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INTRODUCTION:
The subject of communications really begins with the situation shown in
Figure1. Here is an entity called the Source and one called the User- located remotely
from the Source. The Source generates Information and the User desires to learn what
this Information is.
Fig.1 SITUATION FOR COMMUNICATION
The question then arises as to how to send the binary data stream from Source to
User. A Transmission Medium is employed to transport the Information from Source to
User. What is a Transmission Medium? A Transmission Medium is some physical entity.
It is located between the Source and the User and it is accessible to both. The
Transmission Medium has a set of properties described by physical parameters. The set
of properties exists in a quiescent state. However, at least one of these properties can be
stressed or disturbed at the Source end. This is accomplished by somehow imparting
energy in order to stress the property. This disturbance does not stay still, but affects the
parts of the Transmission Medium around it. This disturbance then travels from the
Source end to the User end. Consequently, energy imparted in creating the disturbance is
thereby transferred from the Source end to the User end. Finally, this disturbance or
stressed property can be sensed at the user end.
THE FUNDAMENTAL PROBLEM OF COMMUNICATION:
The fundamental problem of communications is a design problem. The
combination of Transmitter, Transmission Medium and Receiver is termed the
communication link or data link. In the context of our discussion the fundamental
problem of communications is to design a data link appropriate for connecting a given
Source-User pair. Most exercises in obtaining the design solution usually begin with
choosing a Transmission Medium to meet the general requirements of the Source-User
pair. Every Transmission Medium has constraints on its operation, on its performance. It
is these constraints that really decide which Transmission Medium will be employed for
the data link design. It will be worthwhile discussing these constraints.
CONSRAINTS OF COMMUNICATION SYSTEM:
1. Transmission medium attenuation constraints:
In any communication system the data is transmitted by electromagnetic,
electrical, light or any other signal, which have some energy. That is it’s amplitude.
Travelling through the transmission medium the signal may be attenuated due to
interference in medium and may not possible to detect the receiver even at minimal
level. So transmission medium has to be able to deliver at least the minimal
detectable level of signal at receiver end.
2. Interference Constraints:
Interference or noise is some extraneous signal, usually generated outside of the
transmission medium, somehow it get inside of the transmission medium. It realizes
its effect usually by adding itself to the propagating signal.
.
3. Bandwidth Constraints:
Due to attenuation the signal at the output is spread in time. It is the result of the
finite bandwidth of the transmission medium. Because signal is spread in time it is
going to interfere with the output pulses due to input data signals, which will come
after it. So in data link design task the first line of defense against time dispersion lies
in proper selection of the transmission medium.
WHY OPTICAL FIBER?
Considering this discussion of the constraints on the Transmission Medium we
are naturally led to fiber optic cable as an attractive choice for the data link design. Why?
When compared with other candidates for the Transmission Medium commonly
employed today, there is no comparison when it comes to attenuation, interference and
bandwidth.
The optical carrier frequency in the range 1013 to 1016Hz yields a far greater
potential transmission bandwidth than metallic cable. The fiber is totally immune to
virtually all kinds of interference, including lightning, and will not conduct electricity. It
can therefore come in direct contact with high voltage electrical equipment and power
lines. It will also not create ground loops of any kind. It will also not create ground
loops of any kind. As the basic fiber is made of glass, it will not corrode and is unaffected
by most chemicals.
A fiber optic cable, even one that contains many fibers, is usually much smaller
and lighter in weight than a wire or coaxial cable with similar information carrying
capacity. It is easier to handle and install, and uses less duct space.
Optical fibers are quite difficult to tap. Since they do not radiate electromagnetic energy,
emissions cannot be intercepted. And physically tapping the fiber takes great skill to do
undetected. Thus, the fiber is the most secure medium available for carrying sensitive
data.
OPTICAL FIBER SYSTEM:
The optical fiber communication system contains mainly three components 1.
Optical transmitter 2. Fiber optic cable 3. Optical receiver. The transmitter converts an
electrical analog or digital signal into a corresponding optical signal. The source of the
optical signal can be either a light emitting diode, or a solid-state laser diode. The cable
consists of one or more glass fibers, which act as wave-guides for the optical signal. The
receiver converts the optical signal back into a replica of the original electrical signal. In
this paper fiber optic cable itself and it’s manufacturing process is explained in detail.
WHAT IS OPTICAL FIBER?
Optical fibers are long, thin strands of very pure glass about the diameter of a
human hair. They are arranged in bundles called optical cables and used to transmit light
signals over long distances. If you look closely at a single optical fiber, you will see that
it has the following parts as shown in figure2:
Fig.2 PARTS OF SINGLE OPTICAL FIBER
 Core: The inner light-carrying member.
 Cladding: The middle layer, which serves to confine the light to the core.
 Buffer: The outer layer, which serves as a "shock absorber" to protect the core and
cladding from damage.
PHYSICS OF LIGHT PROPAGAITON:
Ray theory:
Ray of light travels more slowly in an optically dense medium then in one that is less
dense and refractive index gives measure of this effect. For the transmission total internal
reflection occurs only if following two conditions are met:
1. The glass inside fiber core must have a slightly higher index of refraction n1 than
the index of refraction n2 of the cladding.
2. Light can be guided down the fiber optic cable if it enters at less than the critical
angle. This angle is fixed by the indices of refraction of the core and cladding and
is given by the formula:
c = arc cosine (n2 /n1).
Of course, it must be noted that a light ray enters the core from the air outside, to the left
of figure3. The refractive index of the air must be taken into account in order to assure
that a light ray in the core will be at an angle less than the some critical angle. This can be
done fairly simply. The following basic rule then applies. Suppose a light ray enters the
core from the air at an angle less than an entity called the external acceptance angle ext.
It will be guided down the core. Here,
ext = arc sin [(n1/ n0) sin
(c)]
Fig.3 LIGHT PROPAGATION IN OPTICAL FIBER
With n0 being the index of refraction of air. This is required maximum “acceptance angle
“and rotating this angle about the fiber axis we will get acceptance cone. Any light aimed
at the fiber end within this cone will be accepted & propagated. Rays of light do not
travel randomly. They are channeled into modes, which are possible paths for a light ray
traveling down the fiber. A fiber can support as few as one mode to as many as thousands
of modes. The numbers of modes that exist depend upon the dimensions of the cable and
the variation of the indices of refraction of both core and cladding across the cross
section.
Refractive Index Profile:
There are mainly two types of refractive index profile:
 Step index profile: It is characterized by a core with a completely constant index
of refraction n1 throughout its bulk and a sudden transition of index to a lower
value at the core wall.
 Graded index profile: In this type of fiber, the material in the core is modified so
that the index of refraction has a maximum value n1 at the axis and lesser values
falling off according to a carefully chosen profile with distance from the axis.
Step index profile consists two modes of propagation. Either single mode or
multimode. Single mode fiber has a narrow core (eight microns or less), and the index of
refraction between the core and the cladding changes less than it does for multimode
fibers. Light thus travels parallel to the axis (as shown in figure 4), creating little pulse
dispersion. Telephone and cable television networks install millions of kilometers of this
fiber every year. Step index multimode fiber has large core up to 100 microns in
diameter. As a result, some of the light rays that make up the digital pulse may travel a
direct route, where as others zigzag as they bounce off the cladding. These alternative
pathways cause the different groupings of light rays, referred to as modes, to arrive
separately at a receiving point. It is shown in figure 4.
Fig.4 DIFFERENT TYPES OF REFRACTIVE INDEX PROFILE
Graded index multimode fiber has variation of index of index gradual as it
extends out from the axis of the core through the core to the cladding (figure 4). There is
no sharp discontinuity in the indices of refraction between core and cladding. The core
here is much larger than in the single-mode step index case discussed above. Multi-mode
propagation exists with a graded index. However, as illustrated the paths of the higher
order modes are somewhat confined. They appear to follow a series of ellipses. Because
the higher mode paths are confined the attenuation through them due to leakage is more
limited than with a step index.
Well than the question now will arise that how this much amount of thin optical
fiber be made and from what kind of material. The next section of the paper is devoted to
give the answer of this question.
CLASSIFICATION OF MANUFACTURING PROCESS OF FIBER:
After the study of refractive index profile, it is clear that a variation of refractive index in
side the optical fiber is a fundamental necessity in the fabrication of the fibers for light
transmission. Hence at least two materials, which are transparent to light, are required.
The graded index fiber requires gradually varying refractive index. So during
manufacturing process refractive index may be varied by suitable doping with another
compatible material. The manufacturing process is classified as following.
Double Crucible Method
Vapor phase deposition method
1.Double crucible method:
The first stage of this process is the preparation of ultra pure material powders,
which are usually oxides, or carbonates of required materials. As shown in figure5
bellow the starting material is fed in either in powder form or by means of rods and
heated together in a platinum or silicacrucible. The double crucible is mounted inside
a vertical, silica lined muffle furnace capable of raising the melt to 1000-1200 degree
temprature. The crucible have nozzles from which the clad fiber is drawn directly. By
varying the concentration of the components the graded fiber can be obtained. Using
this method continuous fiber can be drawn by feeding raw material in crucible.
Fig.5 DOUBLE CRUCIBLE METHOD
2.Vapor phase deposition methods:
This method is also classified in two parts.1.Flame Hydrolysis 2.Chemical Vapor
Deposition. The flame hydrolysis method fabricate the glass fiber either applying a
hydrogen-oxygen flame with proper amount of mixture of halide vapor & depositing
it on a rod or it may fabricate the glass by passing a metal halide gas & oxygen in
rotating tube and hydrolysis process is done by applying the heat continuously at the
outside surface by H2-O2 flame. There are mainly three procedures of flame
hydrolysis explained as following.
1.Outside Vapor Deposition (OVD): In this method glass soot is deposited on to
Fig.6 OUTSIDE VAPOR DEPOSITION
Alumina rod by flame hydrolysis, the metal halide vapors being introduced in to a
methane-oxygen flame and directed on to the rotating rod as shown in figure6 above.
Many layers are built up by scanning the flame along the length of the rod; their
composition being varied so as to produce either a step or a graded variation of refractive
index, first to form core and then cladding layers. The rod is then removed, leaving a
hollow, porous preform. This is heated in chlorine or thionyl chloride and so dried and
sintered to form a solid, clear glass rod and is drown into fiber.
2.Vapor axial deposition (VAD): Core and cladding glasses are deposited simultaneously
on to the end of a seed rod, which is rotated to ensure
Fig.7 VAPOR DEPOSITION METHOD
azimuthally homogeneity and is drawn up into an electric furnace at about 2.5 mm per
minute. There it is heated in an atmosphere of oxygen and thionyl chloride vapor in order
to remove all water and hydroxyl –ion content by chemical reaction. The porous rod of
about 60 mm diameters is then heated in carbon furnace (figure 7) at higher temperature,
where it becomes consolidated, transparent, glassy preform of some 20 mm diameter.
3.Inside vapor deposition (IVD): The vapor deposition takes place by thermal oxidation
on the inside of a hollow tube of pure synthetic silica. Typically the tube is about 1 m
long, 15 mm in diameter and has a wall thickness of about 1 mm. It is mounted on a
glass-working lathe equipped with oxygen hydrogen flame. Metal (Si, Ge etc.) halide
vapors are passed into the tube. All flow rates are carefully controlled and highest purity
is ensured by distillation of the source materials. An interaction between vapors and
oxygen takes place on the inside silica in the heated band with the result that a layer of
SiO2 & dopants is deposited. This is shown in figure8 bellow.
Fig.8 INSIDE VAPOR DEPOSITION
CHEMICAL VAPOR DEPOSITION METHODS:
The chemical vapor deposition method contains mainly Modified Chemical Vapor
Deposition method. In MCVD, oxygen is bubbled through solutions of silicon chloride
(SiCl4), germanium chloride (GeCl4) and/or other chemicals.
Fig.9 MODIFIED CHEMICAL VAPOR DEPOSITION
The precise mixture governs the various physical and optical properties (index of
refraction, coefficient of expansion, melting point, etc.). The gas vapors are then
conducted to the inside of a synthetic silica or quartz tube (cladding) in a special lathe. As
the lathe turns, a torch is moved up and down the outside of the tube. The extreme heat
from the torch causes two things to happen. The silicon and germanium react with
oxygen, forming silicon dioxide (SiO2) and germanium dioxide (GeO2). The silicon
dioxide and germanium dioxide deposit on the inside of the tube and fuse together to
form glass. It is shown in figure as above. MCVD permits faster deposition rate and be
still increased if microwave frequency plasma is created in the reaction zone. Such a
process is known as “Plasma assisted Chemical Vapor Deposition method”. A very fine,
clear glassy deposit is put down only in the plasma region, which is translated through the
tube at up to 0.1m/s rate.
FIBER PULLING AND COATING: The preform, after made by any of above
method, is loaded into the fiber-pulling tower. It gets lowered into a graphite furnace and
the tip gets melted until a molten glob falls down by gravity. As it drops, it cools and
Fig.10 FIBER PULLING AND COATING
forms a thread. The operator threads the strand through a series of coating cups (buffer
coatings) and ultraviolet light curing ovens onto a tractor-controlled spool. The tractor
mechanism slowly pulls the fiber from the heated preform blank and is precisely
controlled by using a laser micrometer to measure the diameter of the fiber and feed the
information back to the tractor mechanism. Fibers are pulled from the blank at a rate of
10 to 20 m/s and the finished product is wound onto the spool. So now fiber is get ready
for required application.
TESTING OF FINISHED OPTICAL FIBER: After the manufacturing of
optical fiber using above-mentioned methods it should be tested that whether it is made
perfect according required parameters? The different parameters of optical fiber like
tensile strength, refractive index profile, fiber geometry, attenuation, information carrying
capacity, dispersion etc. Once the fibers have passed the quality control, they are sold to
telephone companies, cable companies and network providers. Many companies are
currently replacing their old copper-wire-based systems with new fiber-optic-based
systems to improve speed, capacity and clarity.
INTER COMPARISION OF VAPOR PHASE DEPOSITION
METHODS: Well, IVD, MCVD and PCVD methods are mostly same because they
apply the basic principle of deposition of fiber forming soot or vapor inside the tube. So
here is given the comparison of different parameter among MCVD, OVD and VAD:
Comparison
parameter
Attenuation (db/km)
at
different
wavelength (micrometer)
MCVD
0.2 at 1.55 for
single mode, 0.34 at
1.55
for
multi
mode, 0.45 at 1 for
graded index
Usual fiber length 30 for mono mode
(km)
and 10 for single
mode as well as
graded index
Production
Batch
(Batch\Continuous)
Basic
Chemical Oxidation of metal
Process
(Si, Ge, Boron etc.)
halide
Production Rate
Higher compare to
OVD & VAD
OVD
VAD
1 & 1.8 at 1.2 & 1.5 0.7 to 2 at 1.81 for
respectively for all all modes
modes
100 for mono mode, 100 for mono mode,
30 for multi mode 30 for multi mode
and graded index
and graded index
Batch
Continuous
Hydrolysis of metal Hydrolysis of metal
halide
halide
Lower
Lower
CONCLUSION: For the subject of data communication OPTICAL FIBER is the best
option to overcome the fundamental limitations of the transmission medium as well as it
has so many advantages over conventional transmission medium. From the
manufacturing point of view, any one method can’t produce the optical fiber with all the
desired and required parameters so, there should be selected the method according to
application of fiber as well as on which parameter of fiber manufacturer wants to more
concentrate.
References:
1.www.commspecial.com
2.www.telebyteusa.com
3.www.tpub.com
4.www.netoptics.com
5.electronics.howstuffworks.com
6.Optical fiber communications by John M. Senior
7.Optical communication systems by John Gowar