiber optic communication systems, lasers are used to transmit
messages in numeric code by flashing on and off at high speeds. This code can
constitute a voice or an electronic file containing, text, numbers, or illustrations, all by
using Fiber optics. The light from many lasers are added together onto a single Fiber
optic enabling thousands of currents of data to pass through a single Fiber optic cable
at one time. This data will travel through the Fiber optics and into interpreting devices
to convert the messages back into the form of its original signals. Industries also use
Fiber optics to measure temperatures, pressure, acceleration and voltage, among an
assortment of other uses.
Optical Fiber Communication System
Optical Fiber Communication System
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Optical Fiber Communication Scientech 2502
Here, the information source provides an amplified electrical signal to a transmitter
comprising an electrical stage, which drives an optical source to give conversion, may
be either a semiconductor, LASER (Light Amplification by Stimulated Emission of
Radiation) or LED. The transmission medium consists of optical source, which
provides an electrical to optical conversion, an optical Fiber cable used for
transmission of signal and the receiver, consists of an optical detector, which drives a
further electrical stage and hence provides demodulation of optical carrier. This
electrical signal is amplified and applied to the destination example speaker.
Photo diodes (P-I-N or Avalanche) and in some instances photo transistors and photo
conductors are utilized for detection of optical signal and optical to electrical
conversion. The optical carrier may be modulated using an analog or digital
information signal. Analog modulation involves the variation of light emitted from the
optical source in continuous manner. In digital modulation however, discrete changes
in the light intensity are obtained (i.e. ‘On-Off’ pulses) although often simpler to
implement, analog modulation with an optical Fiber communication system is less
efficient, requiring a far higher signal to noise ratio (SNR) at the receiver than digital
modulation. Also, linearity needed for analog modulation is not provided by
semiconductor optical sources especially at high modulation frequencies.
Principle of operation of Optical Fiber:
The principle of operation of optical Fiber lies in the behavior of light. It is a widely
held view that light always travels in straight line and at constant speed. Of course,
the light propagates in straight lines, but when it is reflected inside the optical Fiber
million and trillion times by the clad, each movement comprising of a straight line
and consequently because of such reflections, it acquires the shape of the optical
Fiber. So effectively, it is said to have been traveling along the Fiber. It changes its
direction only if there is a change in the dielectric medium as also illustrated by the
Tyndall’s experiment. To understand the propagation of light within an optical Fiber
it is necessary to take into account refractive index of the dielectric medium.
Refractive index of a medium is defined as the ratio of velocity of light in vacuum to
velocity of light in medium.
Refractive index =
Velocity of light in vaccum
Velocity of light in medium
Since, the velocity of light in any solid, transparent material is less than in vacuum the
refractive index of such material is always greater than 1.0. A ray of light travels
slowly in an optically dense medium than one that is less dense. Now, the direction
that the light approaches the boundary between the two materials is very important.
When a ray is incident on the interface between two dielectrics of differing refractive
indices, refraction occurs. The light is refracted and also partly reflected internally in
the same medium; which is referred as Partial Internal Reflection. It may be
observed that the ray approaching the interface is propagating in a dielectric of
refractive index n1 and is at an angle φ 1 to the normal at the surface of the interface. If
the dielectric on the other side of interface has a refractive index n2 which is less
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Optical Fiber Communication Scientech 2502
than n1 , then the refraction is such that the ray path in this lower index medium is at
angle φ 2 to the normal where φ 2 is greater than φ 1 .
The angle of incidence φ 1 and refraction φ 2 are related to each other and to refractive
indices of dielectrics by Snell's law of refraction which states that:
n1 sinφ 1 = n2 sinφ 2
sin φ1 n 2
=
sin φ 2 n1
It is this change in refractive indices which causes the change in the path of the
incident ray as evident from the Snell’s law. Larger the change in the refractive
indices larger change in the direction of the incident ray. The sine of the angles will
be in the ratio of their refractive indices. As the angle of incident ray increases, the
angle of refraction also increases even faster and when the angle of refraction
becomes 90° thereafter, if the angle of incidence is increased a condition is arrived
where the incident ray is totally reflected in the same medium from where it has
emerged; this is referred as the total internal reflection.
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Optical Fiber Communication Scientech 2502
Total Internal Reflection:
Since, the angle of refraction is always greater than the angle of incidence, when the
incident medium is denser than the refraction medium. Thus, the angle of refraction is
90° and the refracted ray emerges parallel to the interface between the dielectrics.
This is the limiting case of refraction and this angle of incidence is known as critical
angle φ c .
The value of critical angle is given by:
φ 2 = 90°
φ1 = φ c
Substituting this in the equation for Snell’s law gives
n1 sinφ c = n2 sin 90°
n2
∴ sinφ c =
n1
At angles of incidence greater than the critical angle the light is reflected back into the
originating dielectric medium. This behavior of light is termed as Total Internal
Reflection.
Here,
Angle of Incidence = Angle of Reflection
This is the mechanism by which light may be considered to propagate down an optical
fiber with low loss. shown in next figure below illustrates the transmission of a light
ray in an optical fiber via a series of total internal reflection at the interface of the
silica core and slightly lower refractive index silica cladding.
The light ray shown in next figure is known as meridian ray as it passes through the
axis of the fiber core. It is generally used when illustrating the fundamental
transmission properties of optical fiber.
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Optical Fiber Communication Scientech 2502
Acceptance Angle:
Since, only rays with an angle greater than critical angle at the core cladding interface
are transmitted by total internal reflection, it is clear that not all rays entering the fiber
core will continue to propagate down the length.
Shown in next figure illustrates two incident rays I and B. It may be observed that ray
‘I’ enters the fiber core at an angle θ i less than θ a (conical half angle for the fiber
explained herein under) to the fiber axis and is refracted at the air- core interface
before transmission to the core- cladding interface at an angle φ more than the critical
angle φ c . This ray is totally internally reflected and propagated along the fiber. While
incident ray ‘B’ is incident into the fiber core at an angle θ b greater than θ a and will
be transmitted to the core- cladding interface at an angle less than φ c and will not be
totally internally reflected instead will be refracted into cladding and eventually lost
by the radiation. Thus, for rays to be transmitted by total internal reflection within the
fiber core they must be incident on the fiber core within an acceptance cone defined
by conical half angle θ a . Hence, θ a is the maximum angle to the axis at which light
may enter the fiber in order to be propagated and is referred to as the acceptance
angle for the fiber?
Here θ i < θ a < θ b
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Optical Fiber Communication Scientech 2502
Numerical Aperture:
It gives the relationship between the acceptance angle and the refractive indices of the
three media involved viz. the core, the cladding and air.
In the shown in next figure above φ i corresponds to θ i ; when φ i approaches φ c ; θ i
approaches θ a
By Snell's law of refraction:n0 sin θ a = n1 sin(90° − φ c )
= n1 cos φ c
{where φ c is the critical angle}
= n1(1 − sin c 2φ )1/ 2
= n1 (1 −
n 2 2 1/ 2
)
n 21
{as sin φ c =
n2
}
n1
= n12 − n2 2
This term is referred as numerical aperture of the Wave Guide – Optical Fiber.
Numerical Aperture = n0 sin θ a = (n12 − n22 )1/ 2 = (n12 − n22 )
= (n1 − n2 )(n1 + n2 )
Where,
n0 = Refractive index of air
n1 = Refractive index of core
n2 = Refractive index of cladding
The Numerical Aperture is a very useful measure of light collecting ability of a fiber.
It directly relates to the refractive indices of the core and cladding. As we observe
from the above equation, greater the absolute value of the indices of core and
cladding, greater the numerical aperture; similarly, greater the difference between the
refractive indices greater the numerical aperture. In accordance to the requirement of
the numerical aperture, the material for the core and cladding is chosen, keeping in
view the other parameters and requirements for transmission.
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Optical Fiber Communication Scientech 2502
Advantages of FIBER Optic System
●
Enormous Potential Band Width (BW):
The information carrying capacity of a transmission system is directly
proportional to the carrier frequency of the transmitted signals. The optical
carrier frequency in the range 1013 to 1016 Hz. (generally near infrared around
1014 or 1015 Hz) yields a far greater potential transmission B.W. than metallic
cable system. (i.e. coaxial cable Bandwidth up to 500 MHz) or even milli meter
wave radio system, (i.e. system currently operating with modulation Bandwidth
of 700 MHz). Thus the optical Fibers have enormous transmission bandwidths
and high data rate. Using wavelength division multiplexing operation, the data
rate or information carrying capacity of optical fibers is enhanced to many
orders of magnitude. At present the Bandwidth available to fiber system is not
fully utilized by modulation at several GHz over hundred km. and hundreds of
MHz over 300 Km with intervening electronics (repeaters) is possible.
Therefore, the information carrying capacity of optical fiber system has proved
far superior to best copper cable available, by comparison losses in coaxial cable
systems restrict. A much-enhanced Bandwidth utilization for an optical fiber can
be achieved by transmitting several optical signals each at different centre
wavelengths in parallel on the same fiber. This wavelength division multiplexed
operation particularly with dense packing of the optical wavelength (or fine
frequency spacing) offers potential information carrying capacity.
●
Small size and weight :
Optical Fibers have very small diameter in the ranges from 10 micrometers to
50 micrometers. The space occupied by the fiber cable is negligibly small
compared to conventional electrical cables. Hence, when they are covered with
protective coatings they are far smaller & lighter. This is a tremendous boon
towards the alleviation of duct congestion in cities and allowing expansion of
signal transmission in mobiles e.g. aircrafts, ships etc.
●
Electrical Isolation :
Optical Fibers are fabricated from glass or plastic polymers, they are electrical
insulators therefore they do not exhibit earth loop, interference problems,
electromagnetic wave or any high current lightening. This property makes them
suitable for communication in electrically hazardous environment as fiber create
no arcing or spark hazard at abrasions or short circuit & usually fiber do not
contain sufficient energy to ignite vapors or gases. It is also suitable in explosive
environment.
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Optical Fiber Communication Scientech 2502
●
Immunity to Interference and Cross talk :
Optical fibers form a dielectric wave-guide and therefore are free from Electro
Magnetic Interference (E.M.I), Radio Frequency Interference (R.F.I) or
switching transients. It is not susceptible to lightening striker if used overhead
rather than underground. Moreover it is easy to ensure that there is no optical
interference between fibers. Since optical interference among different Fibers is
not possible, cross talk is negligible even many Fibers are cabled together.
●
Signal Security :
The light from optical fibers does not radiate significantly and therefore they
provide a high degree of signal security. Unlike in copper cables, a transmitted
signal cannot be drawn from a fiber without tampering it. Thus, the optical fiber
communication provides 100% signal security. A transmitted optical signal
cannot be obtained from a fiber in a non-invasive manner (i.e. without drawing
optical power form the fiber). In theory, any attempt to acquire a message signal
transmitted optically may be detected. This feature is obviously attractive for
military & banking.
●
Low transmission loss:
Due to the usage of ultra low loss Fibers and the erbium doped silica Fibers as
optical amplifiers ,Optical fibers results in low attenuation or transmission loss
in comparison with the best copper conductor. It facilitates the implementation
of communication links with extremely wide repeater spacing thus reducing
both system cost and complexity. This quality along with already proven
modulation B W capability of fiber cable, it is used in long haul
telecommunication applications. Hence for long distance communication Fibers
of 0.002 dB/km are used. Thus the repeater spacing is more than 100 km.
●
Potential Low Cost :
The glass that generally provides optical fiber transmission medium is made
from sand not a scarce resource. In comparison with copper conductors, optical
fiber offers low cost line communication. This is because many miles of optical
cable are easier and less expensive to install than the same amount of copper
wire or cable.
●
Thinner:
Fiber optics is thinner than copper wire cables, so they will fit in smaller, more
crowded places. This is important for underground cable systems, like in cities,
where space needs to be shared with sewer pipes, power wires, and subway
systems.
●
Non-flammable
Since fiber optics send light instead of electricity, fiber optics are nonflammable. This means there is not a fire hazard. Fiber optics also does not
cause electric shocks, because they do not carry electricity.
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Optical Fiber Communication Scientech 2502
●
Ruggedness and flexibility
The fiber cable can be easily bend or twisted without damaging it. Further the
fiber cables are superior than the copper cables in terms of handling, installation,
storage, transportation, maintenance, strength and durability.
●
Low cost and availability
Since the Fibers are made of silica which is available in abundance. Hence, there
is no shortage of material and optical fibers offer the potential for low cost
communication.
●
Reliability
The optical Fibers are made from silicon glass which does not undergo any
chemical reaction or corrosion. Its quality is not affected by external radiation.
Further due to its negligible attenuation and dispersion, optical fiber
communication has high reliability. All the above factors also tend to reduce the
expenditure on its maintenance.
The disadvantages of optical Fibers are:
●
Price - Even though the raw material for making optical Fibers, sand, is
abundant and cheap, optical Fibers are still more expensive per metre than
copper. Although, one Fiber can carry many more signals than a single copper
cable and the large transmission distances mean that fewer expensive repeaters
are required.
●
Fragility - Optical Fibers are more fragile than electrical wires.
●
Affected by chemicals - The glass can be affected by various chemicals
including hydrogen gas (a problem in underwater cables.)
●
Opaqueness - Despite extensive military use it is known that most Fibers
become opaque when exposed to radiation.
●
Requires special skills - Optical Fibers cannot be joined together as a easily as
copper cable and requires additional training of personnel and expensive
precision splicing and measurement equipment.
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Optical Fiber Communication Scientech 2502
The Optic Fiber:
The simplest Fiber optic cable consists of two concentric layers of transparent
materials. The inner portion the core transports the light, the outer covering (the
cladding) must have a lower refractive index than the core so the two of them are
made up of different materials.
To provide mechanical protection for the cladding an additional plastic layer; called
Primary Buffer is added. Some constructions of optic Fiber have additional layers of
buffers that are then referred to as Secondary Buffers. It is very important to note that
the whole Fiber-Core, Cladding & Primary Buffer is solid and the light is confined to
the core by the Total Internal Reflection due to the difference in the refractive index
of the core as compared to that of cladding.
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Optical Fiber Communication Scientech 2502
Single Mode versus Multi Mode:
As we have already seen that there are particular angles of propagation defined by
cone of acceptance, which can be transmitted down the optic fiber. At these angles,
the electromagnetic wave that the light can set up a number of completes patterns
across the fiber. The number of complete patterns called Modes depends on the
dimensions of the optic fiber core. There are essentially two different types of fiber
optic transmission schemes in use viz.
●
Single Mode
●
Multi Mode
Single Mode:
As the name suggests the single mode cable is able to propagate only one mode
(Electromagnetic wave). This is used in long distance and/or, high-speed
communication. It is beneficial over long distances since it completely eliminates a
problem known as Intermodal Dispersion associated with Multimode cables. All our
long distance telephone conversations are now carried by single mode optic fiber
system over at least some part of the route.
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Optical Fiber Communication Scientech 2502
Multi Mode:
The term multimode means that the diameter of the fiber optic core is large enough to
propagate more than one mode (Electro Magnetic Wave).
Because of the multiple modes the pulse that is transmitted down the fiber tends to
become stretched over distance this is referred to as dispersion & has the effect of
reducing the available bandwidth. These are typically used in applications such as
LAN (Local Area Networks) & FDDI (Fiber Distributed Area Interface)
Optical Fiber Index Profile
Index profile is the refractive index distribution across the core and the cladding of a
fiber. Some optical fiber has a step index profile, in which the core has one uniformly
distributed index and the cladding has a lower uniformly distributed index. Other
optical fiber has a graded index profile, in which refractive index varies gradually as a
function of radial distance from the fiber center. Graded-index profiles include powerlaw index profiles and parabolic index profiles.
Step Index And Graded Index Fibers:
The first type of fiber optic cable put to use was called step index. In this design, the
cladding has a different index of refraction than the core. The light bounces off the
side and is reflected back into the fiber core. The problem with this design is that the
reflected light must travel a slightly longer distance, than that which travels down the
centre of the fiber, thus limiting the maximum transmission rate. This design was
improved with the use of Graded index fiber. In this design, the index of refraction
decreases in proportion to the distance away from the centre of the fiber core. The
light moves more quickly in the outer portion thus compensating for the additional
distance. The change in index has the effect of bending. The light reflects back
towards the core. This change increases the transmission capacity by a reasonable
factor. In the newest single mode design, the diameter of the fiber core is so small that
all the light travels in a straight line. Even the latest fiber optic facility in use today
uses less than 5% of the maximum theoretical capacity of a single mode fiber.
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Optical Fiber Communication Scientech 2502
Some of the optical fibers in use are:●
Multimode step index fibers.
●
Multimode graded index fibers.
●
Single mode step index fibers.
●
Plastic - clad fibers.
●
All plastic fibers.
Dimensions of fiber optic cables are written as a ratio e.g. a cable with cladding
diameter of 125 microns and fiber core diameter of 62.5 or 50 microns will be
referred to as 62.5 /125 or 50 / 125 fibers. That is if the diameter of the core is Dcr and
the diameter of the clad is Dcd both in microns (1 micron = 10- 6 meters), then the
dimensions of the Fiber optic cable will be denoted as Dcr/ Dcd.
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Optical Fiber Communication Scientech 2502
Choice of Operating Frequency:
Once we had the LASER and the new optic fiber available, every thing was in place
for a significant upsurge in communications. This resulted in two driving forces: one
towards the ability to send more data faster and secondly to send the data to greater
distances without being re-amplified.
More Data Faster:
As the transmission rate of data is increased, the required bandwidth increases and
this can be best accommodated by increasing the carrier frequency. This premise has
stood us in good stead over many years. The speech and poor quality music
transmissions on the medium frequency, AM radio, gives way to the higher frequency
of FM radios which accommodate the increased bandwidth necessary for improved
music quality. When television required even higher data rates, we responded by
moving to even higher frequencies. These previous experience rather suggested that
the light used for fiber optic communications should be of the highest frequency
possible. But there was a surprise in store!
Lower Frequencies Mean Lower Losses:
The first experiments used visible light of different colors (frequencies). As the losses
were measured, we found that the higher frequencies caused more losses.
The losses actually increased by the 4th power of the frequency. This means that a
tripling of the frequency would result in the losses increasing by 34 or 81 times. We
therefore have two conflicting influences:
High frequency = High Data Rates
Low frequency = Long Ranges
At the moment, long distance communication is more important than achieving the
ultimate in data transmission rates. Therefore in most real installations, we tend to go
for the relatively low frequencies of infrared light that is just below the visible
spectrum.
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Optical Fiber Communication Scientech 2502
Fiber Windows:
We now have an infrared range between 800 nm – 1700 nm (1 nano meter = 10-9
meter) with one part of it around 1380 nm that is to be avoided due to high losses
because of Hydroxyl Ion Absorption as is clear from the graph optical wavelength
versus losses per kilometer (dB) . It seemed sensible to agree on standard wavelengths
so that equipment from different manufacturers can be made compatible.
This has resulted in three standard wavelength slabs called windows. The windows
were really the result of looking at the available light sources. Some wavelengths of
LED and LASER light are easier and less expensive than others to produce. The
design and manufacture of the optic fiber is then optimized for these frequencies.
Note: The infrared light is very dangerous to eyes which can cause irreversible
damages and since it is invisible, care should be taken to ensure that the optic fiber is
not live.
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Optical Fiber Communication Scientech 2502
Losses in Optic Fiber:
●
Attenuation
●
Material Absorption Losses
●
Linear Scattering Losses
•
Ray Leigh Scatter
•
Mie Scattering
●
Non Linear Scattering
●
Micro Bending and Macro Bending
●
Dispersion
●
•
Inter modal Dispersion
•
Intra modal Dispersion
Attenuation :
Transmission of light is not 100 % efficient. Some photons of light are lost,
causing attenuation of signal. Several mechanisms are involved, including
absorption by materials within the fiber, scattering of light out of the core
caused by environmental factors. The degree of attenuation depends on the
wavelength of light transmitted. Attenuation measures the reduction in signal
strength by comparing output power with input power. Measurements are made
in decibels (dB). It is defined as: Pi
dB loss α = 10 log 10 P
o
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Optical Fiber Communication Scientech 2502
●
Material Absorption Losses :
It is a loss mechanism related to the material composition and fabrication
process of the fiber that result in the dissipation of some of the transmitted
optical power as heat in wave-guide. The absorption of light may be intrinsic
(caused by one or more major components of glass) or extrinsic (caused by
impurities within the glass).
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Optical Fiber Communication Scientech 2502
●
Linear Scattering Losses :
Linear scattering mechanisms cause the transfer of some or all of the optical
power contained within one propagating mode to be transferred linearly
(proportionally) into a different mode. This process tends to result in attenuation
of the transmitted light as the transfer may be to a leaky or radiation mode that
doesn't continue to propagate within the fiber core, but is radiated from the fiber.
It is mainly of two types.
} Ray Leigh Scattering
} Mie Scattering
Ray Leigh Scattering:
When the infrared light strikes a very-very small place where the materials in
the glass are imperfectly mixed, this gives rise to localized changes in the
refractive index resulting in the light being scattered in all directions. Some of
the light escapes the optic fiber, some continues in the correct direction and
some is returned towards the light source. This is called backscatter.
Mie Scattering:
These result from the non - perfect cylindrical structure of the wave-guide. It
may be the caused by the imperfections such as irregularities in the core
cladding interface core, cladding refractive index difference along the fiber
length, diameter fluctuations, strains and bubbles. The scattering created by such
in homogeneities is mainly in the forward direction.
●
Non Linear Scattering :
Optical wave-guide does not behave linearly, several non-linear effects occur,
which in the case of scattering cause disproportionate attenuation usually at high
optical power level. This non-linear scattering causes the optical power from
one mode to be transferred in either the forward or backward direction to the
same, or other modes at different frequency. It depends critically upon the
optical power density within the fiber and hence only becomes significant above
threshold power levels.
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Optical Fiber Communication Scientech 2502
●
Micro Bending and Macro Bending :
A problem that often occurs in cabling of the optical fiber is the twisting of the
fiber core axis on a microscopic scale within the cable form. This phenomenon,
known as micro bending result from small lateral forces exerted on the fiber
during the cabling process and it causes losses due to radiation in both
multimode and single mode fiber.
●
Macro bends :
The light propagates down the optic fiber solely because the incident angle
exceeds the critical angle. If a sharp bend occurs, the normal and the critical
angle move round with the fiber. The incident ray continues in a straight line
and it finds itself approaching the core - cladding boundary at an angle less than
the critical angle and much of light is able to escape.
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Optical Fiber Communication Scientech 2502
●
Dispersion :
When an electrical pulse energizes a LASER, it launches a short flash or light
along the optic fiber. It is an unfortunate fact that the light burst becomes longer
as it moves along the fiber optic cable. The light spreads out. This effect is
called 'Dispersion', shown in next figure the light pulse shown before and after it
has traveled through the cable.
It is going to limit how fast we can send data - how many bits per second we can
transmit through a fiber optic link. In fact it is the main limit to the data
transmission rate for long distance communication system.
If we send flashes of LASER light down a long link in which dispersion is a
problem, the flashes will merge at the far end and the ON/OFF states will not be
distinguished by the receiver. Over a given transmission path, there are only two
remedies. Firstly, we could reduce the transmission rate so that even allowing
for the spreading effect of the dispersion; the ON-OFF states are still clearly
separated. This is not a very exciting solution and would clash with one of the
main reasons for using optic fiber.
There are two types of Dispersion:
}
Inter modal Dispersion
}
Intra modal Dispersion
Inter modal Dispersion:
You will recall that, to be propagated down the core of the optic fiber, the light
must enter at an angle greater than the critical angle. Let us consider just two
such rays of light as they travel along a section of optic fiber.
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Optical Fiber Communication Scientech 2502
Ray ‘A’ will reach the far end before Ray ‘B’ since it is traveling a shorter
distance. Assuming that rays A and B are part of the same pulse of light and
start at the same time, we can now see how the spreading of the pulses can
occur. Each and every ray being propagated at its own angle will arrive at
slightly different times at the far end. This spreading effect will occur all along
the fiber so it is also important to appreciate that the longer the optic fiber, the
greater the dispersion. Transmission rates that are actually possible on an optic
fiber therefore depend in its length. In practice, there are only particular angles
of propagation that can be transmitted down the optic fiber.
Intra modal Dispersion:
This form of dispersion occurs in both multi mode and single mode optic fibers.
It is only really significant in single mode usage since, being very slight; it is
completely swamped by the inter modal dispersion in the multimode case. The
cause is simple enough - the refractive index of material is determined to some
extent by the wavelength of the light source. Can you see how this causes
dispersion? A change in refractive index will change the speed of that particular
wavelength of light. Now if your light source produces different wavelengths at
the same time, we will have components of the transmitted light pulse traveling
at the same time, we will have components of the transmitted light pulse
traveling at different speeds. The total package of light will spread out - hence
the dispersion.
●
Cure for Inter modal Dispersion :
A large core diameter means many modes and severe inter modal dispersion.
The cure for this type of dispersion is quite simple. Reduce the core size; the
number of modes decreases and inter modal dispersion is reduced. We can do
better than just reducing inter modal dispersion, we can completely eliminate it.
Simply make the core so small that only one mode is propagated. A single ray
cannot possibly go at two different speeds so inter modal dispersion cannot
occur. In practice the core is reduced to about 9 µm (micron). The optic fiber
that now carries only a single mode is now referred to as a 'single mode fiber'.
Single mode fiber is used for all long distance and/or high-speed
communications. All long distance telephone conversations are now carried by
single mode fiber optic systems over some parts of the route. The larger core
optic fibers for short and medium distances carry many modes and are called
'Multimode'.
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Optical Fiber Communication Scientech 2502
●
The Cure for Intra modal Dispersion :
The cure is apparently so simple; use a light source that emits only one
wavelength of light. Unfortunately, it has not yet been invented. Our light
sources in current use are the LED and the LASER. Study shown in next figure
and decide which of the two would cause the lesser amount of Intra modal
Dispersion.
The LASER would cause less intra modal dispersion because its light is more
concentrated around the central wavelength. The spread of wavelength measured
between the points where the power output falls to half of the peak power is called the
spectral width. Some LASERS have spectral widths as low as 0.1 nm (nanometre).
The low spectral width together with its high power and fast switching makes the
LASER first choice for long distance communications using single mode optic fiber.
Also there are some losses due to coupling in between the fibers at LED and photo
detector ends.
Applications of Macro bends:
A Live Fiber Detector:
Here is the problem long distance fiber optic systems employ powerful LASER
operating in the infrared region of the spectrum. This infrared light has two properties
that are very significant to the engineers and technicians working on the system. We
have various pieces of test equipment that can be used to check the system. The' live '
fiber detector is able to find which fibers are carrying data in most day to day checks,
but read the instruction manual first to ensure that the instrument is suitable for the
type of optic fiber you are checking.
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Optical Fiber Communication Scientech 2502
This device has a pair of spring-loaded jaws. The fiber under test is slipped in
between them and when the jaws close it will cause the fiber to be bent sufficiently to
cause a macro bend. The escaping light can be detected by the photocell and used to
activate the LED indicator. One flaw in the system is that it relies on the buffer being
transparent to the infrared light.
The Optical Time Domain Reflectometer (OTDR):
The OTDR is a measuring instrument that uses backscatter. It is the most versatile
piece of test equipment that we have for making measurements on fiber optic systems.
It provides us with two different measurements:
● It can measure the magnitude of any losses that occur along optic fiber.
● It can measure distance along the optic fiber.
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Optical Fiber Communication Scientech 2502
Losses:
As the light moves along the optic fiber, the light intensity is attenuated by the losses
in the optic fiber and so the reflections returned to the OTDR receiver become
weaker. Measurement of the amplitude of the returned signals tells us the optic fiber
loss in dB/Km. if a macro bend has occurred, it would show up as a drop in the signal
level at a particular point. (If the optic fiber has been cut, a connector should be fitted
in such a way that end face of the glass causes a reflection of energy. It is also usual
for this to occur at the extreme end of the optic fiber. This cause a localized increase
in energy returned to the Optical Time Domain Reflectometer (OTDR). This
reflection called as Fresnel (the‘s’ is not pronounced) reflection shown up as a small
spike on the display. There is always a Fresnel Reflection at the start of the fiber due
to the connector on the front panel of the OTDR.
Distance:
We obtain timing information by starting the display and the pulse generator at the
same instant. This is achieved by the synchronizing pulse that switches on both the
LASER and the receiver at the same instant. If we know how long it takes for the
backscatter light to return to the OTDR then we only have to know how fast the
infrared light is traveling along the optic fiber to be able to calculate how far the light
has traveled some light returns after say, 500 ns, it follows that it has traveled to a
total of 100 meters. This represents 50 meters out along the optic fiber and 50 meters
back. You will remember that the actual speed of propagation is determined by the
refractive index of the core of the optic fiber.
Speed of propagation = speed of light in free space / refractive index of the core
(The refractive index of the optic fiber being tested must be punched in to the OTDR
otherwise all the distance will miscalculated. The value of the refractive index is
coated by the manufacturer).
The synchronizing pulse simply provides a start to the generator and to the display
circuits to allow them to determine the travel-time of the laser light and the
backscatter.
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Optical Fiber Communication Scientech 2502
Shows in next figure Fiber Optic System together with its appearance on OTDR
screen. Notice that both the macro bends and fusion splices are shown as a sudden
loss of power at a particular point. Indeed, it is not possible to distinguish between
macro bed and a fusion splice just by observing the OTDR display. It just shows a
localized loss. The loss may appear as a vertical drop or a sloping line depending
upon the speed at which the screen being scanned on the OTDR. The connector has a
similar loss but it also has a Fresnel reflection. Typical value of losses:
Fusion splice - 0.05 dB
Connector - 0.2 dB
Macro bend: 0 dB to more than 8 dB depending on the severity of the macro bend.
Two Other Applications of Back scatter.
Distributive Temperature Sensing (DTS):
The amount of backscatter occurring in an optic fiber is dependent upon the
manufacture of the optic fiber, the optic window used, and also upon the temperature
of the optic fiber. Now, when we find a characteristic of the optic fiber that depends
on the temperature, it is but a small step away from using the effect to measure
temperatures. This new technique is called Distributive Temperature Sensing (DTS).
Basically it is an optic fiber connected to equipment operating just like an OTDR that
is then passed through the areas to be measured. If it passes through a refrigerator
(minimum temperature of 190°C or 310°F). shows in next figure e.g. the trace on the
OTDR will show the backscatter level falling to a level dependent upon the
temperature in the refrigerator. Similarly, a heated area (maximum temperature 460°C
or 860°F) would return a higher level of backscatter.
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Optical Fiber Communication Scientech 2502
Security:
You will recall that one of the advantages of the fiber optic system is the high level of
security offered. We know however, that a macro bend would allow the light to
escape and hence the data to be copied. An OTDR monitoring the line would
immediately detect the power loss of the macro bend and be able to measure its
distance along the optic fiber to an accuracy of approximately 0.1 meters (4 inches)
the same immediate detection would occur as with the security matting Shows in next
figure.
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Optical Fiber Communication Scientech 2502
Fiber optic links:
Fiber optic links can be used for transmission of digital as well as analog signals.
Basically a fiber optic link contains three main elements, a transmitter, an optical fiber
and a receiver. The transmitter module takes the input signal in electrical form and
then transforms it into optical (light) energy containing the same information. The
optical fiber is the medium, which takes the energy to the receiver. At the receiver
light is converted back into electrical form with the same pattern as originally fed to
the transmitter.
Transmitter:
Fiber optic transmitters are typically composed of a buffer, driver and optical source.
The buffer provides both an electrical connection and isolation between the
transmitter & the electrical system supplying the data. The driver provides electrical
power to the optical source. Finally, the optical source converts the electrical current
to the light energy with the same pattern. Commonly used optical sources are light
emitting diodes (LED) and LASER beams. Simple LED circuits, for digital and
analog transmissions are shown below.
Shows in next figure Tran conductance drive circuits for analog transmissioncommon emitter configuration. The transmitter section comprises of:
●
Function Generator
●
Frequency Modulator
●
Pulse Width Modulator Block.
The Function Generator generates the input signals that are going to be used as
information to transmit through the fiber optic link. The output voltage available is 1
KHz sinusoidal signal of adjustable amplitude, and fixed amplitude 1 KHz square
wave signal. The modulator section accepts the information signal and converts it into
suitable form for transmission through the fiber optic link.
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Optical Fiber Communication Scientech 2502
The Fiber Optic Link:
Emitter and Detector circuit on board form the fiber optic link. This section provides
the light source for the optic fiber and the light detector at the far end of the fiber optic
links. The optic fiber plugs into the connectors provided in this part of the board. Two
separate links are provided.
The Receiver:
The Comparator circuit, Low Pass Filter, Phase Locked Loop, AC Amplifier Circuits
form receiver on the board. It is able to undo the modulation process in order to
recover the original information signal. In this experiment the TechBook board is
used to illustrate One-Way communication between digital transmitter and receiver
circuits.
Shows in next figure a simple drive circuit for binary digital transmission consisting
of a common emitter-saturating switch.
Modulation:
In order to transmit information via an optical fiber communication system it is
necessary to modulate a property of the light with the information signal. This
property may be intensity, frequency, and phase with either digital or analog signals.
The choices are indicated by the characteristics of optical fiber, the available optical
sources and detectors, and considerations of the over all system.
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Optical Fiber Communication Scientech 2502
Intensity Modulation:
In this system the information signal is used to control the Intensity of the source. At
the far end, the variation in the amplitude of the received signal is used to recover the
original information signal.
The audio input signal is used to control the current through an LED which in turn
controls the light output. The light is conveyed to the detector 1 circuit by optic fiber.
The detector is a phototransistor that converts the incoming light to a small current
which flows through a series resistor. This gives rise to a voltage whose amplitude is
controlled by the received light intensity. The voltage is now amplified within the
detector circuit and if necessary, amplified further by amplifier circuit.
The Analog Bias Voltage:
There are two problems using amplitude modulation with an analog signal. The first is
to do with the signal itself.
If you glance at the shown in next figure you will see that analog wave form moves
positive & negative of the zero line. The second problem is that it is the shape of the
waveform that carries the information. Ideally the emitter characteristic would be a
straight line. Even so we would loose the negative going half cycles as shown in next
figure below:
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Optical Fiber Communication Scientech 2502
The answer is to superimpose the sinusoidal signal on positive voltage called the bias
voltage so that both halves of the incoming signal have an effect on the light intensity.
The combination of linear characteristic would be ideal but the real characteristic is
not completely straight. However it does have a straight section that we can use if we
employ a suitable value of bias voltage. Shown in next figure ideal and practical
situations.
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Optical Fiber Communication Scientech 2502
Digital Modulation:
With digital modulation, discrete changes in light intensity are obtained (i.e. ‘On-Off’
pulses) shown in next figure a block schematic of a typical digital optical fiber link.
Initially, input digital signal from the information source is suitably encoded for
optical transmission. The LED drive circuit directly modulates the intensity of the
light with encoded digital signal. Hence, a digital optical signal is launched into the
optical fiber cable. An amplifier to provide gain follows the phototransistor used as
detector. Finally, the signal obtained is decoded to give the original digital
information.
Digital Bias Voltage:
In case of a digital signal the only information which needs to be conveyed is the
’On’ state and ‘Off’ state. The digital Input signal is entirely positive going as shown
in next figure.
So, there is no negative part of the signal to be lost and further more any distortion
due to non-linearity of the characteristic is of no importance; all we need to know is
whether the signal is ‘On’ or ‘Off’. There is no need therefore to generate a bias
voltage. When amplitude modulation is used with a digital input we employ a
comparator at the receiving end of the fiber to make the waveform square again called
'cleaning it up'.
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Optical Fiber Communication Scientech 2502
Frequency Modulation:
In the traditional form of FM the carrier frequency is changed or modulated by the
amplitude of the analog signal. In a fiber optic system this is not feasible since both
our light sources; the LED and the LASER are fixed frequency devices. In fiber optic
systems FM is achieved by using the original analog input signal to vary the
frequency of a train of digital pulses.
Frequency Modulation
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Optical Fiber Communication Scientech 2502
A circuit called Voltage Controlled Oscillator usually abbreviated as VCO achieves
this. The digital pulses are communicated through the optic fiber and squared up at
the receiver by a comparator in the same way as it was in amplitude modulation
system. At this point, we convert the digital train back to the original analog signal by
means of the Phase Locked Loop Detector (PLL). The PLL circuit performs a very
simple function. It monitors an incoming signal and produces a DC Voltage output. If
the input frequency increases, the DC voltage increases. If the frequency decreases,
the DC voltage decreases in this way the original analog signal is recovered. The
output of the PLL contains many unwanted frequency components. A Low Pass Filter
removes these and then finally the signal is amplified to the desired level.
Pulse width modulation:
Pulse width modulation (PWM) is an alternative to frequency modulation. They are
both digital transmission. In FM, you will remember, the incoming analog signal is
used to change the frequency of the digital stream. In pulse width modulation the
amplitude of the analog signal to be transmitted as the changes in the width of the
pulse. It is an extremely simple system of modulation. Assume an input signal at zero
volts.
The digital stream and the average voltage level would be as shown in figure next.
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Optical Fiber Communication Scientech 2502
Pulse Width Modulation
If the input voltage moves to a positive value, the pulse width will increase and since
the waveform is ‘On’ longer than it is ‘Off’ the average value increases. Similarly, if
the input signal goes negative the width of the pulse will decrease. The average value
of the digital voltage now decreases. You will now appreciate that the average voltage
level is increasing and decreasing in accordance with the input voltage. At the far end
of the transmission system the digital pulses are cleaned up by the comparator and
then simply passed through a low pass filter. The filter removes the square waves but
the average level remains to form the output signal. At this stage, the output signal is
increasing and decreasing in step with the input, but you will remember that the OV
input signal produced a DC level at the output. This DC level must now be removed.
We do this by means of blocking capacitors at the input to the final amplifier.
Recommended Testing Instruments for Experiments:
Oscilloscope 20 MHz Dual Trace Scientech 201
43
Optical Fiber Communication Scientech 2502
Experiment 1
Objective: Setting up Fiber Optic Analog Link
Study of a 650nm fiber optic analog link in this experiment you will study the
relationship between the input signal and received signal.
Equipments Required:
●
Scientech 2502 TechBook with Power Supply cord
●
Optical Fiber cable
●
Cathode ray Oscilloscope with necessary connecting probe
Connection Diagram:
44
Optical Fiber Communication Scientech 2502
Procedure:
●
Connect the Power Supply cord to the main power plug & to TechBook
Scientech 2502.
●
Ensure that all switched faults are ‘Off’.
●
Make the following connections as shown in next figure
•
Connect the 1 KHz sine wave output to emitter l's input.
•
Connect the Fiber Optics cable between emitter output and detectors input.
•
Detector l's output to AC amplifier 1 input.
●
On the board, switch emitter l's driver to analog mode.
●
Switch ON the Power Supply of TechBook and Oscilloscope.
●
Observe the input to emitter 1 (TP5) with the output from AC amplifier 1
(TP28) and note that the two signals are same.
Questions:
●
What is meant by index profile?
●
What is the drawback of multimode Fibers?
●
What is Fiber optics?
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Optical Fiber Communication Scientech 2502
Experiment 2
Objective: Setting up Fiber Optic Digital Link
Study of a 650 nm fiber optic digital link
In this experiment you will study the relationship between the input signal and
received signal.
Equipments Required:
●
Scientech 2502 TechBook with Power Supply cord
●
Optical Fiber cable
●
Cathode ray Oscilloscope with necessary connecting probe
Connection Diagram:
46
Optical Fiber Communication Scientech 2502
Procedure:
●
Connect the Power Supply cord to the main the Power Supply to the board.
●
Ensure that all switched faults are ‘Off’.
●
Make the following connections as shown in next figure.
•
Connect the 1 KHz square wave output to emitter l's input.
•
Connect the fiber optic cable between emitter output and detectors input.
•
Detector 1's output to comparator 1’s input.
•
Comparator l's output to AC amplifier l's input.
●
On the board, switch emitter 1's driver to digital mode.
●
Switch ON the Power Supply of TechBook and Oscilloscope.
●
Monitor both the inputs to comparator 1 (TP13 & 14). Slowly adjust the
comparators bias preset, until DC Level on the input (TP13) lays mid way
between the high and low level of the signal on the positive input (TP14).
●
Observe the input to emitter 1 (TP 5) with the output from AC amplifier 1
(TP28) and note that the two signals are same.
Questions:
●
Why single mode Fibers are used for long distance transmission?
●
What is optical Fiber?
●
What is step index profile?
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Optical Fiber Communication Scientech 2502
Experiment 3
Objective: Study of Intensity Modulation Technique using Analog input signal.
To obtain intensity modulation of the analog signal, transmit it over a fiber optic cable
and demodulate the same at the receiver and to get back the original signal.
Equipments Required:
●
Scientech 2502 TechBook with Power Supply cord
●
Optical Fiber cable
●
Cathode ray Oscilloscope with necessary connecting probe
Connection Diagram:
48
Optical Fiber Communication Scientech 2502
Procedure:
●
Connect the Power Supply cord to the main power plug & to TechBook
Scientech 2502
●
Make the following connections as shown in next figure.
•
Connect the FG output marked 1 KHz sine wave to input if emitter 1.
•
Plug in a fiber optic link from output of emitter 1 LED to the photo
transistor of the detector 1.
•
Detector 1 output TP 10 to input of Amplifier TP 27.
●
In the emitter 1 block switch the mode select to analog.
●
Turn the 1 KHz preset in Function Generator block to fully clockwise
(maximum amplitude) position.
●
Switch on the Power Supply of the TechBook and Oscilloscope.
●
With the help of dual trace Oscilloscope observe the input signal at emitter 1 TP
5 also; observe the output from the detector 1. It should carry a smaller version
of the original 1 KHz sine wave, illustrating that the modulated light beam has
been reconverted back into an electrical signal.
●
The output from detector 1 is further amplified by AC amplifier 1. This
amplifier increases the amplitude of the received signal, and also removes the
DC component, which is present at detector output. Monitor the output of
amplifier 1 TP28 and adjust the gain adjust 1 preset until the monitored signal
has same amplitude as that applied to emitter 1 Input TP 5 .
●
While monitoring the output of Amplifier 1 TP 28 change the amplitude of
modulating sine wave by varying the 1 KHz preset in the Function Generator
block. Note that as expected, the amplitude of the receiver output signal
changes.
Questions:
●
What is the function of transmitter, optical Fiber and receiver?
●
Where Fiber optics links can be used?
●
What is spectral width?
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Optical Fiber Communication Scientech 2502
Experiment 4
Objective: Study of Intensity Modulation Technique using digital Input signal
The objective of this experiment is to obtain intensity modulation of digital signal,
transmit it over fiber optic cable and demodulate the same at the receiver end to get
back the original signal.
Equipments Required:
●
Scientech 2502 TechBook with Power Supply cord
●
Optical Fiber cable
●
Cathode ray Oscilloscope with necessary connecting probe
Connection Diagram:
50
Optical Fiber Communication Scientech 2502
Procedure:
●
Connect the Power Supply cord to the main power plug & to TechBook
Scientech 2502.
●
Make the following connections as shown in next figure.
•
Connect the 1 KHz square wave socket in Function Generator block to
emitter 1 input.
•
Connect an optic fiber link between emitter 1 output & Detector 1 input
with the help of connector provided.
•
Detector output to comparator l's non-inverting (+ve) input.
●
Switch the mode switch in emitter block to digital mode. This ensures that
signal applied to the driver's input cause the emitter LED to switch quickly
between ‘On’ & ‘Off’ states.
●
Examine the Input to emitter 1 TP 5 on an Oscilloscope this 1 KHz square wave
is now being used to amplitude modulate emitter I emitter LED.
●
Examine the output of detector 1 TP 10. This should carry a smaller version of
original I KHz square wave illustrating that the modulated light beam has been
reconverted into an electrical signal.
●
Monitor both input to comparator 1, at TP 13 & 14 and slowly adjust the
"Comparator bias 1 preset until the DC Level on the negative input TP 13 lies
midway between the high & low level of the signal on the positive input TP. 14.
This DC level is comparator's threshold level.
●
Examine the output of comparator 1 TP15 Note that the original digital
modulating signal has been reconstructed at the receiver.
●
Once again carefully flex the fiber optic cable we can see that there is no change
in output on bending the fiber. The output amplitude is now independent of the
bend radius of the cable and that of length of cable, provided that detector output
signal is large enough to cross the comparator threshold level. This illustrates
one of the advantages of amplitude modulation of a light beam by digital rather
than analog means. Also, non-linear ties within the emitter LED & photo
transistor causing distortion of the signal at the receiver output are the
disadvantages associated with amplitude modulating a light source by analog
means. Linearity is not a problem if the light beam is switched ‘On’ & ‘Off’
with a digital signal, since the detector output is simply squared up by a
comparator circuit. To overcome problems associated with amplitude
modulation of a light beam by analog means, analog signals are often used to
vary or modulate some characteristic of a digital signal (e.g. frequency or pulse
width.). The digital signal being used to switch the light beam ‘On’ & ‘Off’. The
next two experiments illustrate how an analog signal can be used to modulate
two specific characteristics of a digital signal.
51
Optical Fiber Communication Scientech 2502
Questions:
●
What is intensity modulation?
●
What is the function of LASER?
●
How the modulated signal is detected?
52
Optical Fiber Communication Scientech 2502
Experiment 5
Objective: The Frequency Modulation System
Equipments Required:
●
Scientech 2502 TechBook with Power Supply cord
●
Optical Fiber cable
●
Cathode ray Oscilloscope with necessary connecting probe
Connection Diagram:
Procedure:
●
Connect the Power Supply cord to the main power plug & to TechBook
Scientech 2502.
●
Ensure that all switched faults are ‘Off’.
●
Make the following connections as shown in next figure
•
Connect Function Generator 1 KHz sine wave signal to frequency
modulator input.
•
Frequency modulator output TP2 to the emitter 1 input at TP5.
•
Connect the optic fiber between the emitter 1 circuit and the detector 1
circuit.
•
Detector 1 output TP10 to comparator 1 input at TP14.
53
Optical Fiber Communication Scientech 2502
•
Comparator 1 output TP15 to the PLL detector input at TP23.
•
PLL detector output at TP26 to the low pass filter 1 input at TP19
•
Low Pass Filter 1 output TP20 to A C amplifier 1 input at TP27
●
Switch emitter l's driver to digital mode. This ensures that fast changing digital
signal applied to the drivers input causes the emitter LED to switch quickly
between ‘On’ & ‘Off’ states.
●
Turn the 1 KHz preset in the Function Generator block to fully anticlockwise
(Zero amplitude) position.
●
Switch ON the Power Supply of the TechBook and Oscilloscope.
●
Monitor the output of the voltage controlled oscillators (VCO) in the frequency
modulator block TP2. Note that the frequency of this digital signal is at present
constant, since the modulating 1 KHz sine wave has zero amplitude.
●
Examine the output of detector 1 (TP10 and check that the transmitted digital
pulses are successfully detected at the receiver).
●
With the help of dual trace Oscilloscope monitor both inputs to comparator1.
Now adjust the bias 1 preset until the bias input at TP13 is halfway between the
top and bottom of the square wave on TP14. You will remember that the
function of the comparator is to clean up the square wave after its transmission
through the fiber optic link.
●
The output of comparator 1 drives the input of the PLL detector which produces
a signal whose average level is proportional to the frequency of the digital
stream. This average level is then extracted by low pass filter 1, and amplified
by AC Amplifier1 to produce the original analog signal at the amplifiers output
TP28. Examine TP28 and note that the output voltage is zero. This is expected
since there is currently no modulating voltage in the transmitter.
●
While monitoring the input to the frequency modulator block TP1 and the
output from AC amplifier 1 TP 28 turn the 1KHz preset to its fully clockwise
maximum amplitude) position. Note that the modulating 1 KHz signal now
appears at the amplifiers output. If necessary, adjust the amplifiers gain, adjust 1
preset until the two monitored signal are equal in amplitude.
●
In order to fully understand how this frequency modulation transmitter/ receiver
system works, examine the inputs and outputs of all functional blocks within the
system, using an Oscilloscope.
Questions:
●
How the FM signal is generated?
●
What are the various detection techniques of FM signals?
●
Why FM is used for short distance communication?
54
Optical Fiber Communication Scientech 2502
Experiment 6
Objective: The Pulse Width Modulation System
Equipments Required:
●
Scientech 2502 TechBook with Power Supply cord
●
Optical Fiber cable
●
Cathode ray Oscilloscope with necessary connecting probe
Connection Diagram:
Procedure:
●
Connect the Power Supply cord to the main power plug & to TechBook
Scientech 2502.
●
Ensure that all switched faults are set to ‘Off’.
●
Make the following connections as shown in next figure.
•
FG' s 1KHz sine wave signal to the Pulse width modulator input TP3
•
Pulse width modulator output TP4 to emitter 1 input TP5
•
Connect the optic fiber between the emitter 1 circuit and detector 1 circuit.
•
Detector 1 output TP10 to comparator & input at TP14.
•
Comparator 1 output TP15 to LPF 1 at TP19.
55
Optical Fiber Communication Scientech 2502
•
LPF 1 output TP20 to A C amplifier 1 input at TP27.
●
Switch emitter 1’s driver to digital mode. This ensures that fast changing digital
signals applied to the drivers input because the emitter LED to switch quickly
between ‘On’ & ‘Off’ states.
●
Turn the 1 KHz preset of Function Generator block to fully anticlockwise (zero
amplitude) position.
●
Switch ON the Power Supply of the TechBook and Oscilloscope.
●
Monitor the output of the pulse width modulator block TP4. Note that the pulse
width of this digital signal is at present constant, since the modulating 1 KHz
sine wave has zero amplitude.
●
Examine the output Detector TP10 and check that the transmitted digital pulse is
successfully detected at the receiver.
●
Monitor both inputs’ comparator 1 TP13 & TP14 and if necessary, slowly adjust
the comparator's bias preset, until the DC Level on the negative input TP13 lies
midway between the high and low level of the signal on the positive input TP14.
●
The average level of comparator l's output is extracted by LPF 1 and then
amplified by AC amplifier which also removes the DC offset. Since, the average
level of the comparator output is proportional to the pulse width, the original
analog signal appears at the amplifiers output TP28. Examine TP28 and note
that the output voltage is zero. This is expected since there is currently no
modulating voltage at the transmitter.
●
While monitoring the input to the pulse width modulator block TP3 and the
output from AC amplifier 1 TP28 turn the 1 KHz preset to its fully clockwise
(maximum amplitude position). Note that the modulating 1 KHz signal now
appears at the amplifiers output. If necessary, adjust the amplifiers gain adjust 1
preset until the two monitored signals are equal in amplitude.
●
In order to fully understand how this pulse width modulation transmitter/
receiver system works, examine the inputs and outputs of all functional blocks
within the system using an Oscilloscope.
Questions:
●
What is PWM?
●
What is the advantage of using PWM in communication systems?
●
What is the function of comparator circuit?
56
Optical Fiber Communication Scientech 2502
Experiment 7
Objective: Study of Propagation Loss in Optical Fiber
To measure propagation or attenuation loss in optical fiber
Equipments Required:
●
Scientech 2502 TechBook with Power Supply cord
●
Optical Fiber cable
●
Cathode ray Oscilloscope with necessary connecting probe
Connection Diagram:
57
Optical Fiber Communication Scientech 2502
Procedure:
●
Connect Power Supply cord to the main power plug & to TechBook Scientech
2502.
●
Make the following connections as shown in next figure.
•
Function Generator’s 1 KHz sine wave output to Input 1 socket of emitter 1
circuit via 4 mm lead.
•
Connect 0.5 m optic fiber between emitter 1 output and detector l's input.
•
Connect detector 1 output to amplifier 1 input socket via 4mm lead.
●
Switch ON the Power Supply of the TechBook and Oscilloscope.
●
Set the Oscilloscope channel 1 to 0.5 V / Div and adjust 4 - 6 div amplitude by
using X 1 probe with the help of variable pot in Function Generator block at
input 1 of Emitter 1.
●
Observe the output signal from detector TP10 on CRO.
●
Adjust the amplitude of the received signal same as that of transmitted one with
the help of gain adjust potentiometer in AC amplifier block. Note this amplitude
and name it V1.
●
Now replace the previous FG cable with 1 m cable without disturbing any
previous setting.
●
Measure the amplitude at the receiver side again at output of amplifier 1 socket
TP 28. Note this value end name it V2.
Calculate the propagation (attenuation) loss with the help of following formula.
V1 / V2 = e- α (L1 + L2)
Where α is loss in nepers / meter
1 neper = 8. 686 dB
L 1 = length of shorter cable (0.5 m)
L 2 = Length of longer cable (1 m)
Questions:
●
How to measure propagation losses?
●
By what optical cable is made up of?
●
What is step index Fiber?
58
Optical Fiber Communication Scientech 2502
Experiment 8
Objective: Study of Bending Loss
The object of this experiment into study bending loss
Equipments Required:
●
Scientech 2502 TechBook with Power Supply cord
●
Optical Fiber cable
●
Cathode ray Oscilloscope with necessary connecting probe
●
Mandrel
Connection Diagram:
59
Optical Fiber Communication Scientech 2502
Procedure:
●
Connect Power Supply cord to the main power plug & to TechBook Scientech
2502.
●
Make the connections as shown in next figure.
•
Function Generator 1 KHz sine wave output to input socket of emitter
Circuit via 4 mm lead.
•
Connect 0.5 m optic fiber between emitter output and detectors input.
•
Connect Detector output to amplifier input socket via 4mm lead.
●
Switch ‘On’ the Power Supply of the TechBook and Oscilloscope.
●
Set the Oscilloscope channel 1 to 0.5 V/ Div and adjust 4-6 div amplitude by
using X 1 probe with the help of variable pot in Function Generator Block at
input of Emitter.
●
Observe the output signal from detector (TP8) on CRO.
●
Adjust the amplitude of the received signal as that of transmitted one with the
help of gain adjusts potentiometer in AC amplifier block. Note this amplitude
and name it V 1 .
●
Wind the fiber optic cable on the mandrel and observe the corresponding AC
amplifier output on CRO, it will be gradually reducing, showing loss due to
bends.
Questions:
●
What is the reason of bending losses?
●
What is core and cladding?
●
What is the function of cladding?
60
Optical Fiber Communication Scientech 2502
Experiment 9
Objective: Measurement of Optical Power using Optical Power Meter
To measure optical power using optical power meter.
Equipments Required:
●
Scientech 2502 TechBook with Power Supply cord
●
Optical Fiber cable
●
Cathode ray Oscilloscope with necessary connecting probe
●
Power Meter Scientech 2551 with Power Supply cord
Connection Diagram:
61
Optical Fiber Communication Scientech 2502
Procedure:
●
Connect the Power Supply cord to mains supply and to the TechBook Scientech
2502.
●
Ensure that all switched faults are ‘Off’.
●
Connect the fiber optic cable between emitters 1's output & power meter's input.
●
On the board, switch emitter l's driver to analog mode. Keep the power meter's
wavelength selector switch in 660 nm
●
Switch ON the Power Supply of the TechBook and power meter.
●
Note the reading displayed in power meter.
●
Switch the wavelength selector switch to 950 nm positions. & note the reading
displayed on power meter.
●
Perform the same experiment with emitter 2.
Questions:
●
How the power is measured using power meter?
●
What is wavelength of light?
●
What do you understand by fiber bending?
62
Optical Fiber Communication Scientech 2502
Experiment 10
Objective: Measurement of propagation loss using Optical Power Meter
To measure propagation loss in optical fiber using optical power meter.
Equipments Required:
●
Scientech 2502 TechBook with Power Supply cord
●
Optical Fiber cable
●
Cathode ray Oscilloscope with necessary connecting probe
●
Power Meter Scientech 2551 with Power Supply cord
Connection Diagram:
63
Optical Fiber Communication Scientech 2502
Procedure:
●
Connect the Power Supply cord to mains supply and to the TechBook Scientech
2501.
●
Keep the mode switch in emitter 1 circuit in analog mode
●
Connect the 0.5m fiber cable in between the emitter LED & I/P of power meter.
●
Switch on the instrument Optical Fiber Communication & power meter (Keep
the wavelength switch in 660 nm, position). Note the reading in power meter.
●
Replace the 0.5m fiber cable with the 1m cables without disturbing any setting.
Again note the reading in power. This reading will be lesser then the previous
one, indicating that the propagation loss increases with increase in length.
●
Perform the same experiment with emitter 2.
Questions:
●
How Propagation Loss in optical fiber is measured?
●
What are the various types of losses in fiber?
●
What is the formula used for measurement of losses?
64
Optical Fiber Communication Scientech 2502
Experiment 11
Objective: Measurement of Numerical Aperture
Measurement of the Numerical Aperture (NA) of the fiber
Equipments Required:
●
Scientech 2502 TechBook with Power Supply cord
●
Optical Fiber cable
●
Numerical Aperture measurement Jig
Connection Diagram:
65
Optical Fiber Communication Scientech 2502
Procedure:
●
Connect the Power Supply cord to mains supply and to the TechBook Scientech
2502.
●
Connect the frequency generator's 1 KHz sine wave output to input of emitter 1
circuit. Adjust its amplitude at 5Vpp.
●
Connect one end of fiber cable to the output socket of emitter 1 circuit and the
other end to the numerical aperture measurement jig. Hold the white screen
facing the fiber such that its cut face is perpendicular to the axis of the fiber.
●
Hold the white screen with 4 concentric circles (10, 15, 20 & 25mm diameter)
vertically at a suitable distance to make the red spot from the fiber coincide with
10 mm circle.
●
Record the distance of screen from the fiber end L and note the diameter W of
the spot.
●
Compute the numerical aperture from the formula given below
W
NA = -------------- = sinθmax
√4L2 + W2
●
Vary the distance between in screen and fiber optic cable and make it coincide
with one of the concentric circles. Note its distance.
●
Tabulate the various distances and diameter of the circles made on the white
screen and computes the numerical aperture from the formula given above.
Inferences: The N.A. recorded in the manufacturer's data sheet is 0.5. The variation
in the observation is due to fiber being used. The Acceptance Angle is given by
2sinθmax. The deviation from the data sheet is again due to fiber being used.
Questions:
●
What is numerical aperture?
●
Write the formula for numerical aperture?
●
What is the significance of numerical aperture?
66
Optical Fiber Communication Scientech 2502
Experiment 12
Objective: Characteristics of E-O Converter using OPM
Study of characteristics of E-O converter using OPM
Equipments Required:
●
Scientech 2502 TechBook with Power Supply cord
●
Optical Fiber cable
●
Cathode ray Oscilloscope with necessary connecting probe
●
Power Meter ST 2551 with Power Supply cord
Connection Diagram:
67
Optical Fiber Communication Scientech 2502
Procedure:
●
Connect the Power Supply cord to mains supply and to the TechBook Scientech
2502.
●
Ensure that all switched faults are ‘Off’.
●
Put emitter 1 block in digital mode. Connect the bias 1 on comparator 1 block
preset TP13 to the emitter 1 I/P at TP5.
●
Adjust the bias l preset to its minimum setting fully counter clockwise. Connect
the Fiber Optics cable between the emitter 1 LED & power meter.
●
Switch ON the Power Supply of the TechBook and Oscilloscope.
●
Note down the reading from power meter.
●
Vary the bias preset so as to vary the voltage applied to emitter 1 LED.
●
Record the change in power meter reading corresponding to change in forward
voltage. Plot the graph between forward voltage and power meter reading.
●
Perform the experiment with emitter 2.
Questions:
●
What is the function of optical power meter?
●
What is the full form of LED?
●
Why LED is not used for long distance transmission?
68
Optical Fiber Communication Scientech 2502
Experiment 13
Objective: V-I Characteristics of Photo LED ( E - O converter)
The aim of this experiment is to plot the characteristic of LED.
Equipments Required:
●
Scientech 2502 TechBook with Power Supply cord
●
Optical Fiber cable
●
Cathode ray Oscilloscope with necessary connecting probe
Connection Diagram:
69
Optical Fiber Communication Scientech 2502
Theory :
LED’s and LASER diodes are the commonly used sources in optical communication
systems, whether the system transmits digital or analog signal. It is therefore, often
necessary to use linear electrical to optical converter to allow its use in intensity
modulation & high quality analog transmission systems. LED's have a linear optical
output with relation to the forward current over a certain region of operation.
Procedure:
●
Connect Power Supply to the board.
●
Ensure that all switched faults are in OFF condition.
●
Put emitter 1 block in Digital Mode
●
Make connections as given below.
•
Connect the bias 1 preset on comparator to the emitter 1 input.
•
Adjust the bias 1 preset to its minimum setting fully counter clockwise.
Now look down the emitter 1 LED Socket and slowly advance the setting
of the bias 1 preset until in subdued lighting the light from LED is just
visible.
●
Connect the DMM between + 12V supply (Red Socket) and tp of Input of
Emitter LED. The DMM will now read the forward voltage (V f)
●
Measure the voltage drop across the 1k (R9) current limiting resistors by
connecting DMM between tp of Input of Emitter LED and tp6 (tp38 if old kit).
The forward current is given by dividing the readings by 1k. This If is known as
threshold current.
●
●
DVM reading
If = ------------------- mA
1000
Vary the bias 1 preset so as to vary the forward voltage (as 1.0, 1.5…4.0), note
the corresponding If (forward current).
Record these values of Vf and If & plot the characteristic between these two.
Questions:
●
What are the characteristics of fiber optics link?
●
What is the function of transmitter?
●
How light signals are converted back to the electrical signals?
70
Optical Fiber Communication Scientech 2502
Experiment 13A
Objective:
Characteristics of Photo Detector (E - O converter)
The aim of this experiment is to plot the characteristic of Photo Detector.
Equipments Required:
●
Scientech 2502 TechBook with Power Supply cord
●
Optical Fiber cable
●
Cathode ray Oscilloscope with necessary connecting probe
Connection Diagram:
Theory:
Photo Transistors and Photo Diodes are the commonly used detectors in optical
communication systems, whether the system receives digital or analog signal. It is
therefore, often necessary to use linear optical to electrical converter to allow its use
in intensity demodulation & high quality analog receiving systems. Photo Diodes
have a linear electrical output with relation to the light intensity over a certain region
of operation.
71
Optical Fiber Communication Scientech 2502
Procedure:
●
Connect Power Supply to the board.
●
Ensure that all switched faults are in OFF condition.
●
Put emitter 1 block in Digital Mode
●
Make connections as given below.
•
Connect the bias 1 preset on comparator to the emitter 1 input.
•
Adjust the bias 1 preset to its minimum setting fully counter clockwise.
Now look down the emitter 1 LED Socket and slowly advance the setting
of the bias 1 preset until in subdued lighting the light from LED is just
visible.
•
Connect the fiber optic cable between emitter output and detectors input.
●
Connect the DMM between + 12V supply (Red Socket) and tp of Input of
Emitter LED. The DMM will now read the forward voltage (V f)
●
Measure the voltage drop across the 75E resistors by connecting DMM between
tp of output of Photo Transistor and Ground. The detector current is given by
dividing the readings by 75E.
●
●
DVM reading0
Id = ------------------- mA
75
Vary the bias 1 preset so as to vary the forward voltage (as 1.0, 1.5…4.0), note
the corresponding If (forward current).
Record these values of Vf and Id & plot the characteristic between these two.
72
Optical Fiber Communication Scientech 2502
Experiment 14
Objective: Characteristics of Fiber Optics communication Link
The aim of experiment is to study the Vin (AC) versus Vo (AC).
Equipments Required:
●
Scientech 2502 TechBook with Power Supply cord
●
Optical Fiber cable
●
Cathode ray Oscilloscope with necessary connecting probe
Connection Diagram:
73
Optical Fiber Communication Scientech 2502
Procedure:
●
Connect the Power Supply cord to mains supply and to the TechBook Scientech
2502.
●
Make the following connections as shown in next figure.
•
Function Generator 1 KHz sine wave output to input socket of emitter 1
circuit via 4mm lead.
•
Connect optic fiber between emitter l’s output and detector 1’s input.
•
Connect Detector 1 output to amplifier 1 input socket via 4nm lead.
●
Switch on the Power Supply.
●
Set the amplitude of the Function Generator to 2V p-p.
●
Observe the transmitted and received signal on CRO. Vo (output voltage) should
be in the same order as Vin (input voltage).
●
Next set Vin to suitable values and note the values of Vo.
●
Tabulate and plot a graph Vo versus Vin & compute Vo/Vin.
Questions:
●
What is the advantage of amplitude modulation in terms of bandwidth
requirement?
●
How amplitude modulation signal is generated?
●
What is the detection process amplitude modulated signals?
74
Optical Fiber Communication Scientech 2502
Experiment 15
Objective: Setting up of Fiber Optics voice link using Intensity Modulation
Study of voice communication through Fiber Optic cable using amplitude modulation
Equipments Required:
●
Scientech 2502 TechBook with Power Supply cord
●
Optical Fiber cable
●
Cathode ray Oscilloscope with necessary connecting probe
Connection Diagram:
75
Optical Fiber Communication Scientech 2502
Procedure:
●
Connect the Power Supply cord to the main power plug & to TechBook
Scientech 2502.
●
Make the following connections as shown in next figure.
•
Connect the FG output marked 1 KHz sine wave to input if emitter 1.
•
Plug in a fiber optic link from output of emitter 1 LED to the photo transistor
of the detector 1.
•
Detector 1 output TP 10 to input of Amplifier TP 27.
•
In the emitter 1 block switch the mode select to analog.
•
Turn the 1 KHz preset in Function Generator block to fully clockwise
(maximum amplitude) position.
●
Switch ON the Power Supply of the TechBook and Oscilloscope.
●
With the help of dual trace Oscilloscope observe the input signal at emitter 1 TP
5 also; observe the output from the detector 1. It should carry a smaller version
of the original 1 KHz sine wave, illustrating that the modulated light beam has
been reconverted back into an electrical signal.
●
The output from detector 1 is further amplified by AC amplifier 1. This
amplifier increases the amplitude of the received signal, and also removes the
DC component, which is present at detector output. Monitor the output of
amplifier 1 TP28 and adjust the gain adjust 1 preset until the monitored signal
has same amplitude as that applied to emitter 1 Input TP 5 .
●
While monitoring the output of Amplifier 1 TP 28 change the amplitude of
modulating sine wave by varying the 1 KHz preset in the Function Generator
block. Note that as expected, the amplitude of the receiver output signal
changes.
●
Disconnect the emitter 1’s input to 1 KHz sine wave socket.
●
Make the following additional connections (as shown in next figure).
●
Audio input block's input to microphone.
●
Connect the output of audio input block to emitter l’s input.
●
AC Amplifier's output to input of audio output block.
●
Observe that same audio output is available on the speaker as fed to the
microphone.
Questions:
●
What is the drawback of FM modulation in terms of bandwidth requirement?
●
How the FM signals are generated?
●
What is the function of AC amplifier?
76
Optical Fiber Communication Scientech 2502
Experiment 16
Objective: Setting up Fiber Optics voice link using Frequency Modulation
The objective of this experiment is to demonstrate voice transmission through optic
fiber using F M.
Equipments Required:
●
Scientech 2502 TechBook with Power Supply cord
●
Optical Fiber cable
●
Cathode ray Oscilloscope with necessary connecting probe
Connection Diagram:
77
Optical Fiber Communication Scientech 2502
Procedure:
●
Connect the Power Supply cord to the main power plug & to TechBook
Scientech 2502.
●
Ensure that all switched faults are ‘Off’.
●
Make the following connections as shown in next figure.
•
Connect Function Generator 1 KHz sine wave signal to frequency
modulator input.
•
Frequency modulator output TP2 to the emitter 1 input at TP5.
•
Connect the optic fiber between the emitter 1 circuit and the detector 1
circuit.
•
Detector 1 output TP10 to comparator 1 input at TP14.
•
Comparator 1 output TP15 to the PLL detector input at TP23.
•
PLL detector output at TP26 to the low pass filter 1 input at TP19
•
Low Pass Filter 1 output TP20 to A C amplifier 1 input at TP27
●
Switch emitter l's driver to digital mode. This ensures that fast changing digital
signal applied to the drivers input causes the emitter LED to switch quickly
between ‘On’ & ‘Off’ states.
●
Turn the 1 KHz preset in the Function Generator block to fully anticlockwise
(Zero amplitude) position.
●
Disconnect the 1 KHz sine wave output from in put of F M block.
●
Make the following additional connections as shown in next figure without
disturbing previous setting.
●
Plug the Microphone in the input of Audio input block.
●
Output of Audio input block to input of FM block.
●
Output of AC Amp block to the Power Supply.
●
Speak in the Microphone and listen the same in the speaker / Headphone.
Questions:
●
What is the drawback of FM modulation in terms of bandwidth requirement?
●
How the FM signals are generated?
●
What is the function of AC amplifier?
78
Optical Fiber Communication Scientech 2502
Experiment 17
Objective: Setting up of Fiber Optic Voice Link using PWM
Study of voice transmission through Fiber Optic cable using PWM
Equipments Required:
●
Scientech 2502 TechBook with Power Supply cord
●
Optical Fiber cable
●
Cathode ray Oscilloscope with necessary connecting probe
Connection Diagram:
79
Optical Fiber Communication Scientech 2502
Procedure:
●
Connect the Power Supply cord to the main power plug & to TechBook
Scientech 2502.
●
Ensure that all switched faults are set to ‘Off’.
●
Make the following connections as shown in next figure
•
FG' s 1KHz sine wave signal to the Pulse width modulator input TP3
•
Pulse width modulator output TP4 to emitter 1 input TP5
•
Connect the optic fiber between the emitter 1 circuit and detector 1 circuit.
•
Detector 1 output TP10 to comparator & input at TP14.
•
Comparator 1 output TP15 to LPF 1 at TP19.
•
LPF 1 output TP20 to A C amplifier 1 input at TP27.
●
Switch emitter 1’s driver to digital mode. This ensures that fast changing digital
signals applied to the drivers input because the emitter LED to switch quickly
between ‘On’ & ‘Off’ states.
●
Turn the 1 KHz preset of Function Generator block to fully anticlockwise (zero
amplitude) position.
●
Switch on the Power Supply of the TechBook and Oscilloscope.
●
Monitor the output of the pulse width modulator block TP4. Note that the pulse
width of this digital signal is at present constant, since the modulating 1 KHz
sine wave has zero amplitude.
●
Examine the output Detector TP10 and check that the transmitted digital pulse is
successfully detected at the receiver.
●
Monitor both inputs’ comparator 1 TP13 & TP14 and if necessary, slowly adjust
the comparator's bias preset, until the DC Level on the negative input TP13 lies
midway between the high and low level of the signal on the positive input TP14.
●
The average level of comparator l's output is extracted by LPF 1 and then
amplified by AC amplifier which also removes the DC offset. Since, the average
level of the comparator output is proportional to the pulse width, the original
analog signal appears at the amplifiers output TP28. Examine TP28 and note
that the output voltage is zero. This is expected since there is currently no
modulating voltage at the transmitter.
●
While monitoring the input to the pulse width modulator block TP3 and the
output from AC amplifier 1 TP28 turn the 1 KHz preset to its fully clockwise
(maximum amplitude position). Note that the modulating 1 KHz signal now
appears at the amplifiers output. If necessary, adjust the amplifiers gain adjust 1
preset until the two monitored signals are equal in amplitude.
80
Optical Fiber Communication Scientech 2502
●
In order to fully understand how this pulse width modulation transmitter/
receiver system works, examine the inputs and outputs of all functional blocks
within the system using an Oscilloscope.
●
Disconnect the PWM input from 1 KHz sine wave socket
●
Make the following additional connection (as shown in next figure) without
disturbing any previous settings
●
•
Plug the microphone into input of audio input block
•
Output of audio input block to input of PWM block
•
Output of AC Amp block to input of audio output block
Observe that the same audio sound is available in the speaker as fed to
microphone.
Questions:
●
What is frequency band for voice signals?
●
By what means the voice signals are converted into electrical signals?
●
Why PWM method is generally preferred for communication system?
81
Optical Fiber Communication Scientech 2502
Experiment 18
Objective: Study of Switched Fault in Intensity Mode
Study of the effect of switched fault number 1& 5 on amplitude modulated system
Equipments Required:
●
Scientech 2502 TechBook with Power Supply cord
●
Optical Fiber cable
●
Cathode ray Oscilloscope with necessary connecting probe
Connection Diagram:
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Optical Fiber Communication Scientech 2502
Procedure:
●
Connect the Power Supply cord to the main power plug & to TechBook
Scientech 2502 .
●
Make the following connections as shown in next figure.
•
Connect the FG output marked 1 KHz sine wave to input if emitter 1.
•
Plug in a fiber optic link from output of emitter 1 LED to the photo
transistor of the detector 1.
•
Detector 1 output TP 10 to input of Amplifier TP 27.
●
Switch ON the Power Supply of the TechBook and Oscilloscope.
●
See that AM System is operating correctly.
●
Adjust the gain adjust potentiometer in the AC amplifier circuit to provide a
sinusoidal signal of same amplitude as input.
●
Switch on fault 1. All other faults are set to ‘Off’.
●
This fault removes the bias normally present on emitter 1's LED in analog mode
so that distortion occurs when analog amplitude modulation takes place.
●
Turn ‘Off’ the fault 1 and check that A.M. system is operating correctly. Adjust
the preset to provide a sinusoidal signal of 4 V peak to peak at the output.
●
Switch ‘On’ fault 5. This shorts the output and negative input of AC amplifier 2.
So that amplifier gain is always +1, irrespective of the position of the gain adjust
2 preset.
●
Observe the output, and vary the gain adjust preset. Is there any change in
output?
●
Switch fault 5 ‘Off’.
●
Switch Power Supply ‘Off’.
Questions:
●
What do you mean by intensity modulation?
●
What is the significance of switch fault?
●
Which type of optical source is used for intensity modulation?
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Optical Fiber Communication Scientech 2502
Experiment 19
Objective: Study of switched Faults in FM System
To study the Effect of fault number 4, 6, & 8 in F.M. System
Equipments Required:
●
Scientech 2502 TechBook with Power Supply cord
●
Optical Fiber cable
●
Cathode ray Oscilloscope with necessary connecting probe
Connection Diagram:
Procedure:
●
Connect the Power Supply cord to the main power plug & to TechBook
Scientech 2502.
●
Ensure that all switched faults are ‘Off’.
●
Make the following connections as shown in next figure.
•
Connect Function Generator 1 KHz sine wave signal to frequency
modulator input.
•
Frequency modulator output TP2 to the emitter 1 input at TP5.
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Optical Fiber Communication Scientech 2502
•
Connect the optic fiber between the emitter 1 circuit and the detector 1
circuit.
•
Detector 1 output TP10 to comparator 1 input at TP14.
•
Comparator 1 output TP15 to the PLL detector input at TP23.
•
PLL detector output at TP26 to the low pass filter 1 input at TP19
•
Low Pass Filter 1 output TP20 to A C amplifier 1 input at TP27
●
Set the emitter 1 to digital mode.
●
Switch ON the Power Supply of the TechBook and Oscilloscope.
●
Check, that FM System is operating correctly.
●
Adjust the preset in AC amplifier block to give a sinusoidal signal of amplitude
same as input.
●
Switch fault 4 ‘On’ all other faults set to ‘Off’. This fault affects the phase
locked loop detector between the voltage controlled oscillator (VCO) and phase
comparator (exclusive OR gate). The result is that the PLL no longer follows
changes in the frequency of the input signal.
●
Observe the system output at TP 28 and the output of PLL block.
●
Switch fault 4 ‘Off’.
●
Switch fault 6 ‘On’. This changes the DC bias on frequency modulator VCO
input from +2.5V to 0V, So that the VCO no longer oscillates irrespective of the
signal applied to its input.
●
Observe the system output and the FM block's output.
●
Switch fault 6 ‘Off’.
●
Switch fault 8 ‘On’. These shorts pin 11 'R' of IC9 to junction of R90 &C32.
●
Observe the output of system and also PLL output.
●
Switch ‘Off’ fault 8 and check the operation of FM System.
Questions:
●
What is the function of VCO?
●
What is FM?
●
What is the function of PLL while detecting the transmitted signals?
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Optical Fiber Communication Scientech 2502
Experiment 20
Objective: Study of switched Faults in PWM System
To study the Effect of Switched Faults 2, 3 & 7 on pulse width modulation system.
Equipments Required:
●
Scientech 2502 TechBook with Power Supply cord
●
Optical Fiber cable
●
Cathode ray Oscilloscope with necessary connecting probe
Connection Diagram:
86
Optical Fiber Communication Scientech 2502
Procedure:
●
Connect Power Supply cord to the mains and to the TechBook Scientech 2502.
●
Ensure that all switched faults are set to ‘Off’.
●
Make the following connections as shown in next figure.
•
FG' s 1KHz sine wave signal to the Pulse width modulator input TP3
•
Pulse width modulator output TP4 to emitter 1 input TP5
•
Connect the optic fiber between the emitter 1 circuit and detector 1 circuit.
•
Detector 1 output TP10 to comparator & input at TP14.
•
Comparator 1 output TP15 to LPF 1 at TP19.
•
LPF 1 output TP20 to A C amplifier 1 input at TP27.
●
Switch on the Power Supply of the TechBook and Oscilloscope.
●
Switch on the Power Supply.
●
Check the correct operation of the PWM system.
●
Switch on fault number 2. This open circuits the feed back loop of the first stage
of detector 2's voltage amplifier, so that the final amplifier output at TP 28
saturates.
●
Observe the output at detector 2 it saturates at + 10V, and observe the system
output, it goes to zero. Try to the explain reason behind it.
●
Switch fault 2 ‘Off’.
●
Switch on fault number 3.
●
This open circuits the positive. In put of first comparator and due to this the
system output goes zero. Try to explain reason behind it.
●
Switch ‘Off’ fault 3.
●
Switch fault 7 ‘On’. This switches ‘Off’ constant current source to PWM so that
output level of modulator is permanently high.
●
Observe the output of PWM, it goes permanently high and output of system to
zero. Try explaining reasons for it.
●
Switch off fault 7 check correct operation of PWM System.
Questions:
●
What is the significance of Switched Faults?
●
What is full form of PWM?
●
What is the function of PWM?
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Optical Fiber Communication Scientech 2502
Experiment 21
Objective: Computer to Computer communication using RS232 interface via Fiber
Optic Link
There are 2 fibers optic Links provided on Optical Fiber Communication. We shall
utilize these two links to communicate from one PC to other & via-versa. That means
it is a Duplex system of communication using fiber optic link. The Software
developed will help to transmit & receive messages from the computers.
Equipments Required:
●
Scientech 2502 TechBook with Power Supply cord
●
Optical Fiber cable
●
Personal computer - 2 Nos. 486 or Pentium, DOS 6.0 or onwards, CD drive
●
RS232 cables for connecting PC's to TechBook - 2 Nos.
Connection Diagram:
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Optical Fiber Communication Scientech 2502
Procedure:
●
Keep one PC towards left and another towards right of Optical Fiber
Communication.
●
Load the software in PC 1 & PC2, with the help of the CD supplied.
●
Keep one of the COM port free on each of PC to connect the RS232 cables.
Keep baud rate of both PC equal, say 57600.
●
Switch off both the PC's.
●
Make the following connections on the Optical Fiber Communication.
•
Connect Fiber Link on CHI (emitter to Detector).
•
Connect Fiber Link on CH2 (emitter to Detector).
•
Connect output of Detector 1 to comparator 1 input.
•
Connect output of Detector 2 to comparator 2 inputs.
•
Connect 1 KHz square wave to input of CHI (emitter).
•
Keep mode switch of both channels to digital and all Switched Faults in
‘Off’ position.
•
Switch on the TechBook. Observe input to emitter 1 and output of
comparator 1. Adjust bias of comparator 1 for square wave output.
•
Switch 1 KHz square wave from the input of CHI to input of CH2 (emitter)
and adjust comparator 2 bias for square wave output. Switch off the
TechBook.
•
Make connections as shown in diagram 17 and Switch on the TechBook.
•
Optical Fiber Communication is ready for connection to PC's. Switch off
the TechBook.
•
Connect PC1 & PC2 to D type connectors. (Any to anyone) switch on the
PC's and the TechBook & start working. Whatever you type in PC1 will be
seen on the transmit column of PC1 and will also be received in the receive
column of PC2 simultaneously & vice versa.
•
Remove any of the fiber links. To transmit & receive of that link is
disconnected.
•
Change baud rate of any of the PCs & you will find that data is not
transmitted. Keep the baud rate same.
•
Reduce the baud rate of both the PCs & you will see that transmit rate is
lower. Switch off the TechBook and the PCs.
Questions:
●
What RS 232C stands for?
●
What is the pin configuration for 25 pin D connector?
●
What is the pin configuration for 9 pin D connector?
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Optical Fiber Communication Scientech 2502
Experiment 22
Objective: Bit Rate Measurement
Determining the bit rate supported by the fiber optic link
Equipments Required:
●
Scientech 2502 TechBook with Power Supply cord
●
Optical Fiber cable
●
Oscilloscope with Power Supply cord
Procedure:
●
Set up the fiber optic digital link as explained earlier, and ensure that the link is
working satisfactorily.
●
Remove the on board TTL output from the emitter input and connect the TTL
output of square wave generator to emitter input.
●
Keep the frequency at 10 KHz.
●
Observe the received output on the Oscilloscope.
●
Vary the frequency of the TTL input observing the output each time (You can
adjust the comparator's bias preset).
●
Note the frequency at which the output is distorted or reduces to zero. The bit
rate supported by the link is twice the frequency reading corresponding to
zero/distorted output in bits per second.
Questions:
●
How to determine the bit rate?
●
What is optical fiber link?
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Optical Fiber Communication Scientech 2502
Experiment 23
Objective: Determining the sensitivity of the Fiber Optic Link
Procedure:
Sensitivity is defined as the minimum power incident on the photo detector in order to
establish the link.
●
Set up the fiber optic digital link as explained earlier using 0.5m cable, and
ensure that the link is working satisfactorily.
●
Remove the on board TTL output from the emitter input and connect the output
of square wave generator to emitter input.
●
Observe the output of the detector on the Oscilloscope.
●
Remove the end of the fiber connected to the detector and connects it to the
optical power meter.
●
Note the reading on the power meter Po. This reading is the power being
transmitted to the receiver from the source.
●
Remove the fiber end which is connected to the power meter and connect it back
to receiver.
●
Slowly reduce the amplitude of the square wave till the output being viewed on
the Oscilloscope reduces to zero.
●
Remove the fiber end from the receiver and connect it to the power meter. Note
the reading on the power meter Ps. This gives the measure of sensitivity of the
receiver.
Questions:
●
Define the sensitivity?
●
What are the elements of fiber optics link?
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Optical Fiber Communication Scientech 2502
Experiment 24
Objective: Determination of Power Margin (Power Budget)
The Power margin is defined by:
Po-Pi-Ps
Where,
Po is the power transmitted
Pi is the power lost in the fiber
Ps the sensitivity of the receiver
Procedure:
●
Assuming that the power lost in the 0.5m fiber Pi is negligible, Po -Ps gives
power margin of the link.
●
Repeat the sensitivity experiment with 1 m fiber optic cable.
●
In this case calculate the Pi as follows:
Connect the 1m fiber to the source and power meter. Note the power meter reading.
We call it p1. Assuming that the power lost in the 0.5m fiber is negligible,
Pi- is P1- Po.
Po-Pi-Ps is the power margin for this link.
Questions:
●
How to determine power Margin?
●
What do you understand by power budget?
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Optical Fiber Communication Scientech 2502
Experiment 25
Objective: To Measure Bit Error Rate.
Apparatus Required:
●
Eye Pattern and BER Measurement Module. (Optional)
●
Scientech 2501/ Scientech 2502 Optical Fiber Communication platform.
●
Patch Cords
●
Optical Fiber Cable
●
+ 5V DC, 500 mA Adapter
Theory:
In telecommunication transmission, the bit error rate (BER) is a Ratio of bits that have
errors relative to the total number of bits received in a transmission. The BER is an
indication of how often a packet or other data unit has to be retransmitted because of
an error. Too high a BER may indicate that a slower data rate would actually improve
overall transmission time for a given amount of transmitted data since the BER might
be reduced, lowering the number of packets that had to be resent.
Connection Diagram:
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Optical Fiber Communication Scientech 2502
Procedure:
This Eye Pattern and BER Measurement Module is to be used with Optical Fiber
Communication Trainer Scientech 2501/ Scientech 2502.
●
Connect Power Supply to Optical Fiber communication Trainer Scientech
2501/Scientech 2502.
●
Ensure that all switched faults are off.
●
Connect the fiber optic cable between emitter output and detector input.
●
Connect +5V DC, 500 mA adapter to the board.
●
Make the connections as shown in next figure.
•
Connect clock frequency of 64 KHz from CLK Generator to CLK In of
Data generator using patch cord.
•
Connect Data out of Data Generator to of Data In of optical Link.
•
Now connect a patch cord from Data In of Optical Link of Module to Input
of emitter section of Optical Fiber Communication Trainer Scientech
2501/ Scientech 2502.
•
Now connect a patch cord from detector section of Optical Fiber
Communication Trainer Scientech 2501/ Scientech 2502 to Data out of
Optical Link of Module.
•
Connect Data Out of Optical Link Section to Signal In of Noise Generator.
•
Make the ground common of Trainer and Module using patch cord.
●
Keep toggle switch towards Bit Error Counter.
●
Switch ON the power of Trainer and Module.
●
Initially Adjust Level pot at middle position.
●
Observe the Error Count on 7-Segment Display of Bit Error Counter by using
the start/ stop switch for any time duration T.
●
Adjust Level pot for minimum and maximum position to observe effect of
variable noise on the error count.
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Optical Fiber Communication Scientech 2502
Calculation and Observations:
Sr.
No.
CLK
Frequency
Time Duration
(td)
Total No. of
Transmitted Bits
Bit Error
Count (E)
Bit Error
Rate = E/N
N= CLK * td
Measuring Bit Error Rate
A BERT (bit error rate tester) is a procedure or device that measures the BER for a
given transmission. The BER, or quality of the digital link, is calculated from the
number of bits received with error divided by the number of bits transmitted.
BER= Bits received with Error /Total bits transmitted
Observations:
●
We can observe that for a fixed clock frequency, as we increase the level of
Noise, Bit Error Count increases.
●
We can also observe that for a fixed level of Noise, Bit Error Count are less in
No. for lower clock frequencies and more for higher clock frequencies.
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Optical Fiber Communication Scientech 2502
Experiment 26
Objective: Study and Observation of Eye Pattern.
Apparatus Required:
●
Eye Pattern and BER Measurement Module. (Optional)
●
Scientech 2501/ Scientech 2502 Optical Fiber Communication platform.
●
Patch Cords
●
Optical Fiber Cable
●
+ 5V DC, 500 mA Adapter
Theory:
The eye-pattern technique is a simple but powerful measurement method for assessing
the data-handling ability of a digital transmission system. This method has been used
extensively for evaluating the performance of wire systems and can also be applied to
optical fiber data links. The eye-pattern measurements are made in the time domain
and allow the effects of waveform distortion to be shown immediately on an
Oscilloscope.
Connection Diagram:
96
Optical Fiber Communication Scientech 2502
Procedure:
This Eye Pattern and BER Measurement Module is to be used with Optical Fiber
Communication Trainer Scientech 2501/ Scientech 2502.
●
Connect Power Supply to Optical Fiber communication Trainer Scientech 2501/
Scientech 2502.
●
Ensure that all switched faults are off.
●
Connect the fiber optic cable between emitter output and detector input.
●
Connect +5V DC, 500 mA adapter to the board.
●
Make the connections as shown in figure.
•
Connect clock frequency of 64 KHz from CLK Generator to CLK In of Data
generator using patch cord.
•
Connect Data out of Data Generator to of Data In of optical Link.
•
Now connect a patch cord from Data In of Optical Link of Module to Input
of emitter section of Optical Fiber Communication Trainer Scientech 2501/
Scientech 2502.
•
Now connect a patch cord from detector section of Optical Fiber
Communication Trainer Scientech 2501/ Scientech 2502 to Data out of
Optical Link of Module.
•
Connect Data Out of Optical Link Section to Signal In of Noise Generator.
•
Make the ground common of Trainer and Module using patch cord.
●
Keep toggle switch towards Eye Pattern Socket.
●
Connect channel 1 (CH Y) of Oscilloscope to Eye Pattern Socket.
●
Connect EXT. TRIG. Of Oscilloscope to CLK In of Data Generator and press
the EXT. TRIG. Switch.
●
Adjust the time base as well as Cal pot of Oscilloscope to get Eye Pattern as
shown in next figure.
●
Observe the Eye pattern for different clock frequencies and different Noise
Level.
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Optical Fiber Communication Scientech 2502
Observation:
Oscilloscope Setting
Eye Pattern: Without error & with error in data
Eye Pattern: Without error & with error in data
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Optical Fiber Communication Scientech 2502
Eye Pattern: Without error & with error in data
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Optical Fiber Communication Scientech 2502
Received Data with no error, less error and more error
Conclusion:
As clock frequency increases the EYE opening becomes smaller.
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Optical Fiber Communication Scientech 2502
Frequently Asked Questions
●
How fiber optics is fabricated?
Fiber optics has been fabricated from materials that transmit light and are made
from a bundle of very thin glass or plastic fibers enclosed in a tube. One end is
at a source of light and the other end is a camera lens, used to channel light and
images around the bends and corners.
●
What is fiber optics?
Fiber optics has a highly transparent core of glass, or plastic encircled by a
covering called "cladding". Light is stimulated through a source on one end of
the fiber optic and as the light travels through the tube, the cladding is there to
keep it all inside.
●
Why single mode fibers are used for long distance transmission?
The single-mode fiber optic is used for high speed and long distance
transmissions because they have extremely tiny cores and they accept light only
along the axis of the fibers. Tiny lasers send light directly into the fiber optic
where there are low-loss connectors used to join the fibers within the system
without substantially degrading the light signal.
●
What is the drawback of multimode fibers?
Multi-mode fibers which have much larger cores and accept light from a variety
of angles and can use more types of light sources. Due to this reason they cannot
be used over long distances transmissions.
●
What is optical fiber?
The fiber optic cable consists of two concentric layers of transparent materials.
The inner portion the core transports the light, the outer covering the cladding
must have a lower refractive index than the core so the two of them are made up
of different materials.
●
List the advantages of optical fiber as waveguide?
Advantages of Fiber Optic System are as follows:
Enormous Potential Band Width (BW)
Small size and weight
Electrical Isolation
Immunity to Interference and Cross talk
Signal Security
Low transmission loss
Potential Low Cost
Thinner
Non-flammable
Ruggedness and flexibility
Low cost and availability
Reliability
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Optical Fiber Communication Scientech 2502
●
List the disadvantages of optical fibers as waveguide?
Disadvantages of optical fibers systems are as follows:
Price
Fragility
Affected by chemicals
Opaqueness
Requires special skills
●
What is meant by index profile?
Index profile is the refractive index distribution across the core and the cladding
of a fiber.
●
What is step index profile?
Step index profile in which core has one uniformly distributed index and the
cladding has a lower uniformly distributed index.
●
What is graded index profile?
Graded index profile, in which refractive index varies gradually as a function of
radial distance from the fiber centre.
●
What is the principle of operation of Optical Fiber?
The principle of operation of optical fiber lies in the behavior of light. It is a
widely held view that light always travels in straight line and at constant speed.
Of course, the light propagates in straight lines, but when it is reflected inside
the optical fiber million and trillion times by the clad, each movement
comprising of a straight line and consequently because of such reflections, it
acquires the shape of the optical fiber. So effectively, it is said to have been
traveling along the fiber. It changes its direction only if there is a change in the
dielectric medium.
●
How refractive index is defined?
Refractive index of a medium is defined as the ratio of velocity of light in
vacuum to velocity of light in medium.
Refractive index =
●
Velocity of light in vaccum
Velocity of light in medium
How refraction occurs?
When a ray is incident on the interface between two dielectrics of differing
refractive indices, refraction occurs.
●
What is partial internal reflection?
If the light is refracted and also partly reflected internally in the same medium
then it is referred as Partial Internal Reflection.
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Optical Fiber Communication Scientech 2502
●
What Snell's law of refraction states?
The angle of incidence φ 1 and refraction φ 2 are related to each other and to
refractive indices of dielectrics by Snell's law of refraction which states that:
n1 sinφ 1 = n2 sinφ 2
sin φ1 n 2
=
sin φ 2 n1
It is this change in refractive indices which causes the change in the path of the
incident ray as evident from the Snell’s law. Larger the change in the refractive
indices larger change in the direction of the incident ray.
●
What is the relationship between incident ray and angle of refraction?
As the angle of incident ray increases, the angle of refraction also increases even
faster and when the angle of refraction becomes 90° thereafter, if the angle of
incidence is increased a condition is arrived where the incident ray is totally
reflected in the same medium from where it has emerged; this is referred as the
total internal reflection.
●
How the critical angle is defined?
Since, the angle of refraction is always greater than the angle of incidence, when
the incident medium is denser than the refraction medium. Thus, the angle of
refraction is 90° and the refracted ray emerges parallel to the interface between
the dielectrics. This is the limiting case of refraction and this angle of incidence
is known as critical angle φ c .
●
What is total internal reflection?
At angles of incidence greater than the critical angle the light is reflected back
into the originating dielectric medium. This behavior of light is termed as Total
Internal Reflection.
●
What is the condition of total internal reflection?
Angle of Incidence = Angle of Reflection
●
What is acceptance angle?
The maximum angle to the axis at which light may enter the fiber in order to be
propagated hence it is referred to as the acceptance angle for the fiber.
●
What is numerical aperture?
It gives the relationship between the acceptance angle and the refractive indices
of the three media involved viz. the core, the cladding and air.
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Optical Fiber Communication Scientech 2502
●
Write the formula for numerical aperture?
Numerical Aperture
n0sin a (n 2 n 2 )1/2
1
2
(n 2 n 2 )
1
2
(n n )(n n )
1 2 1 2
Where,
n0 = Refractive index of air
n1 = Refractive index of core
n2 = Refractive index of cladding
●
What is the significance of Numerical Aperture?
The Numerical Aperture is a very useful measure of light collecting ability of a
fiber. It directly relates to the refractive indices of the core and cladding. As we
observe from the above equation, greater the absolute value of the indices of
core and cladding, greater the numerical aperture; similarly, greater the
difference between the refractive indices greater the numerical aperture.
●
Give the classification of optical fiber?
●
What is single mode fiber?
In single mode fiber only one mode (Electromagnetic wave) is able to
propagate.
●
Where the single mode fiber is used?
This is used in long distance and/or, high-speed communication.
●
Why single mode fiber is used for long distance transmission?
It is beneficial over long distances since it completely eliminates a problem
known as Inter modal Dispersion associated with Multimode cables.
●
What is the dimension of core and cladding for single mode?
Core: 8.3 um
Cladding: 125 um
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Optical Fiber Communication Scientech 2502
●
What is the meaning of multi mode fiber?
The term multimode means that the diameter of the fiber optic core is large
enough to propagate more than one mode (Electro Magnetic Wave).
●
What is dispersion?
Because of the multiple modes the pulse that is transmitted down the fiber tends
to become stretched over distance this is referred to as dispersion.
●
What is the effect of dispersion?
Available bandwidth is reduced.
●
What is the application of multimode fiber?
These are typically used in applications such as LAN (Local Area Networks) &
FDDI (Fiber Distributed Area Interface)
●
What is the dimension of core and cladding for multi mode?
Core: 50 um
Cladding: 125 um
●
What are the various losses in optical fiber?
Losses in Optic Fiber are as follows:
•
Attenuation
•
Material Absorption Losses
•
Linear Scattering Losses
Ray Leigh Scatter
Mie Scattering
•
Non Linear Scattering
•
Micro Bending and Macro Bending
•
Dispersion
Inter modal Dispersion
Intra modal Dispersion
●
What are the reasons for attenuation of signals?
Several mechanisms are involved, including absorption by materials within the
fiber, scattering of light out of the core caused by environmental factors. The
degree of attenuation depends on the wavelength of light transmitted.
●
What is the definition of attenuation?
Attenuation measures the reduction in signal strength by comparing output
power with input power. Measurements are made in decibels (dB). It is defined
as: Pi
DB loss α = 10 log 10 P
o
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Optical Fiber Communication Scientech 2502
●
What material absorption loss indicates?
It is a loss mechanism related to the material composition and fabrication
process of the fiber that result in the dissipation of some of the transmitted
optical power as heat in wave-guide.
●
What is linear scattering means?
Linear scattering mechanisms cause the transfer of some or all of the optical
power contained within one propagating mode to be transferred linearly
(proportionally) into a different mode.
●
Why scattering of light occurs in optical fibers?
This process tends to result in attenuation of the transmitted light as the transfer
may be to a leaky or radiation mode that doesn't continue to propagate within
the fiber core, but is radiated from the fiber.
●
●
Give the types of scattering?
•
Ray Leigh Scattering
•
Mie Scattering
What is backscattering?
When the infrared light strikes a very-very small place where the materials in
the glass are imperfectly mixed, this gives rise to localized changes in the
refractive index resulting in the light being scattered in all directions. Some of
the light escapes the optic fiber, some continues in the correct direction and
some is returned towards the light source. This is called backscatter.
●
What are the causes of Mie Scattering?
These result from the non - perfect cylindrical structure of the wave-guide. It
may be the caused by the imperfections such as irregularities in the core
cladding interface core, cladding refractive index difference along the fiber
length, diameter fluctuations, strains and bubbles. The scattering created by such
in homogeneities is mainly in the forward direction.
●
What is micro bending?
A problem that often occurs in cabling of the optical fiber is the twisting of the
fiber core axis on a microscopic scale within the cable form. This phenomenon,
known as micro bending result from small lateral forces exerted on the fiber
during the cabling process and it causes losses due to radiation in both
multimode and single mode fiber.
●
What happens when sharp bend occurs in fiber?
The light propagates down the optic fiber solely because the incident angle
exceeds the critical angle. If a sharp bend occurs, the normal and the critical
angle move round with the fiber. The incident ray continues in a straight line
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Optical Fiber Communication Scientech 2502
and it finds itself approaching the core - cladding boundary at an angle less than
the critical angle and much of light is able to escape.
●
What is the effect of dispersion on light in optical fiber?
When an electrical pulse energizes a LASER, it launches a short flash or light
along the optic fiber. It is an unfortunate fact that the light burst becomes longer
as it moves along the fiber optic cable. The light spreads out.
●
List the types of dispersion?
There are two types of Dispersion:
●
•
Inter modal Dispersion
•
Intra modal Dispersion
What is the condition for light to be entered into the optical fiber?
Light to be propagated down the core of the optic fiber, the light must enter at an
angle greater than the critical angle.
●
How the spreading of pulses occurs in Inter model dispersion?
When each and every ray is propagated at its own angle will arrive at slightly
different times at the far end. This spreading effect will occur all along the fiber
so it is also important to appreciate that the longer the optic fiber, the greater the
dispersion. Transmission rates that are actually possible on an optic fiber
therefore depend in its length.
●
What is the effect of change in refractive index on light?
A change in refractive index will change the speed of that particular wavelength
of light.
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What is Intra modal Dispersion?
When the light source produces different wavelengths at the same time, the
components of the transmitted light pulse traveling at the same time, and then
the components of the transmitted light pulse traveling at different speeds. The
total package of light will spread out - hence the Intra modal dispersion occurs.
●
How to cure inter modal Dispersion?
A large core diameter means many modes and severe inter modal dispersion.
The cure for this type of dispersion is quite simple. Reduce the core size; the
number of modes decreases and inter modal dispersion is reduced.
●
How inter modal Dispersion is completely eliminated?
To eliminate inter modal dispersion completely simply make the core so small
that only one mode is propagated. A single ray cannot possibly go at two
different speeds so inter modal dispersion cannot occur.
●
Why LASER is used to reduce intra modal dispersion?
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Optical Fiber Communication Scientech 2502
The LASER would cause less intra modal dispersion because its light is more
concentrated around the central wavelength.
●
What is spectral width?
The spread of wavelength measured between the points where the power output
falls to half of the peak power is called the spectral width.
●
Why LASER is used for long distance transmission?
Some LASERS have spectral widths as low as 0.1 nm (nanometer). The low
spectral width together with its high power and fast switching makes the
LASER first choice for long distance communications using single mode optic
fiber.
●
What is full form of OTDR?
Optical Time Domain Reflect meter
●
What is the function of OTDR?
The OTDR is a measuring instrument that uses backscatter. It is the most
versatile piece of test equipment that we have for making measurements on fiber
optic systems.
●
What can be measured with the help of OTDR?
It provides us with two different measurements:
●
1.
It can measure the magnitude of any losses that occur along optic fiber.
2.
It can measure distance along the optic fiber.
Where fiber optics links can be used?
Fiber optic links can be used for transmission of digital as well as analog
signals.
●
Fiber optics links consists of how many elements?
Basically a fiber optic link contains three main elements, a transmitter, an
optical fiber and a receiver.
●
What is the function of transmitter, optical fiber and receiver?
The transmitter module takes the input signal in electrical form and then
transforms it into optical (light) energy containing the same information. The
optical fiber is the medium, which takes the energy to the receiver. At the
receiver light is converted back into electrical form with the same pattern as
originally fed to the transmitter.
In this system the information signal is used to control the Intensity of the
source. At the far end, the variation in the amplitude of the received signal is
used to recover the original information signal.
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Optical Fiber Communication Scientech 2502
Glossary
Acceptance Angle: The angle above which, the core of an optical fiber accepts
incoming light, usually measured from the fiber axis.
Angle of Incidence: The angle between incident ray and the normal to a reflecting or
refracting surface.
Attenuation: Reduction of signal magnitude or loss normally measured in decibels.
Fiber attenuation is normally measured per unit length in decibels per kilometer.
Avalanche Photodiode: A semiconductor photo detector with integral detection and
amplification stages. Electrons generated at p-n junction are accelerated in a region
where they free an avalanche of other electrons. A PD can detect faint signals but
require higher voltage than other semiconductor electronics.
Back Scattering: Scattering of light in the direction opposite to that in which it was
originally traveling.
Bandwidth: The highest frequency that can be transmitted in analog operation or
(especially for digital system) the information carrying capacity of the signal.
Baud: The number of signal level transitions per second in digital data for common
coding schemes, this equals bits per second.
Bit Error Rate (BER): The fraction of bits transmitted incorrectly.
Cladding: The layers of glass or other transparent material surrounding the light
carrying core of an optical fiber. It has lower refractive index than the core, and thus
confines light in the core.
Critical Angle: The smallest angle at which a meridional ray may be totally reflected
within a fiber at the core - cladding interface.
Core: The central part of optical fiber that carries light.
Dark Current: The noise current generated by a photo diode in dark.
Decibel: A logarithmic comparison of power levels, defined as ten times the base ten
logarithm of the ratio of two power levels.
Detector: A transducer that provides an electrical output signal in response to an
input
Index Matching Gel: A gel or fluid whose refractive index is close to the core index
that reduces refractive index discontinuities that can cause reflective losses.
Intensity: Power per unit solid angle.
Infrared: Wave length longer than 700 nm and shorter than about 1 nm. We can not
see infrared radiation but can feel it as heat. Transmission in optical fiber is best in
infrared at wavelengths of 1100-1600 nm.
ISDN: Acronym of Integrated Services Digital Network is a digital standard calling
for 144K bits/sec transmission, corresponding to two 64 K bits/sec digital voice
channels and one 16 K bits/data channel.
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Optical Fiber Communication Scientech 2502
LASER: Acronym of Light Amplification by Stimulated Emission of Radiation. A
device that produces monochromatic coherent light through stimulated emission most
lasers used in fiber optic communication is solid state semiconductor devices.
LED: Acronym of Light Emitting Diode, a semiconductor device which emits light
from a p-n junction (when biased with an electrical current) Light may exit from the
junction strip edge or from its surface (depending on device structure)
Light: Strictly speaking, electromagnetic radiation with properties similar to visible
light includes the invisible near-infrared radiation in most fiber optic communication
system.
Mode: An electromagnetic field distribution that satisfies theoretical requirement for
propagation in waveguide.
Micro bending: Tiny bends in a fiber that allow light to leak out and increase loss.
Macro bending: In an optical fiber all macroscopic deviations of the axis from a
straight line distinguished from micro bending.
Meridian Ray: A light ray that passes through the axis of optical fiber. It is generally
used when illustrating the fundamental transmission properties of optical fiber.
Material Dispersion: Light impulse broadening caused by various wavelengths of
light traveling at different velocities through a fiber. Material dispersion increases
with increasing spectral width of the source.
Numerical Aperture: A characteristic parameter of any given fiber's light gathering
capability defined by the sine of half angle over which a fiber can accept light. It is
multiplied by the refractive index of the medium containing the light.
N.A = n o sin θa = (n12 -n 22 ) 1 2
Noise Equivalent power: The rms value of optical power which is required to
produce an rms signal to noise ratio of 1, and indication of noise level which defines
the minimum detectable signal level.
Optic fiber: This is the length of clear material along which we propagate the light.
Optical Time Domain Reflectometer (OTDR): A method for measuring
transmission characteristics by sending a short pulse of light through fiber and the
resulting back scatter and reflection is measured as a function of time. It is useful in
estimating attenuation coefficient as a function of distance and identifying defects and
other localized losses.
Polarization: Alignment of the electric and magnetic fields that make up an electro
magnetic wave normally refers to the electric field. If all light waves have the same
alignment; light is said to be polarized.
Rayleigh Scattering: Scattering by refractive index fluctuations ( non-homogeneities
in material density or composition) that are small with respect to wavelength.
Reflectance: The ratio of reflected power to incident power. Frequently referred to as
optical density or as a percent in communication applications, generally expressed in
dB.
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Optical Fiber Communication Scientech 2502
Radiometer: An instrument distinct from photometer, to measure power (watts) of
electromagnetic radiation.
Responsivity : The ratio of detector output to input, usually measured in units of
amperes per watt.
SMA: Abbreviation of sub-miniature assembly.
Signal – to - Noise Ratio: The ratio of signal to noise, measured in decibels, an
indication of signal quality in analog system.
Skew Ray: A ray that does not intersect the axis of a fiber is known as skew ray.
Total Internal Reflection: The total internal reflection occurs when light strikes an
interface at angles of incidence (with respect to normal) greater than the critical angle.
Wavelength: The distance an electromagnetic wave travels in the time it takes to
oscillate through a complete cycle. Wavelength of light is measured in nanometers or
micro meters.
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Optical Fiber Communication Scientech 2502
Warranty
1.
We guarantee this product against all manufacturing defects for 24 months from
the date of sale by us or through our dealers.
2.
The guarantee will become void, if
a. The product is not operated as per the instruction given in the Learning
Material.
b. The agreed payment terms and other conditions of sale are not followed.
c. The customer resells the instrument to another party.
d. Any attempt is made to service and modify the instrument.
3.
The non-working of the product is to be communicated to us immediately giving
full details of the complaints and defects noticed specifically mentioning the
type, serial number of the product and date of purchase etc.
4.
The repair work will be carried out, provided the product is dispatched securely
packed and insured. The transportation charges shall be borne by the customer.
Hope you enjoyed the Scientech Experience.
List of Accessories
Quantity
Ø
Patch Cord 16 "(2mm) ...................................................................................10
Ø
Power Supply ...............................................................................................1
Ø
Mains Cord....................................................................................................1
Ø
Head Phone ...................................................................................................1
Ø
Microphone ...................................................................................................1
Ø
Mandrel .........................................................................................................1
Ø
N.A. Plate......................................................................................................1
Ø
N.A. Stand.....................................................................................................1
Ø
Fiber Optic Cable 1 Meter... ..........................................................................1
Ø
Fiber Optic Cable 1/2 Meter... .......................................................................1
Ø
Plastic Box for Cable .....................................................................................1
Ø
Cable RS232 .................................................................................................2
Ø
Crocodile to 2mm Patch Cord........................................................................1
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